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What caused the planets to aggregate as they did?

  1. Jul 24, 2004 #1
    "What caused the planets to aggregate as they did?

    What are the dynamics of gravity, that caused the plantets to evolve, progressively larger from Mercury to Jupiter and then progressively smaller Jupiter to Pluto? :confused:

    What are the dynamics of gravity, that caused the inner plantets to aggregate solid and not gaseous? :confused:

    Newton law states that mass (a bird or its feather) accelerates at the same speed towards a gravitational center, if there is no air resistence.

    A science demonstration popular for centuries had a coin and a feather dropped simultaneously inside a glass tube from which air had been pumped: the two are always seen to drop equally fast.
    Such a demonstration was also made on the surface of the Moon by David Scott, one of the Apollo astronauts. Not only has the Moon no atmosphere, but its gravity is several times weaker, making the fall slower and easier to observe.

    Space is a near vacuum, does dark matter or energy have a play in what has happened? :confused:
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  3. Jul 24, 2004 #2


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    There are several theories regarding solar system evolution. The one that seems most popular nowadays is "planetary migration", a model in which the Solar System did not form as we currently see it, but with the biggest planets closer in to the Sun. Then magnetic breaking caused the large planets to gt accelerated into higher orbits.

    I personally don't buy that one, but it is very popular today.
  4. Jul 24, 2004 #3


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    Where does Mars fit in this?

    the mass distribution of the Planets cannot be explained by simple analysis of Gravitational forces. It may be that you would have to have knowledge of the distribution of mass in the initial disk of matter which formed the Solar System.

    The gases present in a planets atmosphere is determined by the average temperatures and the mass (gravity) of the planet. Thus the small high temperature Mercury has no atmosphere. The larger, colder planets have huge gas atmospheres. Venus has an atmosphere consisting of heavy gases.

    For a gas to be present in the atmosphere of a planet its average velocity (determined by temperature) must be less then the escape velocity of the planet.
  5. Jul 24, 2004 #4
    This paper points two possible ways of planetary formation starting from the originary solar nebula:
    "There are two main formation scenarios envisaged for planet
    formation. The model applied to the solar system assumes
    that planets form initially through the agglomeration of dust
    into rocks and planetary cores, and that finally (for the giant
    planets) these cores use gravity to accrete gas. Alternatively,
    there is the possibility of a more dynamical mechanism us-
    ing gravity directly in the formation process.
    ...this alternative scenario involves the formation
    of giant planets directly, through gravitational instability in
    the protostellar gaseous disc (Boss 2001; Rice et al. 2003;
    Mayer et al. 2002; 2004)."

    That is, in the second scenario the giant gaseous planets are formed directly, without experiencing the rocky phase
  6. Jul 24, 2004 #5
    Hmm.. seems like we'll be sending sismological probes to the gas giants at some point...
  7. Jul 25, 2004 #6
    Mars and the asteroid belt is a anomoly that must have occured after the initial formation. Look at the the mass of the total asteroid belt. Mars is now known to have had a ocean on its surface and there was a series of very large impacts for example, "Hellas Planitia". There should be a missing planet, acccording to how the solar system initially formed.

    Gravity had the initial makings of what our solars system is. Are you then saying that the solar mass and the residual particles from the previous sun, would determine the makeup of a solar system? How would you calculate that? Is there a formula to calculate theoretical solar systems depending on solar mass and residual matter from a super nova?

    Then Mars is an exception it should have a thicker atmosphere.

    So then Jupiter would deplete to its core if it was for some reason moved to Venuses orbit.
  8. Jul 25, 2004 #7
    I can't answer all of that, but I'll try to answer some.

    Not really, it isn't massive enough to maintain a thicker atmosphere. Note the word "maintain" though, as it is theorized that it did have a thicker atmosphere in the past.
    Probably not, since the mass of the gas also counts when determining escape velocity.

    A note: the process of planetary formation is very complex, and there are many parameters that affect the final outcome. For example, the distance from the star is not the only determining factor in the resulting mass of the planet. Similarly, the temperature and mass of a planet are not the only factors in determining the initial thickness of its atmosphere - e.g. comet impacts aslo contribute.
  9. Jul 25, 2004 #8
    After reading up on this planetary migration theory, a question came up in my mind. I can see how many other factors could come into play over eons of time. Then when I read an article like this, I get rolled back to stage one thinking. :rolleyes:
  10. Jul 25, 2004 #9
    These two might not be complete theories, in themselves. How would they explain all the strange orbits and retrograde motion? Why does gravity aggegate steller material to spin in one direction or the other? There seems to be a later process in solar system evolution, where larger gravity forces like close encounters and collisions, shape the end product.
  11. Jul 25, 2004 #10
    What would be the reason for not being able to maintain its atmosphere? :confused: A change in the diameter or orbit of Mars or perhaps a large asteroid to vaporize its ocean. The evidence shows that there was a ocean and there would have to be a atmosphere to hold the ocean in liquid form, at one time. Venus is marginally smaller D=7,521 in size, to the Earth, yet has 90X atmospheric pressure. Mars D=4,222, equivelent to Earth at 30,000 feet. What is that minimum diameter size to maintain a atmosphere?
  12. Jul 26, 2004 #11
    As I said, there are many different factors that contribute to the outcome in various complex ways. It is not enough to consider one or two aspects of the situation. You must consider all aspects of the situation. Moreover, it is not enough to consider the situation at just one time, but rather you must consider the entire history of a solar system.

    Here is what I know about the atmosphere issue. The following may not be accurate, so take this with a grain of salt:

    Earth and Venus are approximately the same size, and hence roughly the same mass. On the other hand, mars is about half the diameter, and therefore has a much smaller mass. But also, a larger surface-area to volume ratio.

    When the planets formed, they all had massive heat stores, obtained from the numerous collisions during their formation. So they were all volcanically active. This activity is known to release large volumes of gas into the atmosphere. So in the past, mars had a thicker atmosphere. As a consequence, it was much warmer than now, and probably had liquid water on the surface. Perhaps earth and venus also had thicker atmospheres, but I can't tell, since I don't know how much they have lost since then.

    Since mars has a large surface-area to volume ratio, and since it is the furthest of the three from the sun, it quickly lost all of its heat store, and became volcanically inactive. At that point, since its atmosphere was no longer being continuously regenerated, it began leaking away into space, due to mars'es lower mass, which didn't provide enough gravitational pull to hold the atmosphere. As the atmosphere got thinner, temperatures dropped. This resulted in the locking of carbon dioxide in the soil, tuis reducing the greenhouse effect, and dropping temperatures even further. In the process, all surface water has either evaporated away due to the lower pressures, or froze.

    On the other hand, Venus, being closer to the sun than Earth, did not loose heat quite as fast as Earth did. So it is still more volcanically active than Earth. This is the reason why Venus'es atmosphere is much thicker. It is also the reason why it is locked in a runaway greenhouse effect, and is loosing heat much much more slowly then Earth.
    Last edited: Jul 26, 2004
  13. Jul 27, 2004 #12
    Actually, it would. Evidence of this has been observed. The only example sticking in my mind at the moment is the planet HD 209458b, which is experiencing atmospheric blowoff from close proximity to it's host star. Other cthonian planets have also been observed.
  14. Jul 27, 2004 #13
    I agree that it can loose a large part of the gass, but whether it will actually loose all its gas, is a question of when will it reach equilibrium, if at all. In any case, if all the gass is lost, you will no longer have a gas giant - you will either have something like mercury, or nothing at all - depending on whether there was a rocky core. And also, I suppose that would take some time.
  15. Jul 27, 2004 #14
    I'm not sure how close a gas giant would have to be to lose all it's gases. In any case, it would depend on where on the main sequence(or off of it) the particular star was, and how massive the core of the gas giant was. It is indeed possible, and Mercury has been suggested to be a cthonian planet
  16. Aug 6, 2004 #15
  17. Aug 6, 2004 #16
    Remember too that solar wind another factor. Solar wind strips the upper layers of the atmosphere off of the planets...abit slowly. The earth is protected from the effects of solar wind because of its magnetic field. Venus is losing its atmosphere slowly because it has a very weak magnetic field and is closer to the sun so the flux of solar wind is higher. But whatever process (probably rampid volcanism) that gave it its thick atmosphere did a swell job. Mars wasn't so lucky (weak magnetic field as well).
  18. Aug 6, 2004 #17
    Yes, I didn't want to mention solar wind with respect to a gas giant loosing its atmosphere when close to the sun, because AFAIK gas giants have very strong magnetic fields (at least some of them), which could protect them from the solar wind.
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