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A New Take on Planet Formation

Figuring out how planets form is an old occupation, with the basic ideas of planetary accretion going back several centuries, though tuned up, to be sure, in the 1970s and tweaked ever since. In a disk of gas and dust orbiting a young central star, dust grains begin to clump together, eventually forming planetesimals. Accretion models assume that these small planetesimals bang into each other and gradually grow. The assumption is that in the inner system at least temperatures are hot and the era of planet formation occurs well after the central star has formed.

Image: Artist’s conception of a protoplanetary disk. Credit: NASA/JPL-Caltech/T. Pyle.

Adjust for distance from the star and subsequent planetary migration in the gas/dust disk and you can come up with a system more or less like ours, with rocky inner worlds and gas giants out beyond the snow line, the latter being the distance from the star where it is cool enough for volatile icy compounds to remain solid. But Anne Hofmeister and Robert Criss (Washington University, St. Louis) are presenting a new model, one in which the Sun and planets form at the same time, and at cold — not hot — temperatures. They argue that their model of a three-dimensional gas cloud explains planetary orbits better than earlier theories. Says Hofmeister:

“This model is radically different. I looked at the assumption of whether heat could be generated when the nebula contracted and found that there is too much rotational energy in the inner planets to allow energy to spill into heating the nebula. Existing models for planetary accretion assume that the planets form from the dusty 2-D disk, but they don’t conserve angular momentum. It seemed obvious to me to start with a 3-D cloud of gas, and conserve angular momentum. The key equations in the paper deal with converting gravitational potential to rotational energy, coupled with conservation of angular momentum.”

Inspiring the new theory was Hofmeister and Criss’ belief that older accretion models could not explain the fact that the planets orbit the Sun in a plane. The planet-building process would have been, after all, chaotic and haphazard, yet it leads to a Solar System demonstrating a large degree of order, with co-planar planetary orbits and axial spins that are for the most part upright. In the cold accretion model the researchers are advancing, a gravitational competition begins:

“The first thing that happens in planet accretion is forming rocky kernels,” Hofmeister says. “The nebula starts contracting, the rocky kernels form to conserve angular momentum, and that’s where the dust ends up. Once rocky kernels exist, they attract gas to them, but only if the rocky kernel is far from the Sun, can it out-compete the Sun’s gravitational pull and collect the gas, as did Jupiter and its friends. But if the rocky kernel is close, like the Earth’s, it can’t out-compete the Sun. We describe this process as gravitational competition. This is why we have the regularity, spacing, and graded composition of the Solar System.”

In other words, the model accounts for the gas giants by saying that rocky protoplanets far enough from the Sun would be able to attract nearby gas, volatiles and dust in ways the inner worlds could not. So the picture appears to be more or less like this: The slow contraction of the nebula that formed both the Sun and the planets allowed the simultaneous creation of both, with rocky protoplanets forming embedded in the dusty debris disk, which the authors believe accounts for their nearly circular co-planar orbits and upright axial spins. Those rocky planetesimals far from the accreting Sun were distant enough to form thick gaseous envelopes. As the pre-solar nebula collapsed, disk debris would have fallen toward the Sun, along the way heating whatever protoplanets it encountered as they in turn spun up as the cloud continued to shrink. The authors believe that this model, “…which allows for different behaviors of gas and dust, explains key Solar System characteristics (spin, orbits, gas giants and their compositions) and second-order features (dwarf planets, comet mineralogy, satellite system sizes).”

The paper is Hofmeister and Criss, “A Thermodynamic and Mechanical Model for Formation of the Solar System via 3-Dimensional Collapse of the Dusty Pre-Solar Nebula,” Planetary and Space Science Vol. 62, Issue 1 (March 2012), pp. 111-131 (abstract). A Washington University news release is available.

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Comments on this entry are closed.

  • Christopher Phoenix March 2, 2012, 17:27

    This model suggests that rocky protoplanets could only attract gas, volatiles, and dust to form gas giants in the outer solar system- but how can the authors explain the many “Hot Jupiters” planet hunters have found in recent years? Do these scorched gas giants form further from their parent star and then wander inward, or is their another explanation? Our solar system is not the only one in the universe, after all. Surely scientists should study other solar systems to gain insight on how those systems and ours might have formed.

  • Paul Gilster March 2, 2012, 18:54

    ‘Hot Jupiters’ are a problem for any of the planet formation models. Current thinking points to planetary migration from beyond the snow line to account for them, but of course these theories are still evolving.

  • Nick March 2, 2012, 23:17

    Cold accretion does strike me as a much better explanation than cometary bombardment of why there’s so much water on earth (and originally on Mars).

  • Pointless Geometry March 3, 2012, 1:12

    Interesting theory, thanks for bringing it to our attention. In this gravity competition I wonder if there is a “most likely to fall into their own sun” list of exoplanets?

  • jkittle March 3, 2012, 10:04

    The simultaneous formation of planets and star is very attractive and does allow for formation of gas giants up close, if they form fast enough. Non of the models does a very good job modeling the transfer of moment via magnetic field interactions

  • Mike March 3, 2012, 14:14

    I wonder if the cooler formation enviroment with gravitational competition model would allow the two outermost rocky planets to collect volatiles during their formation? Perhaps comets didn’t supply all of Earth’s water?
    This new model also suggests a different origin of the Moon. Not sure how it accounts for the Moon’s low density.
    This is all very interesting though I would like to see these ideas applied to the many different exoplanet systems that have been discovered to see how well it describes their formation.

  • Adam Crowl March 3, 2012, 19:25

    Hi All

    At the other end of the mass-scale is this discussion of brown dwarf star formation…
    A Hybrid Scenario for the Formation of Brown Dwarfs and Very Low Mass Stars
    …which builds on previous work which examines the formation of low mass objects – from stars to low end brown dwarfs even super-Jovians – but performs the computations with more resolution. Essentially a short-lived, gravitationally unstable disk forms at large distances from higher-mass stars (Solar mass and up), which fragments gravitationally (Jeans-density is exceeded in small areas) and forms a large number of low-mass objects for every larger star. Most are ejected at low speed via mutual perturbations and near misses by other stars, but some can approach the central star and undergo tidal perturbation and stripping to form lower mass objects. Each low mass star/brown-dwarf also forms its own disk, which in turn can undergo further collapse into smaller objects, forming smaller planets and their moons.

    Will be quite a challenge for both theories to produce distinguishing predictions so they can be compared and tested. I suspect Nature employs both mechanisms to make planets.

  • Name March 3, 2012, 20:28

    Is it possible water rather than being transported fused from hydrogen and oxygen? Under what circumstances do these elements combine to form water?

  • Interstellar Bill March 4, 2012, 14:40

    It will be very interesting to see the results of this model for different stellar masses and metallicities. It also seems that the initial angular momentum of the cloud must greatly influence the cloud size at the time of rock formation. If it’s not temperature that caused rock formation to happen where it did but angular momentum, then distance from the sun’s heat is not what determined the size of the solar system. It would seem that high initial angular momentum would result in a bigger solar system than a low one, at the same stellar mass and metallicity. Perhaps that explains Kepler’s miniature solar systems that would fit inside Venus orbit: low initial angular momentum. After all, we wouldn’t expect five planets to all migrate from farther out and end up so regularly spaced.

    What’s most interesting about this is the unconscious assumption, in all these decades of theorizing, that temperature was king. These authors had the true genius of making the unconscious conscious, and thus overthrew the paradigm that thought it was a fact.

  • ljk September 12, 2012, 9:29

    11 September 2012

    ** Contacts are listed below. **

    Text & Image:

    http://www.cfa.harvard.edu/news/2012/pr201227.html

    PLANETS CAN FORM IN THE GALACTIC CENTER

    At first glance, the center of the Milky Way seems like a very inhospitable place to try to form a planet. Stars crowd each other as they whiz through space like cars on a rush-hour freeway. Supernova explosions blast out shock waves and bathe the region in intense radiation. Powerful gravitational forces from a supermassive black hole twist and warp the fabric of space itself.

    Yet new research by astronomers at the Harvard-Smithsonian Center for Astrophysics shows that planets still can form in this cosmic maelstrom. For proof, they point to the recent discovery of a cloud of hydrogen and helium plunging toward the galactic center. They argue that this cloud represents the shredded remains of a planet-forming disk orbiting an unseen star.

    “This unfortunate star got tossed toward the central black hole. Now it’s on the ride of its life, and while it will survive the encounter, its protoplanetary disk won’t be so lucky,” said lead author Ruth Murray-Clay of the CfA. The results are appearing in the journal Nature [http://arxiv.org/abs/1112.4822v3].

    The cloud in question was discovered last year by a team of astronomers using the Very Large Telescope in Chile. They speculated that it formed when gas streaming from two nearby stars collided, like windblown sand gathering into a dune.

    Murray-Clay and co-author Avi Loeb propose a different explanation. Newborn stars retain a surrounding disk of gas and dust for millions of years. If one such star dived toward our galaxy’s central black hole, radiation and gravitational tides would rip apart its disk in a matter of years.

    They also identify the likely source of the stray star — a ring of stars known to orbit the galactic center at a distance of about one-tenth of a light-year. Astronomers have detected dozens of young, bright O-type stars in this ring, which suggests that hundreds of fainter Sun-like stars also exist there. Interactions between the stars could fling one inward along with its accompanying disk.

    Although this protoplanetary disk is being destroyed, the stars that remain in the ring can hold onto their disks. Therefore, they may form planets despite their hostile surroundings.

    As the star continues its plunge over the next year, more and more of the disk’s outer material will be torn away, leaving only a dense core. The stripped gas will swirl down into the maw of the black hole. Friction will heat it to high enough temperatures that it will glow in X-rays.

    “It’s fascinating to think about planets forming so close to a black hole,” said Loeb. “If our civilization inhabited such a planet, we could have tested Einstein’s theory of gravity much better, and we could have harvested clean energy from throwing our waste into the black hole.”

    Contacts:
    David Aguilar
    +1 617-495-7462
    daguilar@cfa.harvard.edu

    Christine Pulliam
    +1 617-495-7463
    cpulliam@cfa.harvard.edu

    Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.