I was looking a bit at the evolution of Stars and one thing I sometimes see floated is this idea that in the far flung future, when the sun has ballooned into a red giant and the earth burned to a crisp, the solar system's habitable zone has moved far out enough that Saturn's moon Titan has become a pretty nice place to live.

There are some books that work with this idea, having some sort of weird alien species develop on titan long after all life has been extinguished from Earth. Stephen Baxter's book on Titan is a good example.

What I want to know is whether or not a Red giant could be counted upon to provide a suitable habitable zone. Now I understand that Red giants are not long lived, so we'd probably only be looking at around 1 billion-1.5 billion years max that it would maintain itself before giving up the ghost. Can it sustain a stable Habitable zone over that time? I understand that there are stages where it will inflate and deflate in size and ones where it will blast off huge amounts of matter which I assume would send things screwy and would probably sterilize a world before anything interesting happened, but could there be a sweet spot of about a billion years of relative stability when looking at a Red giant formed from a star about sun size or smaller?

I'm imagining some sort of world in the outer solar system that may develop life up to maybe a Eukaryotic stage (being generous here) in a subsurface Ocean before its star puffs out and a habitable zone expands to where it is, allowing liquid water and atmosphere to exist on the surface. At this point might it be conceivable for photosynthesis to emerge, oxygenation of the atmosphere, Multicellular life of increasing complexity, maybe culminating in an intelligent race before the clock runs out? Is this conceivable with a red Giant, would life emerging billions of years before mean that they would be ahead of the curve of some the developments on earth, and would a red Giant possibly be able to offer stable enough conditions over a long enough period for complex life to evolve?


  • $\begingroup$ Red giant phase for the sun is expected to last about 1 billion years. en.wikipedia.org/wiki/Red_giant $\endgroup$ – Gary Walker Apr 29 '15 at 19:58
  • $\begingroup$ Would it be longer for a slightly smaller star that wasn't stuck in the red dwarf doldrums, for example Tau ceti? $\endgroup$ – Khwarezm Apr 29 '15 at 20:02
  • $\begingroup$ Actually a billion years is about the upper bound for red-giant phase duration. Tried to find an on-line that correlates this to mass without much luck. Tau Ceti is so close to our sun in mass, I would not expect a large difference, but I really don't know. $\endgroup$ – Gary Walker Apr 29 '15 at 20:22
  • $\begingroup$ It doesn't stay the same size through the red giant phase. Even if you got a billion years out of it the planet wouldn't stay in the habitable zone for those billion years. $\endgroup$ – Loren Pechtel May 1 '15 at 20:13

It's not a common statement, but a billion years isn't long enough. enter image description here
If Earth is any example --actually it's our only example-- getting from habitable to single celled takes about half of a billion years on its own. Getting from single cells to multicellular took another three billion years.

We've spent three and a half billion years, way over your sun-keeps-ghost-budget, and all we have to show are sponges and fungus.

For the case of Titan, it would be better to start with some microbial contamination on Huygens, or future Titan lander, and go from there. Then we get to skip all that tedium of the first few billion years straight to the far more interesting last billion years. Things could get interesting right after the moon becomes habitable and feasibly, though unlikely, they might get to the point where they decide not to let the Star Wars prequels be made.

  • $\begingroup$ Would the half-billion lead time for first live be relevant? It is not like you are waiting for the crust to cool or the end of the late heavy bombardment. Of course it could be that given the improbability of abiogenesis, even 10 billion years is unlikely to result in life. $\endgroup$ – Gary Walker Apr 29 '15 at 20:44
  • $\begingroup$ @GaryWalker I think a half a billion years might be a fairly liberal estimate already. Also, I don't think we can assume the moon environment would be instantly habitable immediately following the sun turning into a red giant. $\endgroup$ – Samuel Apr 29 '15 at 20:48
  • $\begingroup$ Well, In this scenario I'm assuming that life already developed (or spread to) this hypothetical world, if its somewhat like Titan or Europa or what have you it may be able to harbor life within a subsurface ocean. I'm also assuming that Eukaryotic organisms may have emerged prior to the Red Giant phase, but this may run into trouble if an oxygen catastrophe needs to occur for that to happen, which would entail a long period of filling up the atmosphere with Oxygen. $\endgroup$ – Khwarezm Apr 29 '15 at 20:57
  • $\begingroup$ @Khwarezm Fair enough. Of course, importing life that evolved for the environment which the moon will become is still probably the best idea. $\endgroup$ – Samuel Apr 29 '15 at 21:26
  • $\begingroup$ The fact that it took life on Earth three billion years to step from unicellular organisms to multicellular ones does not necessarily mean it is necessary time for evolution to achieve multicellular life. I think it is perfectly possible that on Earth, there was (for example) initially no clear advantage for such step and it happened essentially randomly. Maybe we could have had multicellular life after first billion years if we were lucky. $\endgroup$ – Irigi May 2 '15 at 11:29


To elaborate on @Samuel 's very fine answer, stars of all but the smallest constant luminosities will have inhabitable zones.

What will the Sun's Luminosity be?
Our Sun's luminosity is expected to increase up to around 5000x current levels.

Where will the new inhabitable zone be?
Since solar irradiation depends linearly upon luminosity and by the $ \frac {1}{r^2} $ of the orbit, that means the inhabitable zone of Red Giant Sun will be:

$$ r = \sqrt {5000} = 70 AU $$

and if you assume the inhabitable zone is +/- 20% of the median distance, this gives you a range of 56 - 84 AU for habitable bodies.

What sort of bodies orbit are out there?
This is beyond the orbital radius of any planet and is in fact beyond the 50 AU limit of the Kuiper Belt.

There are probably some stray planetoids out there and perhaps there is something as large as Earth but we've not detected anything like that yet.

Other issues:
As @Samuel pointed out, a billion years simply isn't enough time for evolution to develop anything interesting from scratch.

But if we looked at this as a colony world of Earth settled by post-Earth humans, we could definitely have transplanted a biosphere there. If we did that, there would be enough time for interesting things to happen from our starter culture.


The big problem (or advantage) is that red giant light spectrum is shifted to infra-red and micro-waves. It would be way less ultra-violet light. So considering the habitable zone the one that will receive around the same power/surface area, it would be more heating light than ionizing light. I would guess the life forms would have to adapt to this condition. For example current live beings wouldn't be able to produce vitamin D. But the production of ozone would also be reduced and since the ozone filter the UV light things could balance out a bit.


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