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Recently, a planet around Proxima Centauri has been found. It is in the habitable zone, in the sense that if the planet was a black body, it would have a surface temperature of −39° Celsius. The planet is very close to its star, which has a number of implications:

  • Radiation is very strong (additionally, Proxima Centauri is a flare star), and probably has stripped the planet of its atmosphere.
  • The planet is probably in a tidal lock, with one side facing the star constantly and the other pointing into cold space.

I'm therefore assuming that the planet has no atmosphere, but can support liquid water somewhere on the dawn area, where the star shines at a steep angle.

What life forms with the greatest similarity to life on earth are possible on this planet? In what points do they have to differ from earth life?

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    $\begingroup$ Welcome to the site, Turion. There are a few issues with what you're asking here. First, there are three markedly different questions in this post. Second, the latter two are infinitely broad and/or opinion-based (the first one is trivial: yes, but maybe not Earth-like). $\endgroup$
    – Frostfyre
    Commented Aug 25, 2016 at 20:35
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    $\begingroup$ @Frostfyre, thanks for the feedback, I have attempted to make the question more concrete. I'd still welcome broad answers. $\endgroup$
    – Turion
    Commented Aug 25, 2016 at 20:38
  • $\begingroup$ Liquid water and no atmosphere seems unlikely. Consider asking on astronomy, but under low pressure water evaporates awfully easy. $\endgroup$
    – Mołot
    Commented Aug 25, 2016 at 20:46
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    $\begingroup$ No. Liquid water in a vacuum is a contradiction. $\endgroup$
    – geometrian
    Commented Aug 26, 2016 at 6:16
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    $\begingroup$ There are several worlds in our solar system where oceans exist and have no atmosphere or barely any, for example, Europa and Ganymede. It is neither unlikely nor a contradiction. Subsurface oceans are more common than open oceans in the solar system. $\endgroup$
    – a4android
    Commented Aug 26, 2016 at 11:59

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There's really no such thing as a planet or moon with no atmosphere. But as it so happens, the object in our solar system most often pointed out to possibly harbour life is Jupiter's moon Europa, atmos$=0.1\mu Pa$, which probably has oceans under a thick layer of ice.

There are creatures in Earth's ocean which live without any interaction with the sun, so if Europa has a molten core like Earth, this could provide jets of lava for light and energy.

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    $\begingroup$ Just to elaborate one point of Feyre's well-put answer: Even without a molten core, Europa could still have oceans. If the tidal forces from Jupiter turn out not to be strong enough to render the core molten as it does on, for example, Io (which happens to be closer to Jupiter), they still very likely generate enough warmth to keep water in a liquid state. $\endgroup$
    – Dan
    Commented Aug 25, 2016 at 23:53
  • $\begingroup$ @Dan but how do you power life in that case? Do the creatures float to the bottom and wait until the day/night cycle squishes them enough? While this would yield some really interesting solutions in theory, I don't think this is very feasible in reality. $\endgroup$
    – John Dvorak
    Commented Aug 26, 2016 at 7:20
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    $\begingroup$ @SteveJessop Even then, there's bound to be more than one way for abiogenesis. The point is you likely need some kind of natural energy and light producing reaction going on anyway for abiogenesis. If life can form, life can survive. $\endgroup$
    – Feyre
    Commented Aug 26, 2016 at 10:43
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    $\begingroup$ "There are creatures in Earth's ocean which live without any interaction with the sun" - True, however the more interesting question is did the evolutionary precursors to those creatures also live without sunlight? If it can be (or has been?) proven that life can exist on Earth without ever having needed to rely upon photosynthesis at any point, that would lend vast amounts of plausibility to the idea that you can have life without habitable surface conditions. $\endgroup$
    – aroth
    Commented Aug 26, 2016 at 12:54
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    $\begingroup$ @JanDvorak Even if the rock is just relatively "hot," it might be enough for the interactions between the rock and the water to provide nutrients necessary for life. This happens on Earth, as Steve Jessop mentions. There can (I believe) be hydrothermal-vent-like features just from the tidal forces without the presence of a molten core. $\endgroup$
    – Dan
    Commented Aug 26, 2016 at 23:11
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The answer to the top question is a likely yes...as we are starting to discover, life seems to exist in a far greater variety of conditions than we could have ever imagined capable of harvesting energy in a large variety of ways.

First, there is no surface liquid water on this planet...without an atmosphere, it would "boil" away exceedingly quickly. Which means all life on this planet is relegated to underground / under ice conditions.

Answer to the remaining questions is purely speculative...but my answer would be kinda. In the far depths of the oceans on Earth exist some creatures that we would be hard stretched to even call earth like. For the most case, what we consider 'Earth like' all finds it's energy roots from the sun (either directly from the sun as in plants, or animals that consume the plants or each other). However in the vast depths, sunlight is no longer available and life relies on some creative techniques to get it's constant supply of energy.

If you are willing to extend the title of 'Earth like' to some of the alien looking creatures at the bottoms of our oceans, then yes...the life on this planet could be 'earth like'. However it's far more likely that this life will be completely alien to us in quite a few manners.

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    $\begingroup$ If the water did "boil" away, would the water vapor become the atmosphere, if the gravity wasn't enough to contain it? $\endgroup$
    – Ranger
    Commented Aug 25, 2016 at 21:52
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    $\begingroup$ @NexTerren - Normally yes, however the question stipulates a large amount of radiation that would continually strip it of whatever atmosphere it gains. If the planet is tidal locked with the star, then it's feasible the dark side of the planet has a bit of an atmosphere from this, but it suffers from the same 'no sun, ever' setup the underground conditions would create. $\endgroup$
    – Twelfth
    Commented Aug 25, 2016 at 21:56
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The primary reference states

Tidal locking does not preclude a stable atmosphere via global atmospheric circulation and heat redistribution [21]

as part of a section devoted to possible habitibility. So I suggest you go beyond the “popular press” digested wimpy version and read the original paper.

Indeed, it answers your other bullet as well:

Because of its close orbit to Proxima, Proxima b suffers from X-ray fluxes that are approximately 400 times that experienced by Earth, but studies of similar systems indicate that atmospheric losses can be relatively small [25]

You can follow up on the references cited in this habitibility discussion for details that you can use in your world-building case.

I’d also like to point out that a tight orbit does not imply tidal locking as is commonly assumed. Look at the case of our own planet Mercury! As this lecture explains, having an odd half multiple for spin/orbit resonance is a more stable situation and now thought to be the common case. This is especially favored for an eccentric orbit, and large planets farther out will drive that eccentricity. In addition, orbital inclination will cause north-south movement of the sun in the sky.

(And the paper points out that even with tidal locking it could very well have enough of a magnetic field to tolerate the flares.)

So in your story, you can be faithfull to all known details of the planet and have an atmosphere and have sunrise/sunset cycle every 22 days, with twilight periods in the arctic and anarctic regions.

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Very interesting question, yet seems to be slightly obvious for me: oxygen, or at least, gases are much more important for sustaining life than water.

Water acts as a material within which basic biochemical reactions occur; oxygen is the fuel that ignites said reactions. Also, water stays in the body for much longer time than oxygen does.

The only proper consideration we may have is underground or aquatic life forms, with little or no needs, or different ways for acquiring oxygen, or other "fuel" substances. This way, it should work, as it's definitely not unusual for life to evolve in such environments.

I would have a pretty strong assumption that the way life appeared on Earth was not directly depending on the atmosphere. (Of course, indirectly it was, but that's another question)

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    $\begingroup$ There are life forms, anaerobic organisms that never touch oxygen (in form of O2). $\endgroup$
    – Cem Kalyoncu
    Commented Aug 26, 2016 at 8:37
  • $\begingroup$ Life on Earth did quite well for over a billion years before the Great Oxygenation Event. We (by which I mean, Earth life) made our own atmospheric oxygen somewhere between 1/3 to 1/2 of the way through our career. But sure, water-based life is probably going to need some things in the water that we think of as dissolved gases since we're also accustomed to seeing them in gaseous state in our atmosphere. $\endgroup$
    – Steve Jessop
    Commented Aug 26, 2016 at 9:03
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In a well covered ocean the cheap energy of the star might not be directly available, but other sources might be. Our kind of like has pretty well covered this planet, including the places humans are just getting capable of checking.

But there are still two qualms I have about saying we have neighbors like us:

1) Has any life spawned there at all. We are only certain of one abiotic life event, it happened pretty early on for our planet but appears to have happened only the once here.

2) If some long hydro carbons start organizing I find it unbelievable that RNA is the way they do it. It's the only way we do it here, but there are many "that's just the way it is" answers in biology that might not be our way somewhere else.

All well and good, but whether it's written in java or perl the app still runs, and I might not be able to tell the difference. I recently couldn't pick some real sea creatures out of a lineup of dreamed up CG ones, so it's quite possible something that lives in water will be at least as familiar as some of our locals.

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There are certainly niches available to life I'd look to the McMurdo Dry Valleys and the deep sea geothermal vent communities to see what organisms can and do survive in the kinds of extreme climates you'd be looking at on such a world.

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