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Birth of a New World: Astronomers Confirm Protoplanet 374 Light Years From Earth

By University of Liege June 28, 2023

Image of the HD 169142 system showing the signal of the forming planet HD 169142 b (around 11 o’clock), as well as a bright spiral arm resulting from the dynamic interaction between the planet and the disc in which it is located. The signal from the star, 100,000 times brighter than the planet, was subtracted by a combination of optical components and image processing (mask in the center of the image). Observations at different times show the planet advancing in its orbit over time. Image obtained with ESO’s VLT/SPHERE instrument. Credit: V. Chrisitaens / ULiège

Located 374 light years away from Earth, HD169142 b has been confirmed as a protoplanet by a team of researchers from the University of Liège and Monash University.

An international team of researchers — including Valentin Christiaens from the University of Liège — has just published the results of the analysis of data from the SPHERE instrument of the European Southern Observatory ( ESO ), which confirms a new protoplanet. This result was made possible thanks to advanced image processing tools developed by the PSILab of the University of Liège. The study is published in the Monthly Notices of the Royal Astronomical Society (MNRAS).

Planets form from clumps of material in discs surrounding newborn stars. When the planet is still forming, i.e. when it is still gathering material, it is called a protoplanet . To date, only two protoplanets had been unambiguously identified as such, PDS 70 b and c, both orbiting the star PDS 70. This number has now been increased to three with the discovery and confirmation of a protoplanet in the disk of gas and dust surrounding HD 169142, a star 374 light years from our solar system.

A protoplanet is an embryonic planet, a large body that is in the process of becoming a planet. It forms from a concentration of gas and dust within a protoplanetary disc, a ring of material that orbits a newly formed star. As this material begins to coalesce, it creates a protoplanet that gradually grows by attracting more of the surrounding material through its increasing gravitational pull.

“We used observations from the SPHERE instrument of the European Southern Observatory’s ( ESO ) Very Large Telescope ( VLT ) obtained on the star HD 169142, which was observed several times between 2015 and 2019,” explains Iain Hammond, a researcher at Monash University (Australia) who stayed at ULiège as part of his doctoral thesis. “As we expect planets to be hot when they form, the telescope took infrared images of HD 169142 to look for the thermal signature of their formation. With these data, we were able to confirm the presence of a planet, HD 169142 b, about 37 AU (37 astronomical units, or 37 times the distance from the Earth to the Sun) from its star — slightly further than the orbit of Neptune .”

Back in 2020, a team of researchers led by R. Gratton had previously hypothesized that a compact source seen in their images could trace a protoplanet. Our new study confirms this hypothesis through both a re-analysis of the data used in their study as well as the inclusion of new observations of better quality.

The different images, obtained with VLT’s SPHERE instrument between 2015 and 2019, reveal a compact source that is moving over time as expected for a planet orbiting at 37 astronomical units from its star. All data sets obtained with the SPHERE instrument were analyzed with state-of-the-art image processing tools developed by the PSILab team at the University of Liège .

The last data set considered in our study, obtained in 2019, is crucial for the confirmation of the planet’s motion,” explains Valentin Christiaens, F.R.S.-FNRS research fellow at the PSILab ( STAR Institute / Faculty of Science ) of the ULiège. “This data set had not been published until now.”

A protoplanetary disc is a flat, rotating disc of dense gas and dust that surrounds a newly formed star. It forms from the original molecular cloud that collapsed to form the star and contains the leftover material that didn’t end up in the star itself. These discs play a crucial role in planetary system formation, as they are the environment in which protoplanets form and grow.

The new images also confirm that the planet must have carved an annular gap in the disc — as predicted by the models. This gap is clearly visible in polarized light observations of the disc.

“In the infrared, we can also see a spiral arm in the disc, caused by the planet and visible in its wake, suggesting that other protoplanetary discs containing spirals may also harbor yet undiscovered planets,” says Hammond.

The polarized light images, as well as the infrared spectrum measured by the research team, further indicate that the planet is buried in a significant amount of dust that it has accreted from the protoplanetary disc. This dust could be in the form of a circumplanetary disc, a small disc that forms around the planet itself, which in turn could form moons. This important discovery demonstrates that the detection of planets by direct imaging is possible even at a very early stage of their formation.

“There have been many false positives among the detections of planets in formation over the last ten years,” says Valentin Christiaens. “Apart from the protoplanets of the PDS 70 system, the status of the other candidates is still hotly debated in the scientific community. The protoplanet HD 169142 b seems to have different properties to the protoplanets of the PDS 70 system, which is very interesting. It seems that we have captured it at a younger stage of its formation and evolution, as it is still completely buried in or surrounded by a lot of dust.”

Given the very small number of confirmed forming planets to date, the discovery of this source and its follow-up should give us a better understanding of how planets, and in particular giant planets such as Jupiter , are formed.

Further characterization of the protoplanet and independent confirmation could be obtained through future observations with the James Webb Space Telescope ( JWST ). The high sensitivity of JWST to infrared light should indeed allow researchers to detect thermal emissions from the hot dust around the planet.

Reference: “Confirmation and Keplerian motion of the gap-carving protoplanet HD 169142 b” by Iain Hammond, Valentin Christiaens, Daniel J Price, Claudia Toci, Christophe Pinte, Sandrine Juillard and Himanshi Garg, 4 April 2023, Monthly Notices of the Royal Astronomical Society: Letters . DOI: 10.1093/mnrasl/slad027

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1 comment on "birth of a new world: astronomers confirm protoplanet 374 light years from earth".

“Given the very small number of confirmed forming planets to date, the discovery of this source and its follow-up should give us a better understanding of how planets, and in particular giant planets such as Jupiter, are formed.”

The paper discussion confirms that a (super)Jupiter massed candidate protoplanet is most likely.

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Protoplanet Theory and Earth’s Formation

No one really knows for sure how the Solar System began. It would be like ask­ing a child to give an account of his birth or a descrip­tion of his conception. Religious scriptures explain the creation of the Earth in compelling ways, but no two accounts agree exactly. Some of them, however, do come quite close to the scientist's idea of creation-or, at least, to the readings of the evidence lodged in the Earth's ancient rocks.

In exploring the origin of the Earth we must at the same time try to explain the beginning of the Solar Sys­tem, for the Earth's past is intimately tied to the history of our nearest neighbors in space.

In 1755 the German philosopher Immanuel Kant published his theory of the heavens, postulating that in the beginning there was an immense, cold whirling cloud of dust and gas. This suggestion is accepted readily by astronomers today. Their extremely powerful modern telescopes show re­mote, dark clouds of dust floating between distant stars -clouds that must even now be similar to the local, swirling cloud that Kant had in mind.

In 1796 Kant's contemporary, the French mathemati­cian Pierre Simon Laplace, took his idea a step further by suggesting how the Solar System might have formed from such a cloud.

The immense mass was set spinning by cosmic forces, Laplace hypothesized. At the same time it began to shrink in size under the gravitational pull of its own matter. At intervals, the contracting cloud shed veils of particles into space, which eventualy condensed into the planets. Shrinking under the force of its own gravity, meanwhile, the central mass became the Sun.

As potent as Laplace's concept was, it fell victim to fundamental physical laws of more recent discovery. Calculations based on these laws show that a shrinking Sun would spin faster and faster as it grew smaller and smaller, until today it would be rotating at a far greater speed than it actually is.

After Laplace's brilliantly imaginative picture was shown to contain flaws, several other seemingly plausi­ble suggestions were put forward by astronomers. One theory assumed the formation of the Sun first, with no planets. Then, a second star passing close by in space tore out a long stream of material. The planets, it was suggested, might then have condensed around the Sun, with the passing star continuing on its way. Unfortu­nately, calculations show that such hot material from the Sun would disperse, rather than form planets. Even if by some unknown process planets were to condense, their orbits would be much more irregular than those found in the Solar System today.

Another theory held that in the distant past of the cosmos, or universe, the Sun had a twin companion, and a passing star collided with its twin. Out of the debris resulting from such a collision, planets might possibly form in orbits around the single remaining sun. But the great distances at which the stars are scattered in space make collisions of this type most unlikely. If such a catastrophe did occur, it seems impossible that planets could form directly from the intensely hot and volatile material of the exploding stars. Both the "close encoun­ter" theory and the "collision" theory fail on one fur­ther count; neither explains how most of the planets have obtained moons.

More recently, cosmologists went back to the sugges­tion of Kant, careful to avoid the pitfall of Laplace. A theory took shape from the combined efforts of astronomers, mathematicians, chemists and geolo­gists. This hypothesis is called the "nebular" or "proto-planet" theory. It gives unity to so many seemingly disparate details of material reality that a majority of cosmologists have become convinced that it cor­rectly accounts for at least the broad features of cosmic evolution.

In the cold depths of the cloud surrounding the proto-Sun, certain atoms of gas combined to form com­pounds, such as water and ammonia. Slowly, solid dust crystals began to grow as did metallic crystals, includ­ing iron and stony silicates. And, gradually, gravita­tional and centrifugal forces at work in the spinning cloud flattened it into the shape of an enormous protoplanetary disc.

If we could have viewed the events at a great distance, our eyes would have beheld something like a gigantic, re­volving vinyl record, with the proto-Sun in the hole at the center.

Within the huge whirling disk, local eddies continued to appear. Some of the swirls were doubtless torn apart in collisions, while others were broken up by the in­creasingly strong gravitational pull of the proto-Sun. In a sense, each small eddy was carrying on a fight for survival. To hold itself together in the face of such dis­ruptive forces, an eddy had somehow to collect a cer­tain critical amount of substance to provide its own center of gravity.

In a kind of cosmic battle within the wheeling system, some local swirls gained material as others lost it. Ultimately a series of large whirling disks developed in the region around the Sun. Each was a proto-planet.

These proto-planets were sufficiently large to hold together under the strength of their own gravitational fields. As each moved through space around the Sun, it acted as a sort of scavenger, sweeping up leftover mate­rial from the original cloud.

At this stage thermonuclear fusion began in the core of the proto-Sun releasing large amounts of energy, and the proto-Sun began to shine. It "burned" fitfully at first, a dull red. In time it was to become the golden yellow star that we see today. Remember that the proto-Sun was about one hundred times larger in diam­eter than the proto-planets. It was this immense differ­ence in size, of course, that caused it to become a star rather than a planet. Its strong gravitational pull was sufficient to trap light hydrogen atoms in its interior, triggering thermonuclear fusion. Such was not the case with the smaller proto-planets.

Somewhere in the region of the proto-Sun, then, proto-Earth was born as a whirling cloud of icy parti­cles and solid fragments-a cosmic dust storm. Only later did this material collect into a ball, sticking to­gether because of the cohesive attraction of water and ice molecules. As proto-Earth orbited around the Sun, it swept up more material by gravitational attraction. Thus the Earth and the other planets formed by the process of accumulation of cold dusts from the region of space near the Sun.

Gradually radioactive elements within the cold ball of dust that was Earth began to give off heat. After mil­lions of years the Earth's temperature became high enough to melt the material at its center. At that time, the heavy metals-iron and nickel-that were spread throughout the ball began to sink to form the molten core of the planet. Afterward, molten rock frequently broke through fissures to the surface. And slowly, molecules of hydrogen, water vapor and other gases escaped from within to create an atmosphere above the planet's surface. But these light gases did not stay with the Earth for long. A second major source of heat was already in action-the rays of the Sun.

The Sun's radiation was now striking the Earth with full intensity, breaking up the molecular compounds in its primitive atmosphere and scattering them into space. Thus most of the atmospheric hydrogen and other light elements escaped from the Earth. This process eventu­ally left behind a high concentration of the heavier, rarer elements of the universe-elements essential for the formation of rocks, plants and our own bodies.

Be­cause of the escape into space over billions of years of such light atoms as hydrogen, the Earth now contains about one thousand times less mass than was present in proto-Earth when it condensed from the dust cloud. The origin of the Moon remains an enigma to scien­tists. Did it form at the edge of proto-Earth? Or did it form elsewhere in space as a separate planet that was later captured by the Earth's gravitational field? Or another theory is that the Moon was the result of a massive asteroid impact with the Earth. Cosmologists favor these last two possibilities rather than the older theory that the Moon was ripped out of that part of the Earth that is now the Pacific Ocean basin. And with the advent of manned exploration of the Moon like to be restarted very soon, it seems likely that the scientific enigma of the Moon will one day be solved.

The story of the Earth has almost reached the point where it can be taken up by a geologist. After the Earth stopped collecting debris from its path in space, its sur­face gradually cooled and became solid. A crust of rock formed; land masses appeared. But the Earth was not yet ready to support life as we know it today; its sur­face was still too hot for living organisms and the atmosphere was heavy with poisonous methane and ammonia. Molten lava flowed from fissures in the crust, allowing the escape of steam that had been trapped in the Earth's molten interior. In fact, many geologists think that this early volcanic activity brought to the surface most of the water that forms the present-day oceans-water originally trapped in icy dust.

As volcanic activity decreased on the Earth, intense ultraviolet radiation from the Sun broke up a portion of the atmospheric water molecules into separate atoms of hydrogen and oxygen. The Earth's gravitational pull wasn't strong enough to retain the lighter hydrogen atoms, and most of them drifted off into space. The heavier oxygen atoms would have remained. Although some free oxygen was thus liberated in the Earth's evolv­ing atmosphere, the gases methane and ammonia must have remained preponderant for a long time, since most of the free oxygen in today's atmosphere is known to exist as the byproduct of photosynthesis in plants, in­cluding the algae of lakes and oceans.

Year by year the Earth became cooler as it radiated heat and proto-Sun faded to the intensity of brightness we know now.

Soon the Earth's atmosphere had cooled enough to cause water vapor in the air to condense and fall back to the surface as rain. At first, the raindrops spattering on the hot surface boiled back in a hiss of steam. Eventually, though, the Earth cooled sufficiently to permit pools of water to collect over the surface. Soon the cooling atmosphere must have begun to yield tremendous amounts of rain.

All the water in the seven seas may have descended in one long continuous deluge. Gradually the shallow areas in the wrinkled crust filled, and oceans appeared on the face of the Earth.

Although scientists are generally convinced that the Earth on which we live has passed through the stages of development outlined in the previous paragraphs, no one, of course, can vouch for the exact chronology. Probably, proto-Earth reached its present size and shape some four and a half billion years ago.

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Article Contents

Early theories, what is a good theory, the accretion theory, the floccule/protoplanet theory, the solar nebula theory, forming planets from a diffuse medium, comments and residual difficulties, the capture theory, satellite formation, orbital evolution, star formation, planetary collision and terrestrial planets, the moon, mars and mercury, smaller bodies, isotopic anomalies in meteorites, the modern laplacian theory.

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The origin and evolution of the solar system

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Michael Woolfson, The origin and evolution of the solar system, Astronomy & Geophysics , Volume 41, Issue 1, February 2000, Pages 1.12–1.19, https://doi.org/10.1046/j.1468-4004.2000.00012.x

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M Woolfson discusses theories of how the Sun and the planets began.

The heliocentric nature of the solar system with its major components — the Sun, planets and satellites — was firmly established well before the end of the 17th century. After the publication of Newton's Principia in 1687 it became possible to apply scientific principles to the problem of its origin.

Most theories that have been advanced in the last 300 years are obviously untenable, but some contain the germs of what might be part of a viable theory. It would not be practical to attempt to deal with all theories in detail in a short review article. Here we shall mention five theories, recently developed or still in the process of development, that have a reasonable scientific basis. Two of them, the Solar Nebula Theory and the Capture Theory, will be described in more detail, emphasizing what they have and have not explained and what their remaining difficulties are. Two early theories will be described first, chosen because they relate closely to the extant ones and illustrate the major problems for theories.

Based on ideas and observations by Descartes, Kant and Herschel, Pierre Laplace (1796) put forward the first really scientific theory (summarized in figure 1 ). A slowly spinning cloud of gas and dust cooled and collapsed under gravity. As it collapsed, so it spun faster and flattened along the spin axis. It eventually took on a lenticular form with equatorial material in free orbit around the central mass. Thereafter material was left behind as a set of rings within which clumping occurred. Clumps orbiting at slightly different rates combined to give a protoplanet in each ring. A smaller version of the scenario, based on the collapse of protoplanets, produced satellite systems. The central bulk of the original cloud collapsed to form the Sun.

An illustration of Laplace's nebula theory. (a) A slowly rotating and collapsing gas-and-dust sphere. (b) An oblate spheroid forms as the spin rate increases. (c) The critical lenticular form. (d) RIngs left behind in the equatorial plane. (e) One planet condensing in each ring.

This monistic theory, that produced the Sun and the planets in a single process, has an attractive simplicity but a fatal flaw. It suggests that most of the angular momentum of the system is in the Sun — which is not so. The Sun with 99.86% of the mass of the system has only 0.5% of the total angular momentum contained in its spin; the remainder is in the planetary orbits. All 19th century attempts to rescue the theory were unsuccessful. The theory, although based on scientific principles, did not agree with observation and so had to be abandoned.

Some time later James Jeans (1917) suggested a dualistic theory, one for which the Sun and planets were produced by different mechanisms. A massive star passed by the Sun, drawing from it a tidal filament (shown in figure 2 ). The gravitationally unstable filament broke up with each condensation forming a protoplanet. The protoplanets, attracted by the retreating star, were retained in heliocentric orbits. At first perihelion passage a small-scale version of the same mechanism led to a filament being drawn from a protoplanet within which protosatellites formed.

An illustration of Jeans' theory. (a) The escape of material from the tidally distorted Sun. (b) Protoplanetary condensation in the ejected filament. (c) Protoplanets attracted by the retreating massive star.

The theory had a good reception — especially as it was supported by some elegant analysis. Jeans found how a tidally affected star would distort and eventually lose a filament of material from the tidal tip. He showed that the filament would fragment through gravitational instability and he also derived a condition for the minimum mass of a filament clump that could collapse. Despite the initial enthusiastic acceptance of the theory, it soon ran into trouble. Harold Jeffreys (1929) , by a mathematical argument involving the concept of circulation, suggested that Jupiter, which has the same mean density as the Sun, should have a similar spin period. The periods differ by a factor of 70. Other simpler, and hence more readily accepted, objections followed. Henry Norris Russell (1935) showed that material pulled from the Sun could not go into orbit at more than four solar radii — well within Mercury's orbit. This was another type of angular momentum problem. Then Lyman Spitzer (1939) calculated that a Jupiter mass of solar material would have a temperature of about 10 6 K and would explode into space rather than collapse. Later, other objections were raised concerning the presence of lithium, beryllium and boron in the Earth's crust, light elements that are readily consumed by nuclear reactions in the Sun.

Jeans tried to rescue his theory by having a cool extended Sun with the radius of Neptune's orbit, but this created new problems — not least that the newly formed planets in a diffuse form would be ploughing through the Sun. He finally conceded that “the theory is beset with difficulties and in some respects appears to be definitely unsatisfactory”.

The Laplace and Jeans theories were scientifically based but finally succumbed to scientific criticism. They both had angular momentum problems although of different kinds. Nevertheless all the modern theories described here involve ideas that they introduced. They also illustrate important problems that theories must address to be considered as plausible.

Those producing cosmogonic theories usually provide lists of “facts to be explained” but, as the scientific historian Stephen Brush concluded, such lists often emphasize those facts that the individual's theory best deals with. This could well be true. To avoid that possibility, I give below the union of all “facts” suggested by various workers. They are separated into groups according to whether they are gross features or relate to details of the system.

Gross features :

the distribution of angular momentum between Sun and planets

a planet-forming mechanism

planets to form from “cold” material

direct and almost coplanar orbits

the division into terrestrial and giant planets

the existence of regular satellites.

Secondary features:

the existence of irregular satellites

the 7° tilt of the solar spin axis to the normal to the mean plane of the system

the existence of other planetary systems.

Finer details of the solar system:

departures from planarity of the system

the Earth-Moon system variable directions of planetary spin axes

Bode's law or commensurabilities linking planetary and satellite orbits

asteroids: origin, compositions and strutures

comets: origin, compositions and structures

the formation of the Oort cloud

the physical and chemical characteristics of meteorites

isotopic anomalies in meteorites

Pluto and its satellite, Charon

Kuiper-belt objects.

The least that a theory should deliver is convincing explanations of the gross features. A theory without a slowly spinning Sun and a planar system of planets with regular satellite systems for some is, at best, implausible.

If alternative plausible theories are available then one may resort to the principle first enunciated by the English philosopher William of Occam (1285–1349), known generally as Occam's razor. Loosely translated from the Latin this implies that “if alternative theories are available that explain the observations equally well then the simpler is to be preferred”.

The goal then is to find a simple theory based on well-established scientific principles, that explains what is known and that cannot be refuted by scientific arguments. We shall now look at the ideas that have been put forward over the last half century, roughly in their date order of presentation.

In 1944, Soviet planetary scientist Otto Schmidt suggested a new kind of dualistic theory. It was known from telescopic observations that cool dense clouds occur in the galaxy and Schmidt argued that a star passing through one of these clouds would acquire a dusty-gas envelope. Schmidt believed from energy considerations that, for two isolated bodies, material from one body could not be captured by the other and so he introduced a third body nearby, another star, to remove some energy. The need for a third body made the model rather implausible but, as Lyttleton showed in 1961, Schmidt's argument was invalid since the cloud was of large extent and the star-plus-cloud behaved like a manybody system. Lyttleton proposed capture of material by an accretion mechanism first suggested by Bondi and Hoyle (1944) and illustrated in figure 3 . The cloud material moves relative to the star at speed V , greater than the escape speed. Deflected interacting streams, such as at point G, lose their component of velocity perpendicular to the original direction of motion and the residual speed can then be less than the escape speed.

Streams of material arriving at point G cancel their velocity components perpendicular to the axis.

Lyttleton used parameters for the model that gave the mass and angular momentum of captured material compatible with that of the planets, although no process was suggested for producing planets from the diffuse envelope. However, Lyttleton's parameters were implausible. The temperature of the cloud was 3.18 K, in equilibrium with galactic radiation, and the relative velocity of cloud and star was 0.2 kms -1 . A cloud temperature of 10–20 K or even greater is more consistent with observation, and the relative speed is more likely to be of order 20 kms -1 . The proposed mechanism does no more than suggest a source of planetary material. It cannot be regarded as a convincing theory, especially as planet formation from diffuse material presents additional difficulties, as we shall see later.

In 1960, McCrea suggested a theory that linked planetary formation with the production of a stellar cluster and also explained the slow rotation of the Sun. McCrea's starting point was a cloud of gas and dust that was to form a galactic cluster. Due to turbulence, gas streams collided and produced regions of higher-than-average density. The high-density regions, referred to as “floccules”, moved through the cloud and combined whenever they collided. When a large aggregation formed, it attracted other floccules in its region so producing a protostar. Since floccules joined the accreting protostar from random directions, the net angular momentum of the protostar was small; for a particular set of parameters it would be only a few times the present angular momentum of the Sun and the excess can be removed after formation by various physical processes.

It was assumed that star-forming regions were isolated and McCrea showed that the angular momentum contained in a region due to the original floccules was much greater than that residing in the protostar. The missing angular momentum was assumed to be taken by smaller aggregations of floccules that were captured by the protostar to form a set of planets.

In the original form of the theory, each floccule had about three times the mass of the Earth so many of them had to combine to form the giant planets. The resultant planetary aggregations contained much more angular momentum than the present planets. McCrea turned this apparent problem into an asset. As the protoplanet collapsed it would have become rotationally unstable and behaved as described by Lyttleton (1960) and shown in figure 4 . The protoplanet would have broken into two parts with a mass ratio of about 8:1. The smaller part, moving faster relative to the centre of mass, could escape from the solar system, with most of the angular momentum. In a neck between the two separating parts, small condensations would form and be retained by the larger part as a satellite family. To explain the terrestrial planets, McCrea had to assume that the fission process took place in a dense core of the protoplanet. In the inner part of the solar system, with higher escape speeds, both parts were retained and formed the pairs Earth-Mars and Venus-Mercury.

The fission of a rapidly spinning protoplanet with the formation of protosatellite droplets.

With some parameters deduced from the present solar system and others chosen to give the best possible results, the Sun plus planets and satellites system could be explained. Nevertheless the theory has severe problems. First, the floccules were unstable, with lifetimes much less than the time between floccule collisions. In response, McCrea (1988) produced a modified form of the theory where the initial condensations, now called “protoplanets”, were of Saturn's mass and stable. The initial system would not have been coplanar and indeed there could have been retrograde orbits although, with motion in a resisting medium and collisions to remove a minority population of retrograde objects, the system could have evolved to the present state. However, what is highly suspect is the idea that the angular momentum not present in the protostar must necessarily reside in a planetary system. It is much more likely that the “missing” angular momentum would reside in relative motions of protostars than in planetary systems.

Over the past 30 years a paradigm has arisen — a model that has wide acceptance and is the basis of thinking about contingent matters. This is the Solar Nebula Theory (SNT).

In the 1960s it became clear that many features of meteorites were interpretable in terms of condensation from a hot vapour, encouraging the view that early solar system material had been in a hot gaseous form. In addition, in the 1960s Victor Safronov was working on planet formation from diffuse material and in a seminal paper translated into English ( Safronov 1972 ) he summarized this work. Driven by these twin developments a new Solar Nebula Theory (SNT) quickly took off as a major research activity. It was believed that new knowledge and approaches should enable the original problems of Laplace's nebula theory to be solved.

An early worker on the SNT concluded quite quickly: “At no time, anywhere in the solar nebula, anywhere outwards from the orbit of Mercury, is the temperature in the unperturbed solar nebula ever high enough to evaporate completely the solid materials contained in interstellar grains,” ( Cameron 1978 ). Although this undermined an important raison d'être for the revival of nebula ideas, by this time the work was in full flow and proceeded without interruption.

Work on the redistribution of angular momentum has been central in the development of the SNT. Lynden-Bell and Pringle (1974) described a mechanism in which, given turbulence and energy dissipation in a disk, the disk would evolve to conserve angular momentum by inner material moving inwards while outer material moved outwards. This is tantamount to the outward transfer of angular momentum. However, it does not solve the basic angular momentum problem. Material joining the central condensation gradually spirals inwards so that it is always in a near-Keplerian orbit around the central mass. A useful way of thinking about the spin angular momentum of the Sun is to equate it to onequarter of a Jupiter mass orbiting at the Sun's equator. If the Sun could form in its present condensed configuration by material spiralling inwards, which it could not, then it would still have hundreds of times its present angular momentum. Realistically, without having much less angular momentum it could not form at all. Various mechanisms have been suggested for transferring angular momentum ( Larson 1989 ). An example is by gravitational torques due to spiral arms in the disk ( figure 5 ). To be effective this requires a massive nebula, which is undesirable for other reasons, but any mechanism giving a spiralling motion for material does not solve the problem.

The gravitational effect of a massive trailing spiral arm is to add orbital angular momentum at P and subtract it at Q.

An effective mechanism for removing angular momentum from a pre-existing star involves a loss of ionized material from the star plus a strong stellar magnetic field, both likely in a young active star. Ionized material moves outwards locked to a magnetic field line. The field rotates with the star so the ionized matter moves outwards with constant angular speed; the increased angular momentum it acquires is removed from the star. It remains attached to the field line until the kinetic pressure of the ion flow exceeds the magnetic pressure that, in the case of a dipole field, varies as r -6 . Analysis shows that, with plausible stellar winds and fields, some 90% or so of the original angular momentum can be removed in this way.

T-Tauri emission, at the deduced rate of 10 -7 M ⊙ year -1 for a period of 10 6 years, is often cited as a model for mass loss. However, spectroscopic evidence shows that T-Tauri emitted material is only lightly ionized and hence would be feebly coupled to the field. In addition, low-mass stars, for which no T-Tauri emission occurs, also spin slowly so a second mechanism would be needed for these stars.

Forming the Sun requires inward movement of material while the magnetic field mechanism for removing angular momentum requires outward movement. If a way could be found whereby the nebula core would grow and simultaneously lose highly ionized material which coupled to a strong stellar magnetic field (∼10 5 times as strong as the present solar field) then the angular momentum problem would be solved. For example, one could envisage a bipolar inflow of neutral material adding to the mass of the star with an equatorial loss of ionized material to remove angular momentum — although it seems unlikely that such a pattern would arise naturally. To summarize, while it is not possible to say that the angular momentum problem cannot be solved, it has certainly not been convincingly solved as yet although general papers on the evolution of disks appear from time to time (e.g. Pickett and Durisen 1997 ).

There are two possible planet-forming scenarios for the SNT. In the first, the nebula disk had about a solar mass and a density and temperature such that regions of it contained a Jeans critical mass and spontaneously collapsed to produce planets. This gives planets, but so many that there is a challenging disposal problem. SNT theorists no longer seriously consider this possibility.

The other scenario is with a disk of mass between 0.01 M ⊙ and 0.1 M ⊙, similar to that considered by Safronov (1972) whose work has been developed by others. Recent observed infrared excess radiation from young stars is almost certainly due to the presence of dusty disks. These observations, taken as supporting the SNT, also impose a constraint; stars older than a few million years do not show infrared excess radiation. It has been inferred, and generally accepted by the SNT community, that planet production has to be completed within 10 million years of disk formation.

What emerges is a multi-stage process:

(i)Dust within the disk settles into the mean plane. For dust grains as small as normal ISM grains this process would take too long. Weidenschilling et al. (1989) suggested that grains were sticky so that large dust particles formed, thus drastically shortening the settling time. There is controversy about the need for sticky dust but general agreement that the dust disk must form in a reasonably short time.

(ii)The dust disk is gravitationally unstable and fragments to form kilometre-size bodies, called “planetesimals”. The early nebula might have had to be turbulent to allow transfer of angular momentum but a quieter nebula is now required to allow the planetesimals to form.

(iii)Planetesimals accumulate to form planets. This is the awkward part of the process. Planets would form in the terrestrial region within 107 years but, according to Safronov's theory, it would take 1.5 × 10 8 years to produce a Jupiter core and 10 10 years or more to produce Neptune — more than twice the age of the solar system.

There are conflicting requirements here. Short formation times require a turbulent environment to bring planetesimals together quickly while, for planetesimals to amalgamate, approach speeds must be low. Stewart and Wetherill (1988) suggested conditions that would lead to runaway growth. These include local density enhancements in the disk, viscous forces to slow down planetesimals and the application of an energy equipartition principle so that larger bodies would move more slowly and hence be able to combine more readily. These are ad hoc assumptions but reduce formation times to within the allowed period — except for Uranus and Neptune. In the first programme of a recent BBC television series The Planets, an SNT theorist said, “according to our theories, Uranus and Neptune do not exist”! (iv) Planetary cores accrete gaseous envelopes. This would take about 10 5 years for Jupiter.

Satellite formation is taken as a miniature version of planet formation although angular momentum transfer is not such a serious problem in this case. The ratio (intrinsic orbital angularmomentum of the secondary body)/(intrinsic equatorial spin angular momentum of the primary body) is 7800 for Jupiter-Sun and 17 for Callisto-Jupiter so that only a modest outwards transfer of angular momentum is required.

The difficulties of angular momentum transfer and planet formation have not been convincingly resolved after 30 years of concentrated effort so the SNT per se has not progressed beyond these basic problems.

Papers are produced from time to time on planet formation, usually involving special assumptions that are not justified other than that they lead to a desired outcome. For example Pollack et al. (1996) , by numerical simulations involving the simultaneous accretion of solid planetesimals and gas, gave the formation times of Jupiter, Saturn and Uranus as a few million years. The major assumption they made was that the growing planet was in a disk of gas and planetesimals with uniform surface density and that planetesimals had to remain within the feeding zone of the planet. More recently Chambers and Wetherill (1998) have simulated the formation of terrestrial planets on the assumption of a pre-existing Jupiter and Saturn but, even then, the period covered by the simulation is an unacceptable 3 × 10 8 years. There is no model for planet formation that has commanded general support from the SNT community which describes a progression from a believable initial condition through a series of well-founded physical processes to planetary formation.

The division of planets into terrestrial and giant categories is related to the temperature of their formation. Mercury is formed where only iron and silicate grains can survive and the Mercury region would have been iron-rich. However, there is no simple explanation for the seemingly erratic pattern of densities of the terrestrial planets. Beyond the orbit of Mars, ice grains would have been stable, so allowing massive planetary cores to form that attracted extensive atmospheres.

On the question of angular momentum transfer the situation is perhaps less favourable than for planet formation. Again papers appear giving rather general results which are not, and cannot be, directly related to the problem of a slowly spinning Sun.

The SNT should yield the solar spin axis strictly perpendicular to the mean plane. An explanation for the 7° tilt could be perturbation by a passing star that disturbed the orbital planes of the planets subsequent to their formation. There are some tricky problems with this explanation. Neptune's orbit is almost perfectly circular and any perturbation that significantly changed its inclination would also have greatly changed its eccentricity. There is, however, a ready explanation for the tilts of the planetary spin axes. Planetesimals, or larger aggregations, will build up planets by collisions from random directions and spin axes could be in almost any direction, although the preponderance of direct planetary spins may require explanation.

The Capture Theory (CT) ( Woolfson 1964 ) actually predated the advent of the SNT by several years but its arrival was largely unnoticed. The basis of the CT, as first presented, is illustrated in figure 6 which shows a point-mass model, an early one of its kind, in which interpoint forces simulated the effects of gravity, gas pressure and viscosity. It depicts a tidal interaction between the Sun and a diffuse cool protostar, of mass 0.15 M ⊙ and radius 15 AU. As Jeans had deduced, the protostar distorts and eventually a filament of material escapes from the tidal tip. The model was too coarse to show filament fragmentation, but individual mass points were captured by the Sun. This model, which involved mechanisms analysed by Jeans, was free of all the criticism that had been raised against the original tidal model. The angular momentum of the planetary orbits comes from the protostar-Sun orbit and the range of perihelia given by the model, up to 38 AU, matches that of planetary orbital radii. Since the material is cold it satisfies the chemical constraints. The orbital planes are close to the Sun-protostar orbital plane although, due to protostar spin throwing material slightly out of the plane, there would be some variation of inclinations.

The disruption of a model protostar. Captured points are marked with their orbital perihelion distances (10 12 m) and eccentricities (in brackets). Escaping points are marked H (hyperbolic orbits).

It was seven years before the next CT paper was published. This paper ( Dormand and Woolfson 1971 ) improved the original model by exploiting the dramatic increase in available computer power. The paper confirmed the validity of the capture process and showed, from several simulations, that the calculated radial distributions of planetary material agreed reasonably well with that in the solar system ( figure 7 ). From the properties of the filament it seemed that six or so protoplanet condensations would be expected. Much later, by the use of a smoothed particle hydrodynamics (SPH) approach, Dormand and Woolfson (1988) modelled filament fragmentation that was found to take place much as Jeans had described.

The mass distribution from four Sun-protostar encounters together with the smoothed-out distribution for the solar system

The modelling showed that the protoplanets began moving towards the aphelia of very eccentric orbits. If the collapse time of a protoplanet was substantially less than its orbital periods (>100 years) then this would enable it to condense before being subjected to disruptive tidal forces at perihelion. The collapse of a Jupiter-like protoplanet, under the conditions of CT formation, was modelled in detail by Schofield and Woolfson (1982) . This indicated planetary collapse time as short as 20 years with reasonable model parameters.

While the planets could survive, they were subjected to considerable tidal forces during their first orbit. Consequently they would go into their final collapse stage in a distorted form that included a tidal protuberance. The characteristic of a nearly free-fall collapse is to amplify any distortion so that what began as a tidal bulge turned into a tongue or filament. Condensations within this filament would give a family of regular satellites. Williams and Woolfson (1983) found good quantitative agreement between predictions based on this model and the properties of the regular satellite families of Jupiter, Saturn and Uranus. Actually, this mechanism is similar to that suggested by Jeans for satellite formation — a small-scale version of his planet-forming process. The Jeans tidal theory had insuperable angular momentum problems for planets but not for satellite formation.

Dormand and Woolfson (1974) , investigating the effect of a resisting medium around the Sun, found that protoplanet orbits quickly round off. In one simulation, with a medium with five times Jupiter's mass, it was found that Jupiter rounded-off in 10 5 years, Saturn in 3 × 10 5 years and Uranus and Neptune in 2 × 10 6 years. The times depend on the density of the medium and were also approximately proportional to the inverse of the planet's mass. They are comfortably less than the inferred lifetimes of disks around young stars if, indeed, the resisting medium acts as a disk.

The periods of Jupiter and Saturn and those of Uranus, Neptune and Pluto are close to being commensurate. Melita and Woolfson (1996) showed that orbital evolution in a resisting medium leads to resonance locking between pairs of planets. During the evolution of the orbits with energy loss, the periods reach some commensurability. Thereafter an automatic feedback mechanism ensures a difference between the energy lost by the outer planet and its gain of energy from the inner planet such that the resonance is maintained. This does not give Bode's law — but it does explain commensurabilities that have a firmer physical foundation.

The original solar spin axis could have been in any direction. However, during the dispersal of the resisting medium — mostly by being pushed outwards by radiation pressure and the solar wind - larger solid grains would have spiralled inwards due to the Poynting-Robertson effect. As they joined the Sun, their angular momentum contribution pulled the solar spin axis towards the normal to the mean plane. Absorption of a fraction of a Jupiter mass in this way would give the spin axis nearly, but not quite, normal to the mean plane — not a problem for, but a natural consequence of, this model.

The basic CT provides an explanation of the tilts of the planetary spin axes as due to strong tidal interactions between planets that approached closely while their orbits were still highly eccentric. Woolfson (2000) describes a point-mass model of a proto-Uranus with a radius of 0.25 AU in an orbit of semi-major axis 35.6 AU and eccentricity 0.69 interacting with a model Jupiter on an orbit with semimajor axis 14.8 AU and eccentricity 0.826. Jupiter passes over Uranus with nearest approach 1.15 AU and the spin axis of Uranus changes from being normal to the original orbit to being at an angle of 98.7° to the almostunchanged new orbit. Other planetary spin-axis inclinations are readily explained in this way.

The CT is a dualistic one and offers no explanation for the slow solar spin, something that must always be of concern to the cosmogonist. To address this concern, Woolfson (1979) described a model for star formation within a galactic cluster and similar ideas have been investigated by Pongracic et al . (1991) . The model followed the evolution of a collapsing dark cool cloud within which turbulent energy steadily increased. The collision of turbulent gas elements gave compressed hot regions that cooled much faster than they re-expanded. If the free-fall time of the cool dense region was less than the coherence time for the whole cloud, during which matter was completely redistributed within it, then a star could form. Producing stars this way, with subsequent accretion to form more massive stars, gave spin rates for different classes of stars similar to those observed. Additionally, the rate of star formation and the variation of the masses of formed stars with time agreed with observations from young clusters. The predicted mass index of stars, that gives the stellar mass distribution, also agreed with observation. Given at least one star-forming model that explains solar spin in the context of the spin characteristics of all stars, it is reasonable for a dualistic theory to confine itself to the problem of planetary orbital angular momenta.

The basic CT gives planets formed from cold material, in direct almost coplanar orbits of the right dimensions and accompanied by natural satellites. However, there were problems with the original model. Dormand and Woolfson (1971) reported that, according to their model, terrestrial planets would have gone too close to the Sun and so have been disrupted.

The first orbital round-off calculations by Dormand and Woolfson (1974) were two-dimensional but later they explored a threedimensional scenario. They found, as expected, reducing orbital inclinations but they also found other, unexpected, orbital behaviour. Due to the medium's gravitational influence the eccentric orbits precessed in a complex way. The original inclined orbits did not intersect in space but, because of differential precession, pairs of orbits did occasionally intersect. Strong interactions could occur if planets arrived together near a point of intersection. A tidal interaction between a proto-Uranus and proto-Jupiter was previously described, but Dormand and Woolfson (1977) considered much stronger interactions where either one or other of the planets was ejected from the solar system or where there was a direct collision. Straightforward calculations showed that characteristic times for strong interactions were similar to those for orbital round-off.

Dormand and Woolfson took an initial system with six major planets, the present four plus two others denoted by A and B in table 1 . The characteristics of A and B are speculative but the conclusions that follow are insensitive to the parameters chosen. From table 2 , it appears that at least one major event was more likely than not in the early solar system.

Planets in the early solar system according to the Capture Theory

Characteristic times for (a) planet 1 to be expelled from the system, (b) planet 2 to be expelled and (c) a collision

Dormand and Woolfson (1977) modelled a collision between protoplanets A and B and showed that A could be expelled from the solar system while B was sheared into two parts that would have rounded off to the present orbits of the Earth and Venus. The largest terrestrial planets were interpreted as two non-volatile residues of a disrupted major planet.

The possible outcomes for the planetary satellites were that they could leave the solar system, go into independent heliocentric orbits, or be retained or captured by one or other of the B fragments. Thus, in one computational model the Earth fragment captured a satellite of A into a very stable orbit with an eccentricity of 0.4. The capture readily occurred in the presence of other bodies that removed energy from the Earth-satellite (Moon) system.

This scenario explains a curious feature of the Moon. The Moon's far side lacks large mare features, so characteristic of the near side. Since altimetry from lunar orbiters shows the presence of large basins on the far side, the usually accepted and sensible conclusion is that the solid crust was thicker on the far side so that magma was unable to reach the surface. Complicated explanations for this have been advanced yet simple tidal effects should lead to a thicker crust on the near side. Planetary collision is a straightforward explanation. Collision debris, travelling at more than 100 kms-1, would have bombarded the satellites and abraded their surfaces. A thickness of a few tens of kilometres of the Moon's original surface could have been removed in this way - but only from the planet-facing hemisphere.

Protoplanets A and B would have had small perihelia and, because of large solar tidal forces, families of large satellites. A satellite origin for Mars explains its hemispherical asymmetry. The surface features of Mars, and their relationship to its spin axis, were explained by Connell and Woolfson (1983) who also considered the early water-rich evolution of that planet. Mercury too could be an escaped satellite, originally of similar mass to Mars but so heavily abraded that its surface completely reformed and it was left with a high density (Woolfson 2000).

The CT model does not predict large satellites for the outer planets. Neptune's large satellite, Triton, is also anomalous in its retrograde orbit. Woolfson (1999) described a computational model in which Triton was an escaped satellite from the collision. This collided with an existing regular satellite of Neptune, Pluto, which was expelled into a heliocentric orbit like its present one while Triton was captured by Neptune. The collision sheared off a portion of Pluto to give its satellite, Charon.

Debris from the planetary collision would have had the greatest concentration in the inner part of the system. Near-surface volatilerich material from the colliding planets would have moved out furthest and, interacting with protoplanets near the aphelia of their original elliptical orbits, have provided a comet reservoir beyond the present planetary region. Inner larger members of this reservoir form Kuiper belt objects. Others, perturbed outwards by occasional close passages of stars or giant molecular clouds, formed the Oort cloud. Perturbations now remove Oort cloud comets and replenish them from the inner reservoir.

Debris closer in provided the early heavy bombardment within the solar system for which there is so much evidence. Those bodies that were in “safe” orbits remain today as asteroids or as captured irregular satellites.

Models of a planetary collision (Woolfson 2000) show a collision-interface temperature in excess of 3 × 10 6 K. With a wide range of temperatures available there would have been an abundance of molten and vaporized material to explain chondrule formation and rapid cooling to give unequilibrated mineral assemblages within chondrules. There are interesting isotopic anomalies in meteorites including important ones for oxygen, magnesium, neon, silicon, carbon and nitrogen. An intriguing anomaly in some meteorites is neon-E, almost pure 22 Ne, assumed to be the daughter product of 22 Na with a half-life of 2.6 years. This sodium isotope was produced by nucleosynthesis and trapped in a cold rock within a few years.

Most explanations of isotopic anomalies deal with them individually on an ad hoc basis. The excess 16O in some meteorites is ascribed to formation from 12 C in some far region of the galaxy, then transport in grains to the solar system and then exchange with normal oxygen.

One widespread anomaly within the solar system is the D/H ratio — 2 × 10 -5 for Jupiter, 1.6 × 10 -4 for the Earth, a few times the Earth value for some meteorites and 100 times the Earth value on Venus. Michael (1990) showed that the early evolution of intermediate-mass protoplanets could lead to differential loss of D and H and a D/H ratio as high as that of Venus. The consequence of a colliding planet having such a high D/H ratio was quantitatively examined by Holden and Woolfson (1995) . A triggering temperature of 3 × 10 6 K sets off a nuclear reaction chain, at first involving D but later other nuclei as the temperature rises. All the isotopic anomalies referred to above can be well explained as mixtures of processed and unprocessed material; there is no need for ad hoc explanations. For example, figure 8 shows the variation of the concentration of oxygen isotopes (and 17 F and 18 F that decay quickly to 17 O and 18 O) with temperature during the nuclear reaction. At ∼5 × 10 8 K the system explodes, the collision region expands and cools and reactions virtually cease. The oxygen content of processed material is almost pure 16 O; mixing it with unprocessed material explains the anomaly.

The variation of the concentration of stable oxygen isotopes and radioactive fluorine isotopes with temperature. ( Holden and Woolfson 1995 .)

The Solar Nebula Theory is clearly related to the original Laplace model but the Modern Laplacian Theory ( Prentice 1974 ) follows the Laplace scenario much more closely.

To solve the problem of a slowly spinning Sun, Prentice followed a suggestion of Reddish and Wickramasinghe (1969) and assumed that the Sun formed from grains of solid molecular hydrogen settling within a dense cool cloud to which they were strongly coupled. The gravitational energy of the collapse vaporized the grains to give a cloud of hydrogen of radius 10 4 R⊙ with a dense core formed by fasterfalling CNO grains. By the time the radius of the cloud equalled that of Neptune's orbit, the boundary material was in free orbit. At this stage Prentice introduced turbulent stress. Supersonic turbulence within the cloud gave density variations and less dense regions were propelled outwards from the surface by buoyancy effects in the form of needle-like elements. Motion outwards would have been fast but inward motion slower, giving a higher density in the surface region ( figure 9 ). Prentice showed that an instability would occur from time to time at the cloud equator so that material would be lost in the equatorial plane in the form of rings, much as Laplace postulated. All the rings had a similar mass, about 10 3 M ⊙, with temperatures falling off with increasing ring radius. Prentice postulated that the several rings within the orbit of Mercury were vaporized, for a terrestrial ring there would have been silicate and metal grains with total mass 4 M ⊙ and in major planet regions there would have been additional ice grains giving a total ring mass of 11–13 M ⊙.

Needle-like elements due to supersonic turbulence. Material in the shaded region slowly falls back to the surface of the proto-Sun.

Prentice presented an analysis in which solid material fell towards the axis of each ring and then came together to form a single planet or planetary core. In the major planet region the cores were sufficiently massive to accrete gas. While this gas contracted, a smaller scale version of the process, including supersonic turbulence, was taken to produce planetary systems.

This theory is by far the most complex of the current theories but despite its attention to the fine details of the system it does have severe drawbacks. The several rings within Mercury would have had an angular momentum several hundred times that of the Sun so they would not fall into the Sun. It can be shown that the rings would not have been stable and have had lifetimes much shorter than the time required for material within them to aggregate. The process by which material falls towards a ring axis is based on rather dubious mechanics requiring quite large solid bodies to be strongly coupled to a very diffuse gas. Finally, the system produced by this model would be highly coplanar and could not explain the tilt of the solar spin axis.

The current paradigm, the SNT, has not yet been successful in explaining the structure of the solar system at a very basic level. The observation that young stars are accompanied by dusty disks does not necessarily confirm the validity of the SNT because it predicts and depends upon a disk. Indeed, it is difficult to envisage a star-forming process that would not provide extraneous material that would form a disk. The important thing is not the disk but whether or not it gives planets. Nevertheless all observations are interpreted in terms of the SNT. For example, the nebula concept naturally suggests that radioactive isotopes were uniformly distributed in the early solar system. Hence, by looking for daughter products of particular decays in various types of object one can get relative times for when they became closed systems. The timings thus deduced are confusing and inconsistent — although the measurements are of good quality. Conformity reigns supreme and there is reluctance to consider that the SNT may not be valid. A more fruitful approach would be to find out what the experiments and observations are indicating rather than trying to force them into a theoretical strait-jacket. To quote Richard Feynman: “The test of all knowledge is experiment. Experiment is the sole judge of scientific ‘truth’.” This is applicable to cosmogony where “experiment” is usually observation.

By contrast the CT provides a coherent selfconsistent model where single events explain many observations and events occur in causally related sequences. Figure 10 shows a schematic flow diagram for the CT including a planetary collision. Explanations have been given for all but one of the 20 features referred to previously in this article — the existence of other planetary systems. It turns out that CT interactions would probably be common in an evolving stellar cluster. Recently there has been much discussion of the embedded phase in the evolution of a galactic cluster (Gaidos 1995) where stellar density can be of order 10 5 pc- 3 . Recent work, as yet unpublished, has not only realistically modelled planetary formation in great detail, showing the formation of single-planet or multiple- planet systems, but also indicated that the predicted frequency of planetary systems is consistent with recent observations.

A schematic representation of the Capture Theory and related events.

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14.3 Formation of the Solar System

Learning objectives.

By the end of this section, you will be able to:

• Describe the motion, chemical, and age constraints that must be met by any theory of solar system formation
• Summarize the physical and chemical changes during the solar nebula stage of solar system formation
• Explain the formation process of the terrestrial and giant planets
• Describe the main events of the further evolution of the solar system

As we have seen, the comets , asteroids , and meteorites are surviving remnants from the processes that formed the solar system. The planets, moons, and the Sun, of course, also are the products of the formation process, although the material in them has undergone a wide range of changes. We are now ready to put together the information from all these objects to discuss what is known about the origin of the solar system.

Observational Constraints

There are certain basic properties of the planetary system that any theory of its formation must explain. These may be summarized under three categories: motion constraints, chemical constraints, and age constraints. We call them constraints because they place restrictions on our theories; unless a theory can explain the observed facts, it will not survive in the competitive marketplace of ideas that characterizes the endeavor of science. Let’s take a look at these constraints one by one.

There are many regularities to the motions in the solar system. We saw that the planets all revolve around the Sun in the same direction and approximately in the plane of the Sun’s own rotation. In addition, most of the planets rotate in the same direction as they revolve, and most of the moons also move in counterclockwise orbits (when seen from the north). With the exception of the comets and other trans-neptunian objects, the motions of the system members define a disk or Frisbee shape. Nevertheless, a full theory must also be prepared to deal with the exceptions to these trends, such as the retrograde rotation (not revolution) of Venus.

In the realm of chemistry, we saw that Jupiter and Saturn have approximately the same composition—dominated by hydrogen and helium. These are the two largest planets, with sufficient gravity to hold on to any gas present when and where they formed; thus, we might expect them to be representative of the original material out of which the solar system formed. Each of the other members of the planetary system is, to some degree, lacking in the light elements. A careful examination of the composition of solid solar-system objects shows a striking progression from the metal-rich inner planets, through those made predominantly of rocky materials, out to objects with ice-dominated compositions in the outer solar system. The comets in the Oort cloud and the trans-neptunian objects in the Kuiper belt are also icy objects, whereas the asteroids represent a transitional rocky composition with abundant dark, carbon-rich material.

As we saw in Other Worlds: An Introduction to the Solar System , this general chemical pattern can be interpreted as a temperature sequence: hot near the Sun and cooler as we move outward. The inner parts of the system are generally missing those materials that could not condense (form a solid) at the high temperatures found near the Sun. However, there are (again) important exceptions to the general pattern. For example, it is difficult to explain the presence of water on Earth and Mars if these planets formed in a region where the temperature was too hot for ice to condense, unless the ice or water was brought in later from cooler regions. The extreme example is the observation that there are polar deposits of ice on both Mercury and the Moon; these are almost certainly formed and maintained by occasional comet impacts.

As far as age is concerned, we discussed that radioactive dating demonstrates that some rocks on the surface of Earth have been present for at least 3.8 billion years, and that certain lunar samples are 4.4 billion years old. The primitive meteorites all have radioactive ages near 4.5 billion years. The age of these unaltered building blocks is considered the age of the planetary system. The similarity of the measured ages tells us that planets formed and their crusts cooled within a few tens of millions of years (at most) of the beginning of the solar system. Further, detailed examination of primitive meteorites indicates that they are made primarily from material that condensed or coagulated out of a hot gas; few identifiable fragments appear to have survived from before this hot-vapor stage 4.5 billion years ago.

The Solar Nebula

All the foregoing constraints are consistent with the general idea, introduced in Other Worlds: An Introduction to the Solar System , that the solar system formed 4.5 billion years ago out of a rotating cloud of vapor and dust—which we call the solar nebula —with an initial composition similar to that of the Sun today. As the solar nebula collapsed under its own gravity, material fell toward the center, where things became more and more concentrated and hot. Increasing temperatures in the shrinking nebula vaporized most of the solid material that was originally present.

At the same time, the collapsing nebula began to rotate faster through the conservation of angular momentum (see the Orbits and Gravity and Earth, Moon, and Sky chapters). Like a figure skater pulling her arms in to spin faster, the shrinking cloud spun more quickly as time went on. Now, think about how a round object spins. Close to the poles, the spin rate is slow, and it gets faster as you get closer to the equator. In the same way, near the poles of the nebula, where orbits were slow, the nebular material fell directly into the center. Faster moving material, on the other hand, collapsed into a flat disk revolving around the central object ( Figure 14.11 ). The existence of this disk-shaped rotating nebula explains the primary motions in the solar system that we discussed in the previous section. And since they formed from a rotating disk, the planets all orbit the same way.

Picture the solar nebula at the end of the collapse phase, when it was at its hottest. With no more gravitational energy (from material falling in) to heat it, most of the nebula began to cool. The material in the center, however, where it was hottest and most crowded, formed a star that maintained high temperatures in its immediate neighborhood by producing its own energy. Turbulent motions and magnetic fields within the disk can drain away angular momentum, robbing the disk material of some of its spin. This allowed some material to continue to fall into the growing star, while the rest of the disk gradually stabilized.

The temperature within the disk decreased with increasing distance from the Sun, much as the planets’ temperatures vary with position today. As the disk cooled, the gases interacted chemically to produce compounds; eventually these compounds condensed into liquid droplets or solid grains. This is similar to the process by which raindrops on Earth condense from moist air as it rises over a mountain.

Let’s look in more detail at how material condensed at different places in the maturing disk ( Figure 14.12 ). The first materials to form solid grains were the metals and various rock-forming silicates. As the temperature dropped, these were joined throughout much of the solar nebula by sulfur compounds and by carbon- and water-rich silicates, such as those now found abundantly among the asteroids. However, in the inner parts of the disk, the temperature never dropped low enough for such materials as ice or carbonaceous organic compounds to condense, so they were lacking on the innermost planets.

Far from the Sun, cooler temperatures allowed the oxygen to combine with hydrogen and condense in the form of water (H 2 O) ice. Beyond the orbit of Saturn, carbon and nitrogen combined with hydrogen to make ices such as methane (CH 4 ) and ammonia (NH 3 ). This sequence of events explains the basic chemical composition differences among various regions of the solar system.

Example 14.1

Rotation of the solar nebula.

With P initial equal to 1,000,000 years, P final , the new rotation period, is 64 years. This is a lot shorter than the actual time Pluto takes to go around the Sun, but it gives you a sense of the kind of speeding up the conservation of angular momentum can produce. As we noted earlier, other mechanisms helped the material in the disk lose angular momentum before the planets fully formed.

Check Your Learning

The period of the rotating nebula is inversely proportional to D 2 D 2 . As we have just seen, P final P initial = ( D final D initial ) 2 . P final P initial = ( D final D initial ) 2 . Initially, we have P initial = 10 6 yr and D initial = 10 4 AU. Then, if D final is in AU, P final (in years) is given by P final = 0.01 D final 2 . P final = 0.01 D final 2 . If Jupiter’s orbit has a radius of 5.2 AU, then the diameter is 10.4 AU. The period is then 1.08 years.

Formation of the Terrestrial Planets

The grains that condensed in the solar nebula rather quickly joined into larger and larger chunks, until most of the solid material was in the form of planetesimals, chunks a few kilometers to a few tens of kilometers in diameter. Some planetesimals still survive today as comets and asteroids. Others have left their imprint on the cratered surfaces of many of the worlds we studied in earlier chapters. A substantial step up in size is required, however, to go from planetesimal to planet.

Some planetesimals were large enough to attract their neighbors gravitationally and thus to grow by the process called accretion . While the intermediate steps are not well understood, ultimately several dozen centers of accretion seem to have grown in the inner solar system. Each of these attracted surrounding planetesimals until it had acquired a mass similar to that of Mercury or Mars. At this stage, we may think of these objects as protoplanets —“not quite ready for prime time” planets.

Each of these protoplanet s continued to grow by the accretion of planetesimals. Every incoming planetesimal was accelerated by the gravity of the protoplanet, striking with enough energy to melt both the projectile and a part of the impact area. Soon the entire protoplanet was heated to above the melting temperature of rocks. The result was planetary differentiation , with heavier metals sinking toward the core and lighter silicates rising toward the surface. As they were heated, the inner protoplanets lost some of their more volatile constituents (the lighter gases), leaving more of the heavier elements and compounds behind.

Formation of the Giant Planets

In the outer solar system, where the available raw materials included ices as well as rocks, the protoplanets grew to be much larger, with masses ten times greater than Earth. These protoplanets of the outer solar system were so large that they were able to attract and hold the surrounding gas. As the hydrogen and helium rapidly collapsed onto their cores, the giant planets were heated by the energy of contraction. But although these giant planets got hotter than their terrestrial siblings, they were far too small to raise their central temperatures and pressures to the point where nuclear reactions could begin (and it is such reactions that give us our definition of a star). After glowing dull red for a few thousand years, the giant planets gradually cooled to their present state ( Figure 14.13 ).

The collapse of gas from the nebula onto the cores of the giant planets explains how these objects acquired nearly the same hydrogen-rich composition as the Sun. The process was most efficient for Jupiter and Saturn; hence, their compositions are most nearly “cosmic.” Much less gas was captured by Uranus and Neptune, which is why these two planets have compositions dominated by the icy and rocky building blocks that made up their large cores rather than by hydrogen and helium. The initial formation period ended when much of the available raw material was used up and the solar wind (the flow of atomic particles) from the young Sun blew away the remaining supply of lighter gases.

Further Evolution of the System

All the processes we have just described, from the collapse of the solar nebula to the formation of protoplanets, took place within a few million years. However, the story of the formation of the solar system was not complete at this stage; there were many planetesimals and other debris that did not initially accumulate to form the planets. What was their fate?

The comets visible to us today are merely the tip of the cosmic iceberg (if you’ll pardon the pun). Most comets are believed to be in the Oort cloud, far from the region of the planets. Additional comets and icy dwarf planets are in the Kuiper belt, which stretches beyond the orbit of Neptune. These icy pieces probably formed near the present orbits of Uranus and Neptune but were ejected from their initial orbits by the gravitational influence of the giant planets.

In the inner parts of the system, remnant planetesimals and perhaps several dozen protoplanets continued to whiz about. Over the vast span of time we are discussing, collisions among these objects were inevitable. Giant impacts at this stage may have stripped Mercury of part of its mantle and crust, reversed the rotation of Venus, and broke off part of Earth to create the Moon (all events we discussed in other chapters).

Smaller-scale impacts also added mass to the inner protoplanets. Because the gravity of the giant planets could “stir up” the orbits of the planetesimals, the material impacting on the inner protoplanets could have come from almost anywhere within the solar system. In contrast to the previous stage of accretion, therefore, this new material did not represent just a narrow range of compositions.

As a result, much of the debris striking the inner planets was ice-rich material that had condensed in the outer part of the solar nebula. As this comet-like bombardment progressed, Earth accumulated the water and various organic compounds that would later be critical to the formation of life. Mars and Venus probably also acquired abundant water and organic materials from the same source, as Mercury and the Moon are still doing to form their icy polar caps.

Gradually, as the planets swept up or ejected the remaining debris, most of the planetesimals disappeared. In two regions, however, stable orbits are possible where leftover planetesimals could avoid impacting the planets or being ejected from the system. These regions are the asteroid belt between Mars and Jupiter and the Kuiper belt beyond Neptune. The planetesimals (and their fragments) that survive in these special locations are what we now call asteroids, comets, and trans-neptunian objects.

Astronomers used to think that the solar system that emerged from this early evolution was similar to what we see today. Detailed recent studies of the orbits of the planets and asteroids, however, suggest that there were more violent events soon afterward, perhaps involving substantial changes in the orbits of Jupiter and Saturn. These two giant planets control, through their gravity, the distribution of asteroids. Working backward from our present solar system, it appears that orbital changes took place during the first few hundred million years. One consequence may have been scattering of asteroids into the inner solar system, causing the period of “heavy bombardment” recorded in the oldest lunar craters.

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What is the Protoplanet theory?

How did the Solar System’s planets come to be? The leading theory is something known as the “protoplanet hypothesis”, which essentially says that very small objects stuck to each other and grew bigger and bigger — big enough to even form the gas giants, such as Jupiter.

Who proposed the protoplanet theory?

W. H. McCrea

In 1960, 1963, and 1978, W. H. McCrea proposed the protoplanet hypothesis, in which the Sun and planets individually coalesced from matter within the same cloud, with the smaller planets later captured by the Sun’s larger gravity.

How does the protoplanet theory explain the origin of the solar system?

The Protoplanet theory

The planets are smaller blobs captured by the star. The small blobs would have higher rotation than is seen in the planets of the Solar System, but the theory accounts for this by having the ‘planetary blobs’ split into planets and satellites .

What are the contributions of protoplanet hypothesis?

Migrating Planets: The protoplanet hypothesis explains most of the features of the Solar System ; however, the outer solar system is still strange, especially the properties of Pluto/Charon. One explanation is that the Solar System was not born in the configuration that we see today.

When was the protoplanet theory proposed?

The floccule/protoplanet theory. In 1960 , McCrea suggested a theory that linked planetary formation with the production of a stellar cluster and also explained the slow rotation of the Sun.

Is Mercury a protoplanet?

Basically, Mercury is pretty much a planet-scale cannonball and not much else . An artist’s impression of the collision between two massive protoplanets early in solar-system history. Such a “big splat” might have left Mercury with a thin silicate mantle overlying a huge, iron-rich core.

What are the 4 theories of the universe?

It’s thought that at such an incomprehensibly dense, energetic state, the four fundamental forces— gravity, electromagnetism, and the strong and weak nuclear forces —were forged into a single force, but our current theories haven’t yet figured out how a single, unified force would work.

What are examples of theories?

Examples include: Physics: the big bang theory, atomic theory, theory of relativity, quantum field theory . Biology: the theory of evolution, cell theory, dual inheritance theory.

Can time be defined?

Physicists define time as the progression of events from the past to the present into the future . Basically, if a system is unchanging, it is timeless. Time can be considered to be the fourth dimension of reality, used to describe events in three-dimensional space.

What are the types of theories?

Different Types of Psychological Theories

• Grand Theories. Grand theories are those comprehensive ideas often proposed by major thinkers such as Sigmund Freud, Erik Erikson,4 and Jean Piaget. …
• Emergent Theories. …
• Behavioral Theories. …
• Humanistic Theories. …
• Personality Theories. …
• Social Psychology Theories.

What are the 3 types of theory?

Although there are many different approaches to learning, there are three basic types of learning theory: behaviorist, cognitive constructivist, and social constructivist .

What is a simple definition of theory?

A theory is a carefully thought-out explanation for observations of the natural world that has been constructed using the scientific method, and which brings together many facts and hypotheses .

What are the five types of theory?

Over the years, academics have proposed a number of theories to describe and explain the learning process – these can be grouped into five broad categories:

• Behaviourist.
• Cognitivist.
• Constructivist.
• Experiential.
• Social and contextual.

What makes a theory a theory?

In everyday use, the word “theory” often means an untested hunch, or a guess without supporting evidence. But for scientists, a theory has nearly the opposite meaning. A theory is a well-substantiated explanation of an aspect of the natural world that can incorporate laws, hypotheses and facts .

What are the 6 major psychological theories?

The six Grand Theories in Psychology are: Psychoanalysis, Behaviorism, Cognitivism, Ecological, Humanism, and Evolutionary . The theorists of the well-known theories are (Freud, Erickson), (Watson, Skinner), (Piaget, Vygotsky), (Bronfenbrenner), (Rogers, Maslow), (Lorenz).

What are the two components of theory?

The components of theory are concepts (ideally well defined) and principles . A concept is a symbolic representation of an actual thing – tree, chair, table, computer, distance, etc.

What is the main purpose of theory?

Definition. Theories are formulated to explain, predict, and understand phenomena and, in many cases, to challenge and extend existing knowledge within the limits of critical bounding assumptions.

What are key characteristics of a theory?

A scientific theory should be:

• Testable: Theories can be supported through a series of scientific research projects or experiments. …
• Replicable: In other words, theories must also be able to be repeated by others. …
• Stable: Another characteristic of theories is that they must be stable. …
• Simple: A theory should be simple.

What is theory according to authors?

McQuail (1983) writes that a theory consists of a set of ideas of varying status and origin which seek to explain or interpret some phenomenon . Kurt Lewin (1958), a theory is a way of explaining the ordering and recurrence of various events in the ecosphere.

What is theory according to philosophers?

From Wikipedia, the free encyclopedia. A philosophical theory or philosophical position is a view that attempts to explain or account for a particular problem in philosophy . The use of the term “theory” is a statement of colloquial English and not reflective of the term theory.

What is the purpose of theory in research?

Theories are usually used to help design a research question, guide the selection of relevant data, interpret the data, and propose explanations of the underlying causes or influences of observed phenomena .

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Planets Still Forming Detected in a Protoplanetary Disk

Just as the number of planets discovered outside our solar system is large and growing — more than 3,700 confirmed at last count — so too is the number of ingenious ways to find exoplanets ever on the rise.

The first exoplanets were found by measuring the “wobble” in their host stars caused by the gravitational pull of the planets, then came the transit technique that measured dips in the light from stars as planets passed in front of them, followed by the direct imaging of moving objects deemed to be planets, and numerous more.

A new technique can now be added to the toolkit, one that is useful only in specific galactic circumstances but is nonetheless ingenious and intriguing.

By detecting unusual patterns in the flow of gas within the protoplanetary disk of a young star, two teams of astronomers have confirmed the distinct, telltale hallmarks of newly formed planets orbiting the infant star.

In other words, the astronomers found planets in the process of being formed, circling a star very early in its life cycle.

These results came thanks to the Atacama Large Millimeter/submillimeter Array ( ALMA ), and are presented in a pair of papers appearing in the Astrophysical Journal Letters.

Richard Teague, an astronomer at the University of Michigan and principal author on one of the papers , said that his team looked at “the localized, small-scale motion of gas in a star’s protoplanetary disk. This entirely new approach could uncover some of the youngest planets in our galaxy, all thanks to the high-resolution images coming from ALMA .”

To make their respective discoveries, each team analyzed the data from various ALMA observations of the young star HD 163296, which is about 4 million years old and located about 330 light-years from Earth in the direction of the constellation Sagittarius.

Rather than focusing on the dust within the disk, which was clearly imaged in an earlier ALMA observation, the astronomers instead studied the distribution and motion of carbon monoxide (CO) gas throughout the disk.

As explained in a release from the National Radio Astronomy Observatory, which manages the American operations of the multi-national ALMA , molecules of carbon monoxide naturally emit a very distinctive millimeter-wavelength light that ALMA can observe. Subtle changes in the wavelength of this light due to the Doppler effect provide a glimpse into the motion of the gas in the disk.

If there were no planets, gas would move around a star in a very simple, predictable pattern known as Keplerian rotation.

“It would take a relatively massive object, like a planet, to create localized disturbances in this otherwise orderly motion,” said Christophe Pinte of Monash University in Australia and lead author on the other of the two papers.

And that’s what both teams found.

Detecting planets within a protoplanetary disk — or finding theorized planets within those disks — is a big deal.

That’s because information about the characteristics of very young planets orbiting young stars can potentially add substantially to one of the long-debated questions of planetary science: How exactly did those billions upon billions of planets out there form?

The leading theory of planet formation, the “core accretion model,” has planets forming slowly — with dust, small objects and then planetesimals smashing into a rocky core and leaving matter behind. In this model, the planet building takes place in a region close to the protoplanet’s stars.

Another theory looks to gravitational instabilities in the disk, arguing that giant planets can form quickly and far from their host stars.

The distribution of current solar system planets and beyond can give some clues based on the size, type and distribution of those planets. But planets migrate and evolve, and they have never been studied before they had a chance to do much of either.

The techniques currently used for finding exoplanets in fully formed planetary systems — such as measuring the wobble of a star or how a transiting planet dims starlight — don’t lend themselves to detecting protoplanets.

With this new method for looking into those early protoplanetary disks, the hunt for infant planets becomes possible. And the results in terms of understanding planet formation look to be very promising.

“Though thousands of exoplanets have been discovered in the last few decades, detecting protoplanets is at the frontier of science,” said Pinte.

This is not the first time that ALMA images of protoplanetary disks have been used to identify what seem to be protoplanets.

In 2016, a team led by Andrea Isella of Rice University reported the possible detection of two planets, each the size of Saturn, orbiting the same star that is the subject of this week’s report, HD 163296.

These possible planets, which are not yet fully formed, revealed themselves by the dual imprint they left in both the dust and the gas portions of the star’s protoplanetary disk.

But at the time that paper was published, in Physical Review Letters , Isella said the team was focused primarily on the dust in the disks and the gaps they created, and as a result they could not be certain that the features they found were created by a protoplanet.

Teague’s team also studied the dust gaps in the disk of HD 163296, and concluded they provided only circumstantial evidence of the presence of protoplanets. What’s more, that kind of detection could not be used to accurately estimate the masses of the planets.

“Since other mechanisms can also produce ringed gaps in a protoplanetary disk,” he said, “it is impossible to say conclusively that planets are there by merely looking at the overall structure of the disk.”

But studying the behavior of the gas allowed for a much greater degree of confidence.

The team led by Teague identified two distinctive planet-like patterns in the disk, one at approximately 80 astronomical units (AU) from the star and the other at 140 AU. (An astronomical unit is the average distance from the Earth to the sun.) The other team, led by Pinte, identified the third at about 260 AU from the star. The astronomers calculate that all three planets are similar in mass to Jupiter.

The two teams used variations on the same technique, which looked at anomalies in the flow of the gas – as seen in the shifting wavelengths of the CO emission — that would indicate it was interacting with a massive object.

Teague and his team measured variations in the gas’s velocity. This revealed the impact of several planets on the gas motion nearer to the star.

Pinte and his team more directly measured the gas’s actual velocity, which is better precise method when studying the outer portion of the disk and can more accurately pinpoint the location of a potential planet.

“Although dust plays an important role in planet formation and provides invaluable information, gas accounts for 99 percent of a protoplanetary disks’ mass,” said coauthor Jaehan Bae of the Carnegie Institute for Science.

So while those images of patterns within the concentric rings of a protoplanetary disk are compelling and seem to be telling an important story, it’s actually the gas that is the key.

This is all an important coup for ALMA , which saw its first light in 2013. The observatory was not designed with protoplanet detection and characterization as a primary goal, but it is now front and center.

Coauthor Til Birnstiel of the University Observatory of Munich said the precision provided by ALMA is “mind boggling.” In a system where gas rotates at about 5 kilometers per second, he said, ALMA detected velocity changes as small as a few meters per second.

“Oftentimes in science, ideas turn out not to work or assumptions turn out to be wrong,” he said. “This is one of the cases where the results are much more exciting than what I had imagined.

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8.2: Origin of the Solar System—The Nebular Hypothesis

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• Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
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Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.

Planet Arrangement and Segregation

As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition

The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria:

• Enough mass to have gravitational forces that force it to be rounded
• Not massive enough to create a fusion
• Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209

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protoplanet

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1949, in the meaning defined above

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Protoplanet Imaged During Formation

AB Aurigae b was directly imaged in joint operations between the Subaru and Hubble telescopes and provides evidence for the disk instability theory of planet formation.

In joint observations between the  Subaru Telescope  and the  Hubble Space Telescope , a protoplanet called AB Aurigae b has been directly imaged during its formation. AB Aurigae is only a two-million-year-old star, and this baby planet is actually nine times the mass of Jupiter and orbiting 13.9 billion kilometers away. That’s three times the distance between the Sun and Neptune. And finding this protoplanet where it is definitely breaks a bit more of our theories on planetary formation. Or as lead author Thayne Currie said:  AB Aur b sheds new light on our understanding of the different ways that planets form.

You see, while we know that planets migrate, we didn’t think that planetary cores could form this far away from the star. This would mean that a gas giant’s core would form closer in and then migrate out to collect the gases. In our previous story, models show that Jupiter migrated inward at some point to collect all the heavier elements in its atmosphere, and now this baby world shows us that gas giants  can  form out the great distance Jupiter would have needed before migrating inward.

And that means instead of the “standard” core accretion method we’ve talked about before, where rocky worlds spin and spin and crash and collect up material and then migrate outward for their gases, we now have to seriously consider the disk instability hypothesis. That’s where the massive protoplanetary disk, full of gases, cools down over time and then breaks into one or more collapsing clumps of planetary mass. Sort of similar to how stars form, but only if you don’t look too closely at the process.

Honestly, this discovery is huge, and not just in the size of the exoplanet kind of way. Planetary scientists have been debating the core accretion versus disk instability methods for decades now. And it took the combined power of Subaru’s extreme adaptive optics system, infrared spectrograph, visible light camera, and 13 years of observations plus the ability of Hubble to separate the planet from its star and provide a baseline for the system.

The results of this discovery were published in  Nature Astronomy .

More Information Hubblesite  press release Subaru Telescope  press release “ Images of embedded Jovian planet formation at a wide separation around AB Aurigae ,” Thayne Currie et al., 2022 April 4, Nature Astronomy

This story was written for the Daily Space podcast/YouTube series. Want more news from myself,  Dr. Pamela Gay , and  Erik Madaus ? Check out  DailySpace.org .

If you enjoy reading stories like these and would like to support me as a writer,  please consider signing up to become a Medium member . It’s $5 a month and gives you unlimited access to stories on Medium. Thank you for reading!

This article was originally published by Beth Johnson on  medium.com .

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