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How common is it for stars to have rocky bodies orbiting them?

  1. Jun 24, 2011 #1
    Hi all,

    I've read somewhere that at least 20% of our galaxy's stars have planets orbiting them? But what is the share if we include all kinds of rocky bodies? How likely is it that stars suck in everything without leaving any leftover material what so ever?

    I'm curious because the Drake equation seem to have a flaw in that it requires planets in order for life to form. Wouldn't all the required elements would also be available on smaller objects as well? The environmental conditions might not be as hospitable, but that's a separate question.

    Thanks as always!
  2. jcsd
  3. Jun 24, 2011 #2
    Almost every star will have some rocky material around it---it's just a question about how much. In the case of sub-planet sized objects, its very hard to say because there's no data. From a theoretical perspective, every star will leave some material around after it forms---not all stars necessarily have all of the heavy carbonaceous and silicate compounds to form rocky structures however.

    Data from the new Kepler satellite suggests that about 20% of all stars have 'multiple planets'. Most likely the number of stars with at least 1 planet is much larger.

    About Drake's equation---there are lots of reasons to expect Life to require planets to form. But there are people who think that life could form otherwise---on asteroids, comets, or even just in interstellar clouds (but many of those people do very questionable 'science'). What it comes down to, is that there is only one place we know life to exist, and that is on a rocky planet. It is thus logical to start with the idea that rocky planets are the primary location of interest.

    Some of the reasons why rocky planets would be necessary:
    Smaller bodies will not attract enough material, of enough varieties to form life; larger bodies are too hot/heavy for large compounds to form.
    The formation of complex molecular structure requires both strong energy sources (for the chemical reactions) and a stable, protective environment to shield the products---this is hard to find elsewhere.
    Life as we know it, is entirely dependent on liquid water---again, only possible on rocky planets that are able to sustain an atmosphere.
    The list goes on.
  4. Jun 26, 2011 #3
    Note that, by analogy with our gas giants' tidally-kneaded moons that seem to contain liquid water between their frozen crust and rocky cores, mega-moons around Neptunian giants could be 'terrestial' beyond the cool end of the normal 'goldilocks' zone. IMHO, something like Mars could stay tectonically active as far out as Jupiter if it were an inner moon...

    Uh, I suggested 'Neptunian' because Jupiter and Saturn have lethal magnetospheres...
  5. Jun 26, 2011 #4


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    I think rocky alone isn't likely to be enough. As far as we now know, it's going to also need liquid water.
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