I'm sorry I am not quite sure this question satisfies this point:

Questions that are purely hypothetical, for example a question such as 'Could a black hole destroy the universe' or 'What if our solar system had two suns' (however, feel free to visit World Building)

But actually I am not asking hypothetically. Given the timespan it took to develop life on earth, I wonder if we can project this to life having already developed once or going to develop again in the solar system. I'm referring to the fact that during suns lifespan the habitable zone shifts towards other areas. This won't last only for a few centuries, right? But for millions to billions of years. Some of the planets and other celestial bodies contain the basic elements to host life (as we know it) already. And then there's stuff floating around in the belts that could bombard those bodies.

E.g. this article claims a study found:

Researchers studying the crater said investigations by the Mars Science Laboratory have confirmed the presence of an ancient lake that existed in the Gale crater for up to 10 million years, indicating that parts of Mars may once have been habitable for thousands of years.


I'm not sure how to go from 10 million years to thousands of years. Thousands of years might not have been enough for substantial life to develop, but millions of years sounds more possible.

Just for reference this video got me to the question:

(of course it's just a fun video)

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    $\begingroup$ This is absolutely "nobody knows" type question. We only have a single example of life developing, it seemed to do so pretty quickly after liquid water appeared on Earth. But with only a sample of 1, we can't do much statistical analysis. $\endgroup$
    – James K
    Jul 22, 2021 at 6:51
  • $\begingroup$ Hmm, I thought more in the sense of: - from since habitability was reached to development of first life - take that span and check if some body will be in the habitable zone for the same amount of time (or longer) - also consider if some body already has all elements vs. needs outside source I mean of course no one will know if it will actually develop, aka if just being in the habitable zone is enough. $\endgroup$
    – steros
    Jul 22, 2021 at 7:01
  • $\begingroup$ See en.wikipedia.org/wiki/Fermi_paradox and en.wikipedia.org/wiki/Drake_equation. They apply to the full Galaxy rather than just the Solar System, but the underlying questions are very similar, especially the difficulty in the estimates and the very very wide range of both inputs and results. See also en.wikipedia.org/wiki/Rare_Earth_hypothesis. $\endgroup$
    – jcaron
    Jul 22, 2021 at 22:41
  • $\begingroup$ @jcaron Thanks for the comment, but actually I was thinking a different direction. ProfRob updated his answer and now it's much close to what I had in mind. $\endgroup$
    – steros
    Jul 22, 2021 at 23:50

2 Answers 2


The answer is, it could be non-zero (some would argue it must be non-zero), but since we don't know what the probability of life emerging on Earth was, it is impossible to quantify.

This is why this question is normally turned around - if we find life elsewhere in the Solar System (and it is independently developed), then what is the probability that life can emerge given reasonable circumstances?

The occurrence of a second instance would allow us to put a number on the probability. That number would be high enough that, since the Sun is a fairly ordinary star and that Solar System architectures may also be reasonably frequent, life ought to be common throughout the Galaxy.

There won't be any drastic changes in the habitable zone location for the next few billion years - it slowly moves outwards as the Sun's luminosity increases. Perhaps Mars will warm up a bit and become habitable again? If so, there is certainly the time available (still a couple of billion years before the Sun becomes a red giant and Mars gets much hotter) for primitive lifeforms to develop, given that life on Earth seems to have come into existence very soon (less than a few hundred million years) after the conditions became suitable.

The asteroid belt and outer planets? Much less likely. The habitability or not of the Jovian and Saturnian moons is unlikely to be affected by gradual changes in the solar luminosity during the main sequence and the habitable zone remains well inside the orbit of Jupiter.

Once the Sun does become a red giant, then the habitable zone does move outwards maybe as far the Neptunian moons and Kuiper belt (Stern 2003). There are plenty of icy bodies with organic materials that might have the raw materials to form life. The difficulty here might be the rapidly changing luminosity of the Sun as it approaches the end of it's life. It will not be stable on timescales of billions of years like it is as a main sequence star. Other authors disagree on the exact location of the habitable zone during this phase, probably because of its exact definition and because of difficulties in modelling the final stages of the Sun (see below). Below is a plot from Ramirez & Kaltenegger (2016) showing how the habitable zone (between the solid lines) changes rapidly during the final billion years of the Sun's life. There is a period of about 100 million years when the habitable zone is between 5 and 15 au (i.e. the Jovian moons), and an even briefer period before that when where there could be habitability out to 30 au.

Ramirez and Kaltenegger 2016

But again, given we don't know exactly how quickly life can develop, although there is an opportunity for life, we can't quantify what the probability is.

Note that all of of the above is uncertain in three ways. First, not everyone agrees on what defines the habitable zone and what temperature range it should encompass. Second, we don't know exactly how the Sun will behave when it becomes a red giant because much depends on the mass-loss rate, for which there is no fundamental theory at present. Third, the behaviour of the atmospheres and albedos of the planetary bodies or moons, which can change, will play an important role. For example, Mars is actually in the habitable zone now, according to simplistic calculations that just use the solar luminosity, and will be for many billions of years. It could just be suitable for life as we find it on Earth (antarctic microbes). On the other hand, if we demand a temperature range of 270K-300K, then Mars may only be habitable for a very brief period in the future (11.6-11.7 billion years from the birth of the Sun, according to Schroder et al. 2001) and this may be insufficient time for life to develop.

  • $\begingroup$ Thanks for your answer. Maybe probability is the wrong term? I don't want to extrapolate any chances from one occurrence (on earth). That would be a bit difficult. I simply want to take the time span and maybe a few factors like temperature into account. So Mars might have been habitable before. Thus it might become habitable again, e.g. ice would melt. But would it be long enough to at least match earths timeframe? $\endgroup$
    – steros
    Jul 22, 2021 at 7:20
  • $\begingroup$ @steros well your comment illustrates the difficulties. The Sun was less luminous in the past, so Mars was outside the habitable zone. However, Mars was much more habitable in the past because it had liquid water, an atmosphere and was warmer. Anyway, I think I have added some detail along the lines you suggest. $\endgroup$
    – ProfRob
    Jul 22, 2021 at 7:23
  • $\begingroup$ Okay I see thank you. Maybe it's a bit too "unscientific" for this stack and should move to worldbuilding 🤔 $\endgroup$
    – steros
    Jul 22, 2021 at 7:24
  • $\begingroup$ @steros Part of it is astrophysics, part of it is planetary science, since the influence of an atmosphere on whether a planet is habitable can be enormous. I've given you the astrophysical bit. $\endgroup$
    – ProfRob
    Jul 22, 2021 at 7:47
  • $\begingroup$ @steros, I think that your question is: can life be extinct then NATURALLY restart again when conditions change? Because, there are people believing that hunanity can make Mars habitable (i.e. Terraforming Mars). This is not part of your question, right? $\endgroup$
    – Ng Ph
    Jul 22, 2021 at 8:28

There's nothing wrong with asking a question to which the answer is "nobody knows" -- so long as you are willing to accept that the answer is essentially "nobody knows".

The only thing we do know is that the probability you are asking about is non-zero because life does exist on the Earth. Some argue that the probability you are asking about is very close to zero. This is the Rare Earth hypothesis -- the Earth happened to pick the one out of a trillion life lotto ticket.

Others argue that the probability you are asking about is very close to one. This is the basis of the mediocrity principle which implies that life is common; that there's nothing special about the Earth. Astronomers have detected Earth-like exoplanets orbiting in the habitable zone of other stars.

That completely fuzzy range between nearly zero and nearly one is just life as we know it. This falls into the "known unknown" category. What about life as we don't know it? That's an unknown unknown.

  • $\begingroup$ Non only the 1/trillion lucky lotto ticket (to harbor life). To harbor it long enough so that A form of life starts to ask questions, of which "who organized this lotto?". That's a 1/zillion chance. $\endgroup$
    – Ng Ph
    Jul 22, 2021 at 18:56
  • $\begingroup$ @NgPh Strictly speaking, even if we assume the rare Earth hypothesis to be true, that does not tell us anything about the probability of us happening to develop the required level of intelligence to ask questions. For all we know, both the rare Earth hypothesis and the strong anthropic principle are true, we quite simply have insufficient evidence to render a conclusion on either point. $\endgroup$ Jul 22, 2021 at 21:07
  • $\begingroup$ Thanks for your answer but I understand my question must be misleading. I am not asking about this kind of aspect. I think it has been discussed a lot and there is a lot material available (which you linked already). $\endgroup$
    – steros
    Jul 22, 2021 at 23:44

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