Core Accretion Model vs Disk Instability Model

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From the two main models given, which is the more plausible model for the formation of the planets in our solar system?

Poll ended at Thu May 19, 2005 3:17 pm

The Core Accretion Model
3
100%
The Disk Instability Model
0
No votes
 
Total votes: 3

Kaustav
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Core Accretion Model vs Disk Instability Model

Post by Kaustav »

Astronomers currently have two main theories of how the planets formed in our solar system.

a) The Core Accretion Model

Grains of dust in the solar nebula attract each and clump together through gravity to form planetesimals. In the inner solar system, due to the heat generated by the protosun, heavier elements such as iron, nickel, etc accrete to form planetesimals and hence the solid, dense nature of the terrestrial planets (i.e. Mercury, Venus, Earth, Mars). The outer Jovian planets (i.e. Jupiter, Saturn, Uranus and Neptune) formed through a similar process of core accretion. However, due to the lower temperature in the outer solar system the cores contained much more ice making them generally less dense. These cores reached a point where they started to attract large amounts of Hydrogen and Helium in the outer solar system and thus formed what we call the gas giants.

b) Disk Instability Model

Some astronomer theorise that the Jovian planets all formed through pure contraction of gas under gravitational attraction. They postulate that due to the non-uniform nature of the material in the solar nebula, areas of gas started to clump and rotate around their common centre of gravity (due to the conservation of momentum). As the process accelerated and more gas treamed in, dense gaseous cores formed and the thus the gas giants were born.


The problem with both models is that the planets such as Neptune and Uranus couldn't have formed as gas giants so far out in the solar system. So how did they get where they are? Finally, of the two models presented above, which one is the more plausible model for planetary formation?
Last edited by Kaustav on Sat May 07, 2005 8:29 pm, edited 1 time in total.
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Post by joe »

Hi Kaustav,

God knows. A couple of things though....if the protosun is attracting hydrogen gas and the outer planets are attracting gas, why aren't the terrestrials attracting gas? And what is happening to the terrestrials in the DI Model while the gas giants are forming from gas in the nebula? We need to know.

Regards,

:?
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Condensation temperature

Post by Kaustav »

Hi Joe,
Ah, your question doesn't have a simple answer and it should be noted that both the core accretion model and disk instability model are only unproven theories. Most of the astronomy professors I know who specialise in solar system formation theory quite frankly tell me that "we don't know how the planets formed, but we do have some theories based on observational evidence and extrapolations from the behaviour of elements and materials previously studied. So in answer to your questions some high school science reminders need to be brought to attention;

To understand how the planets formed, we must understand the conditions that prevailed within the solar nebula. Density of material in the solar nebula outside the region of the protosun was low, as was the pressure of the nebula gas. As we learned in high school science classes, if the pressure is sufficiently low, a substance cannot remain in liquid form but must end up as either a solid or gas. So for a given pressure, what determines whether a substance is a solid or gas is its condensation temperature (CT). If the temperature (T) of a substance is above its CT the substance is a gas; if the T of a substance is below its CT then is solidifies into small specks of dust or snowflakes.

Substances such as H2O, CH4, NH3 have low CTs, from about 100 to 300K (Kelvin). Rock forming substances have much higher CTs between 1300-1600K. Gas clouds which the solar nebula formed from had an initial T of about 50K so all these substances should have existed in solid form. It follows that the early solar system was abundantly full of these elements in the form of small ice particles and solid dust grains. Now the thing is Hydrogen (H) and Helium (He), the most abundant elements in the solar nebula, have CTs near that of absolute zero so they always existed as gas during the formation of the solar system.

As the solar nebula underwent contraction to form the protosun the inner most parts rose in T to about 2000K, however T in the outermost regions remained below 50K. So in the innermost solar nebular water, methane and ammonia vaporised. Only materials with high CTs such as iron, silicon, magnesium, sulphur, calcium, aluminium, nickel remained solid. In contrast ice coated grains and particles survived in the cooler outer portions of the solar nebula. Small chunks would have formed in the inner solar nebula from collisions between grains. Chemical bonds kept them together and over millions of years these chunks coalesced into planetesimals. They had diameters of about 1Km or so. These were large enough to be held together by gravitational attraction. As time ticked the planetesimals coalesced to form larger protoplanets, roughly the size of our moon. During the final stages these protoplanets accreted to form the terrestrial planets.

Meteor samples on Earth give us good clues to this stage in the evolution of the inner solar system as they are some of the oldest known objects. Most meteorites contain chondrules as well as dust grains. They're small glassy and roughly spherical globules. Liquid tends to form spherical drops, so the shape of the chondrules show that they were once molten. Scientists have attempted to form chondrules in the lab and have shown that they're formed through sudden rises and equally sudden drops in temperature. This means they couldn't have been melted by the temperatures of the inner solar nebula which remained hot for thousands of years. Instead, chondrules must have been formed from sudden, high impact events that quickly melted material and permitted it to cool.

Like the inner planet, the outer planet may have begun to form from the accretion of planetesimals. The key difference is they would have formed from ice and rocky grains. The elements of which ices are made are much more abundant than those that form rocky grains. Much more solid material would have been available to form planetesimals in the outer solar system than in the inner solar system. As a results much larger solid objects would have formed compared to the ones in the inner solar system. Such mixtures of icy and rocky materials would have formed the core of the Jovian planets. Due to the lower T in the outer solar system, the H and He gas was moving very slowly and was easily captured by the strong gravitational forces of cores. Thus the cores of the Jovian planets would have continued to attract gas and grow. This is what we call the Core Accretion model. Note that in the inner solar system due to the higher temperatures, the H and He gas was much more exited and moving around faster. The small terrestrial planets did not have the gravitational attraction to capture the hotter H and He gas and thus they have very thin envelopes (atmospheres) compared to the Jovian planets.

Calculations based on the CE model over millions of years, as rocky materials and gas accumulated, the mass of the core and the enveloping gas became equal. From this critical point onwards, the envelope pulled in all the gas it could get, dramatically increasing the protoplanets mass and size. This run away process could have continued until all the available gas was used up. The result was a huge planet with a very thick H rich envelope surrounding a rocky core with 5 to 10 times the mass of Earth.

There are, however, still many unanswered question about planet formation in our solar system. The CE model doesn't account for why and how the other gas giants formed so far out. Perhaps they formed much closer in and were perturbed by Jupiter's massive gravity and flung out the farther orbits. No one really knows if the CE Model of the DI model are correct but more astronomers go for the CE model these days.

A big spanner was thrown in the work ever since we started to detect extra solar planets. The oddest thing is that when you look at some of the newly discovered extra solar planets, they're usually MASSIVE Jupiter sized worlds and they're insanely close to their parent stars. Some of them are closer to their stars than Mercury is to our own sun which sort of blows most of the models we have for the formation of our solar system out of the water. As we detect more and more extra solar worlds, I think better and stronger theories of solar system formation will come along. This why it is so important to study extra solar planets as understanding their formation will help us understand the formation of our own solar system.
Last edited by Kaustav on Sat May 07, 2005 10:57 pm, edited 1 time in total.
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Post by joe »

Kaustav,

You didn't write that....did you? And "most of the astronomy professors...tell me"? How many do you know?

If you did write it I take my hat off and if you do know enough profs to say "most" then god bless you.

:P
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Post by Kaustav »

Hi Joe! LOL
I sincerely hope I know all that I just wrote as I'm being examed on it at UCL on Tuesday. I don't know too many Astro professors right now... off the top of my head perhaps about 10... and out of those only two are planetary specialists. Anyway, I hope I managed to answer your question adequately. I found your questions quite helpful for my revision! Thanks :-) Can you ask me about the five ways in which Astronomers can detect extra solar planets? I think I've got all the facts for that memorised now.

There's probably MUCH more to planetary formation theory but for now that's all I need to know to pass my exams! The whole area of extra solar planetary research is facinating and with new facilities like ALMA in the Atacama dessert being built it'll help astronomers cut through all the thick dust and gas (because ALMA working in the milimeter and sub-milimeter range of the EM spectrum) in planetary formation regions of the ISM and really get in to the heart of areas such as solar nebulas and reveal some facinating new discovers that will hopfully go towards bringing a better understanding to solar system formation (and much more besides).

I'm still befuddled over how the Jovian planets are so far out when compared to the extra solar planets we've seen so far (~ 160 planets so far) where many of them are VERY close to their parent stars. I don't understand why and how they got so close where as in our own solar system it's not like that. Does anyone reading this know? I'd be very interested to know, especially if you have any useful pointers to articles and papers on the Internet.

Kaustav.
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Post by joe »

Hi Kaustav,

I was aware that you were doing that course and the very best of luck with your exams. You explain yourself very clearly and simply so you shouldn't go wrong. Thanks for taking the time to answer. An alternative view of planetary formation is taken up by Ernest J. Sternglass, a so-called Primeval Atom theory. It involves the division of an initial electron/positron pair into Galaxy cluster, galaxies, stars, planets, etc. and claims to account for all the anomolies in the Standard Model(s). It's a bit left field and might not be a good idea to muddy the waters just before an exam but it is a good alternative read.
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Post by KendalAstronomer »

To answer the qustion on why there is less gas around the terrestrial planets than the gas giants - over the course of the evolution of the solar system, gas will be evaporated by solar radiation and scavenged by the solar wind. How much goes depends on the closeness to the Sun - more radiation and denser solar winds close in - as well as the strength of the individual planet's magnetosphere. A good magnetosphere like Earth's will deflect much of the damage the solar wind can cause, but only above a certain radius. No magnetosphere can lead to a situation like Mars (not helped by Mars' small size making its atmosphere more vulnerable to evaporation).

An interesting fact that is sometimes mentioned is that the point at which the terrestrial planets stop and the gas giants formed (the asteroid field between Mars and Jupiter) is about the furthest point at which liquid water could exist on a planet due to radiation.

To answer another point mentioned earlier, the poster could indeed know a lot of proffessors in this field - we at UCL now have a staggering 27 due to staff retention... frightening.
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Post by joe »

Hi KendalAstronomer,

Sorry, just to be pedantic :) can a gas evaporate? But I know what you mean anyway. It was just a little unclear in Kaustav's first post as to what was happening across the board at each epoque. The problem remains though that it doesn't make sense for extrasolar planets of gas giant proportions to be so near their star and hold on to their enormous atmospheres, unlike - as you mention - the terrestrials. One thing I'm not completely sure of though is what the cores of the gas giants consist of. The Core Accretion Model seems to be the more widely accepted one but is it not true that their cores are suspected to be of "metallic" gas and therefore contrary to the theory?
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Post by KendalAstronomer »

Hi joe, yes, ignore me when I try to use particular words like "evaporate", just my mind wandering or bad habits I've picked up...

The cores of Gas Giants are referred to as having 'metallic' state gas inner cores as a result of two things. We know they have gas - and have to extrapolate down, saying if the gas is this dense at the radii we see it, how dense do we expect it to get. Secondly, we know that Jupiter in particular has a magnetosphere - a gigantic magnetosphere - that needs some sort of metallic motions to create it. So there's definately some high pressure materials down there and some metals. There is nothing in this to say there aren't heavier elements deeper down, like an iron core. It is just that the gas giants have an atmosphere that is so dense, there is no transition between the low altitude, high density gas and what on Earth would be termed the mantle.

The core accretion model suggests that the primordial soup gathers together in huge globs. Those globs then undergo a process known as differentiation, where the denser materials fall to the core and the lighter ones rise. In a gas giant's case, after this has happened, if there is enough of a body of gas remaining, it will compress the lower layers of the atmosphere into new states. This is where the metallic Hydrogen and Helium layers apparently come from.

The terrestrial planets in this model, would be bodies where the high density gas remains gaseous because so much above it has been lost, and so little pressure remains. The mantle develops a skin - the crust - and the little stone in the middle made of the heavier elements is seen as being seperate from the atmosphere.

Whether or not this is true is not known. We can't do seismic tests and all our probes get crushed before experiencing the metal state layer. So all conjecture, but fun to do (honest)

As for exoplanets. We know even less. There is a vague idea about how composition could cool down a Jupiter type planet, but the ideas haven't been worked on too much due to computational limitations. There are certain ions in the atmosphere of Jupiter that are good emitters of infrared. If they were more abundant, they could keep a planet cool in theory - but the thing would have to be a gas giant to begin with, so we're left with a chicken and egg thing. We can't get the cooled down planet without some special chemicals available, but those chemicals don't normally appear abundantly without the high densities of a Gas Giant.

Then there's always the question of did they form there or did they fall in from further out and not have time to "evaporate" [please fill in appropriate word]? If so how? Who knows? Not me! But will keep on looking...
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