New model provides different take on planetary accretion

Feb 28, 2012 By Tony Fitzpatrick
This image of the Eagle Nebula, made with data from the Kitt Peak telescope, corresponds more closely to the authors’ model than tothe traditional model. In their model planets form in a cold, three-dimensional cloud of gas and dust. IMAGE: T. A. RECTOR & B. A. WOLPA, NOAO, AURA

(PhysOrg.com) -- The prevailing model for planetary accretion, also called fractal assembly, and dating back as far as the 18th century, assumes that the Solar System’s planets grew as small grains colliding chaotically, coalescing into bigger ones, colliding yet more until they formed planetesimals. The planetesimals then collided until they formed planets as varied as the Earth and Jupiter.

The model assumes that this occurred in an extremely hot (as high as 1,600 degrees Celsius) environment for the inner , fostered by a dusty, two-dimensional disk post-dating the Sun.

The basic modern model, developed by Russian astronomer Victor Safronov, and further developed by planetary scientist George Wetherill, is called the Solar Nebular Disk Model and was made available in English in the early 1970s. It has remained essentially the same over the past 40 years.

But not everyone is convinced the model is correct. How could such a chaotic, haphazard process as fractal assembly lead to the regularities of the Solar System with all of the planets in a single plane, rotating in the same sense, spinning and orbiting around the Sun?

For the discontents, a new model, offered by Anne Hofmeister, PhD, research professor of earth and planetary sciences and Robert Criss, PhD, professor in earth and planetary sciences at Washington University in St. Louis, presents a different scenario. Their explanation is published in the March issue of Planetary and Space Science.

Using classical physics, the laws of thermodynamics and mechanics, Hofmeister, with assistance from Criss, presents an accretion model that assumes a three-dimensional (3-D) gas cloud. This pre-solar nebula collapses and forms the Sun and planets at essentially the same time, with the planets contracting toward the Sun.

The temperature is cold, not hot. The thermodynamic and mechanical model of 3-D accretion explains planetary orbits and spins, unlike the 2-D model, which does not.

Hofmeister and Criss explain compositional gradients across the Solar System in terms of lighter molecules diffusing faster than heavier ones. The model connects planet mass to satellite system size via gravitational competition.

Explaining planetary orbits and spins

“This model is radically different,” Hofmeister says. “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.”

No energy left over for heat

“In the new model, heat production is not important in planetary formation,” Hofmeister says.

Criss says the prevailing notion that gravitational collapse is a hot process is a mis-interpretation of thermodynamics. He offers an analogy of a beaker of water placed outside in the winter. It slowly starts to freeze. Freezing water actually releases a latent heat, he says, because order (ice, a crystal) is being made from disorder (liquid).

The planetary scientists say that the solar system we know today could not have formed out of a flat, hot disk that postdates the Sun, like the one shown in this artist’s impression. IMAGE: ESO/L. CALÇADA

The heat released is considerable, but it cannot warm the beaker because “it’s released only as fast as the environment will take it away,” Criss says. “If the heat would warm the water above 32 degrees Fahrenheit, the ice would melt. People clinging to the old accretion models want to make the ice and heat the beaker, too.”

Gravitational competition

The authors say 2-D models don’t explain why the inner Solar System is comprised of rocky planets and the outer gas giants.

“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.”

Gravitational competition also offers a new view of formation of the moon that does not require an extremely low probability giant impact.

Not limited to the Solar System

Hofmeister says there is a continuum between single stars, binary stars, multiple stars, planets and even extrasolar planets.

“In all cases, the process is gravitational accretion of these cold, 3-D clouds making things contract and spin out, and that’s where the energy comes from,” she says. “It’s all happening in very cold temperatures, in 3-D instead of 2-D.”

Criss says there is plenty of observable evidence that the 2-D is wrong.

“It patently doesn’t make sense that a bunch of random collisions between heavy, solid objects are going to produce a Solar System with planets orbiting the Sun in a beautiful plane, with everything having upright spins,” he says. “That’s like setting off a nuclear bomb and expecting all the trees in the world to end up neatly stacked.

Moreover, the Hubble pictures show stars being born in the Eagle nebula, and they’re formed in a cold 3-D cloud.”

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Hugh023
5 / 5 (2) Feb 28, 2012
This seems very reasonable to me . . .
baudrunner
1.8 / 5 (4) Feb 28, 2012
Wow, lots of content. Is the planet formation process the same right after the big bang as it is in a mature universe? Does the cloud of dust spit out suns and planets like the star making nurseries out there in Hubble images? And, most importantly..

How can you not factor heat into the equation? Heat melts matter under pressure creating a liquid-like plasma centre of a planet that swirls out of sync with the surface, and that can affect not just its spin, but the tidal forces that determine their relationships with their neighbors. The last stage of solar system formation.

Dictionaries had better change their definition of entropy, by the way. I've always thought so. Systems do NOT have a natural tendency toward chaos and disorder, but the exact opposite.
Ironhorse
3.8 / 5 (5) Feb 28, 2012

... Dictionaries had better change their definition of entropy, by the way. I've always thought so. Systems do NOT have a natural tendency toward chaos and disorder, but the exact opposite.


Actually the units of entropy are joules per unit kelvin. Entropy increases if energy goes up without increase in temperature (ie. disorder) or if temperature goes down with with energy unchanged or increasing. A nebula shrinking down into a star system initially releases energy hence the cold temperatures. When the star or planetary cores heat up the temperature increase increases chaotic motion in such a way that entropy increases.
Quantum theory if I remember correctly, states that you can have a local decrease in entropy so long as the entropy somewhere else increase to compensate. You just have to draw the box around you're experiment large enough.
Lurker2358
1 / 5 (1) Feb 28, 2012
Well, what do ya know? This is much more like the things I've said in the past regarding SS formation, both in terms of competing gravity and dimensional analysis.

It's particularly fulfilling regarding proposed convergence of tilted orbital planes, a 3D process to be sure, which I used to attempt to explain why Uranus' axis and moons' orbits are so heavily tilted. While I need not be right in the details, I was clearly right in the concept. It clearly requires a 3D process to explain Uranus' present condition, which is one of the biggest glaring clues that the standard Nebular Disk model of SS formation was wrong.

And of course, not "all" of the orbiting bodies are on the exact same plane just yet, Pluto, Sedna, and comets are still highly inclined, but this is just more evidence that the disk model was always wrong; that a 3D process was involved, and isn't even yet entirely finished, BTW.

This now explains tilted axis without the need for giant collisions at all.
aroc91
5 / 5 (2) Feb 28, 2012
Quantum theory if I remember correctly, states that you can have a local decrease in entropy so long as the entropy somewhere else increase to compensate.


That's not the least bit limited to quantum theory. Life is a localized decrease in entropy because the Earth is an open system.
barakn
1 / 5 (1) Feb 28, 2012
When measuring the entropy of a system, you have to measure the whole system. In the case of a planetary system forming, this would include the infrared energy that is escaping and also the electromagnetic energy arriving from outside sources. Thus we are talking about a region of space millions of light years wide. However much the entropy might decrease in the material composing dust, planetesimals, planets, or the star due to rearrangements of said material, the surrounding area's entropy increases by a corresponding amount as it is flooded with infrared radiation.
Kinedryl
1 / 5 (2) Feb 28, 2012
The distribution of inner planets inside of solar system is not random, as J. Kepler noted already, which implies, the Solar Nebular Disk Model will be more relevant here. The density fluctuations inside of dense gas will follow the dodecahedral geometry of hypersphere particle packing, which we can seen inside of fluctuations of dark matter and this geometry could even take place during formation of planets from sufficiently dense protoplanetary disk. This model could explain the retrograde tilt of Venus and Uranus planets, too. The outer planets were rather formed with accretion, i.e. from bottom up, because they emerged from more sparse areas of protoplanetary disk.
Kinedryl
1 / 5 (3) Feb 28, 2012
Errata: the "Solar Nebular Disk Model" should be a "three-dimensional (3-D) gas" indeed.
On the same idea the dense aether model of black hole formation from dark matter is based. In standard cosmology the sparse divided matter has been formed first, the more massive objects had come later and the dense black holes appeared with accretion at the very end. But the observational evidence of most distant galaxies is exactly the opposite - they're formed merely with black hole itself (quasars or AGNs). From this reason the opposite idea is more relevant for massive galaxies: the clouds of dark matter condensed until reached the critical density and from this moment the dense cloud has started to evaporate like so-called white black hole into galaxy. This evaporation occurred first trough whole surface of quasar, later when Schwarzchild's criterion was reached the event horizon was formed and the evaporation continued trough polar jets, which gave the flat shape to the newly formed galaxy
Kinedryl
1 / 5 (2) Feb 28, 2012
IMO both models are equally correct and for large galaxies or protoplanetary disks the 3-D gas model is more relevant (top to bottom mechanism), whereas for smaller objects the accretion has take place. In accordance with it, inside of small stellar clusters the central black holes are usually missing...

IMO the habitable zone of galaxies and planetary system occupies just the places, where both mechanisms occurred with the same relevance. It requires to have of the galaxy and planetary system of certain size: not too big or not too small one. After them the habitable zone of maximal complexity emerges just at the place, where both mechanisms were just balanced. Such a zone enables the formation of relative large moons around planets with optimizes the planetary speed of rotation and tilt and so on, which greatly enhances the speed of life evolution... The pure 3-D gas provides only minimal number of moons, whereas the Solar Nebular Disk model provides large amount of small moons.
jibbles
Feb 28, 2012
This comment has been removed by a moderator.
Anda
5 / 5 (1) Mar 01, 2012
Kinedryl... are u a clone of Callippo or of Rawa1?
antialias_physorg
5 / 5 (1) Mar 01, 2012
It's an interesting simulation (and I'm not saying it's wrong) but this part seems strange:
It patently doesnt make sense that a bunch of random collisions between heavy, solid objects are going to produce a Solar System with planets orbiting the Sun in a beautiful plane, with everything having upright spins,

I think it does make sense that you'd end up with all planets in a plane eventually - because every deviation from the plane will result in a net force of all the other planets dragging it back into the plane.
Even with all planets starting off in completely different planes do we get a slow settling into (or close to) the plane of the largest planet.
As for the spins: There are planets and moons in the solar system that exhibit various spin directions (Uranus is pretty much sideways and Venus is retrograde).
Kinedryl
1 / 5 (1) Mar 01, 2012
There are planets and moons in the solar system that exhibit various spin directions (Uranus is pretty much sideways and Venus is retrograde)
This is just the thing, which the dense gas model of protoplanetary disk could explain, if you try to imagine, how this dense vortex is rolling in spiral-like fashion similar to accretion disk around black holes.

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