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If microscopic life evolves on a planet which is subsequently ejected from orbiting its star, then the planet would cool down. How deep into the interior of the planet could organisms reach, assuming the mantle had cooled down enough?

For example, on Earth there are rock-boring bacteria that have tunneled their way 3 km into the crust.

The limiting factor for depth of bacteria on Earth seems to be temperature, but presumably if the mantle had cooled down then at some depth other factors would come into play such as pressure or rock hardness or composition.

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closed as off topic by Chris White, Waffle's Crazy Peanut, David Z Feb 3 '13 at 3:44

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Please clarify. "Reach" by what mechanism? And survive? – Stuart Robbins Jul 23 '11 at 18:27
Edited and hopefully clarified. – XOPs Jul 23 '11 at 18:44
There is a recent paper (Abbot & Switzer 2011) which presents an estimate that the liquid water oceans can exist below glaciers of order 1 to 100 km thick for such a planet. – EHN Jul 24 '11 at 18:00

It depends on several factors.

  • (a) Age of the planet (which determines how much heat it gets through radioactive decay)
  • (b) Size of the planet (more massive planets can generate significant amounts of heat through radioactive decay for far longer periods of time)
  • (c) Availability of nearby nutrients that can be decomposed to release energy for the organism's metabolism. The further down you go, the more massive the elements are (and also, the more homogeneous the elements are too). Homogeneity is not necessarily a good thing for life, as organisms can often "use" energy by acting on various chemical/pressure/temperature gradients.
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