Life and Gas Giants
Can a gas giant be a host to life both in its own
atmosphere and on the surface of a moon in the while orbiting
inside the habitable zone?
Very good question. Only on the surface of the moon orbiting a
planet inside its habitable zone. The gas giant itself does not orbit
in any zone. Also the period for such life to evolve would be much
shorter than here because the star is later on the H-R diagram.
David H. Levy
I think the answer is possibly, but it depends on a great number of
conditions. In short, I do not think we have any way of reliably saying
YES or NO to your question at this time (and I would say anyone that is
giving you a firm answer has not thought about it enough).
Part of the question involves what you mean by life. That is a huge
question just by itself. I think the likelihood of finding "simple"
life (so like bacteria) on a gas giant moon is reasonably good. Given
the right conditions life very different from what we think of as normal
could arise. Given the simply staggering number of stars (and hence
planets) in the galaxy, let alone the universe, I have a hard time
imagining the rest of the universe as bereft of life. If we were betting
on the likelihood of finding at least some kind of simple life on a moon
somewhere in this solar system, I would bet on the "yes" side. That said,
I have lost plenty of bets and we have not found any life outside of earth
at the moment.
Simple life may even be possible in gas giants themselves. Would it be
large multi-celled organisms that inhale O2, exhale CO2, enjoying
temperatures in the narrow range of 10-40 Celsius, needing sunshine and
a firm place to stand? Probably not (I think Carl Sagan's idea of giant
"floaters" was clever, but probably not very reasonable either). But
just because the environment of a gas giant is so different from what
we think of as needed for life does not preclude it (in my mind) from
having the potential for at least some simple life in the right
conditions. Perhaps nature is clever enough to even have more
complicated life on gas giants in the right circumstances.
So I suppose my unqualified answer (I am not an astrobiologist) is this:
Given the right set of circumstances and the shear number of planets and
moons in the universe, I would say it is very likely for at least simple life
to exist on both a moon and a gas giant somewhere.
There is a very, very interesting story about life outside of earth that
we do know fairly well. While we have not discovered life elsewhere in
the solar system, we have taken it out with us and seen that at least
some organisms are capable of surviving the vacuum of space. Apollo 12
was able to retrieve microbes from earlier probes launched to the moon.
These microbes had to survive for years in a harsh vacuum, extreme
temperature and without any support. So have we found extra-solar life?
Not yet, but it is reasonable that some very simple life may actually be
more ubiquitous than we often think.
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Update: June 2012