Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Outer planets always gas planets?

  1. Oct 21, 2004 #1


    User Avatar

    In another thread: "When is a planet a planet?"


    Someone mentioned that normally the outer planets in a solar system are gas-gaints and the inner planets are big "rocks". Why is that? Why can the gas-gaints not be the inner planets?
  2. jcsd
  3. Oct 21, 2004 #2
    The preferred model for the Solar system formation is called the Condensation theory, an improvement over the old Nebular theory. Quoting from this page:
    "An advanced theory, called the condensation theory, includes the nebular theory but also incorporates interstellar dust as an essential ingredient in the formation of the planets. This theory claims that the dust grains of the interstellar medium helped cool the nebular cloud by radiating heat away, and also acted as a foundation upon which atoms could attach. These properties of the interstellar dust grains aided in the collapse of the nebula and in the formation of planets"

    So why are outer planets gaseous and inner rocky in the solar system? It seems that is a question of temperature. Inner planets are closer to their sun, this extra heat seems that can dissipate the gas around these bodies when they were protoplanets. the same page explains it this way:

    "Another factor in the development of the four inner terrestrial planets and the outer gaseous planets was temperature. After the protoplanets had formed, the central regions of the solar nebula were collapsing and forming the Sun, as described in Module 2. The young Sun caused the temperature of the closer inner protoplanets to be higher than the outer protoplanets. As a result, the kinetic energy of the gaseous molecules was too high for them to coalesce, and they simply dissipated. At the outer planets, however, the molecules were cold, and were moving slowly enough for gravity to overcome their movement. Over the course of several million years, the planets grew into the planets we know today. "
    Last edited by a moderator: Apr 21, 2017
  4. Oct 21, 2004 #3


    User Avatar
    Science Advisor
    Gold Member

    It largely has to do with the distance from the star at which the material becomes cool enough to solidify. While still a gas, the atoms are moving too fast and are too diffuse to clump or be gravitationally captured in great quantities and radiation pressure from the star pushes them away. Once they solidify, clumping and gravitational attraction takes over. Thus, inner planets are mainly composed of high melting point materials [rock] while outer planets are mainly composed of low melting point materials [gas].
  5. Oct 22, 2004 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I don't think this has been demonstrated. That is the case in our solar system, but we have not found terretrial planets around other stars yet (they're out of the range of our technology so far). So, we don't have any other examples like our own solar system yet (although we have some potential candidates).

    Presumably, they can. Many of the extrasolar planets that have been found are "hot Jupiters"...gas planets found close to their star (much less than 1 AU).
  6. Oct 23, 2004 #5


    User Avatar

    Do these finding not demonstrate then that the condensation theory is wrong. As I understood, from previous replies, that due to the heat of the star the light gasses (H, He) escape to space.
    It seems that this theory is written to fit the current situation is our solar system?
  7. Oct 23, 2004 #6


    User Avatar
    Staff Emeritus
    Gold Member
    Dearly Missed

    Some of the light gasses escape, but much of it remains, in the outer regions of the forming cloud. That was the explanation for outer planets like Jupier, Saturn, Uranus and Neptune being gas giants. The other factor is the size of the cloud.
  8. Oct 23, 2004 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Check out the following attachment. It is a chart showing the known extrasolar planets, plotting their masses against distance from their primary.

    Jupiter and Saturn are marked in their respective positions for comparison.

    One thing to be noted is that it does not appear to be rare for massive planets to form close to their sun.

    We however can't put too much emphasis in the fact that Jupiter falls near the edge of the grouping, and that Saturn falls well outside it. This is likely due to the limitations of the present methods of detecting extrasolar planets, Which tend to favor the detection of very massive planets or not as massive planets with smaller AU. The combination of Saturn's comparitively small (to Jupiter) mass and its large AU most likely places it outside the sensitivity of present methods.

    Attached Files:

  9. Oct 23, 2004 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Pluto/Charon is not a gas giant, neither is Sedna, nor any of the EKB objects ... and that raises the question of what is a planet?

    I think it's important to distinguish between how 'planets' are formed and where they are (and how they are!) today. E.g. maybe 'hot Jupiters' weren't formed close to the stars they now orbit so closely? At another extreme, one can only wonder how to classify the 'planets' that orbit pulsars!
  10. Oct 24, 2004 #9
    I remember to have read in some website that Condensation theory is not applicable to all the known exosolar planetary systems

    In fact, the Solar system seems to be a rara avis in the universe. This paper called "How special is the Solar System?"


    "We have noted that the solar system is an outlier in the pa-
    rameter space of known planetary systems."
  11. May 17, 2011 #10
    The impression I have been getting is, as most here say, that gas and ice giants form beyond the 'snow line' (where gas cools enough to go molecular/solid) in 10E5 years or so, whilst the silicates (rocky bits) form "terrestrial style" planets inside the ice line but very much more slowly, in 10E7 to 10E8 years.
    However, IF the gas in the protoplanetary disc does not dissipate quickly, that is after the 10E5 or 10E6 years of gas/ice giant formation, then these early giant planets can migrate inwards, scattering any early rocky planetoids as they go, giving rise to rocky planetoids well out beyond the ice line and with gas/ice giants falling all the way in to close orbits to where the gas HAS already been dissipated and there we see them stopped for many star/planet combinations.
    The bit no-one has yet explained, is how the protoplanetary gas cloud has stayed long enough for the gas/ice giant planetary migration to take place, after their formation was complete, unless more/extra gas in-falls within the snow line? Anyone explain?
  12. May 17, 2011 #11
    One plausible answer is that they can, but if you have Jupiter-sized planets in the inner solar system then the odds of an earth like planet being around long enough to develop life becomes extremely low.

    One thing that would be interesting to find out is whether solar systems like ours are rare or common.
  13. May 17, 2011 #12
    FWIW, I remember the output from an early 'planet formation' simulation in, IIRC, Icarus, which gave a zoo of systems-- Some rocky on the inside, some with close giants, some all mixed up. Clearly, those 'hot jupiters' did not match the small sample then available (us) and were culled from later code...
  14. May 19, 2011 #13
    From two-fish "One plausible answer is that they can," but I don't think this can work, because at present hot Jupiter orbital distances (generally <1 au) ALL original gas would NEVER condense since it is too hot, so the Jupiters can ONLY form beyond the snow/ice line which seems to lie beyond 3-4 au for most protoplanetary discs? They then migrate inwards by exchange of angular momentum with gas/dust still within the snow/ice line boundary.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook