A world between fire and ice

This really is an exciting time for planet hunters. A few weeks ago, the discovery of a system with as many as 7 planets was announced around HD 10180. Now, it’s been announced that a planet around another star, Gliese 581,* is perfectly located in the star’s habitable zone. The audacious claim that this planet, Gliese 581g, is certain to host life has even been made!

Now, I wouldn’t go so far as to say that this planet is certain to host life. It seems remarkably premature to say such a thing, if I’m honest. All the same, the prospects here are really quite exciting. Gliese 581g is expected to have a mass of around 3.1 – 4.3 Earth masses. Assuming a similar composition to Earth, it would have a radius between 1.2 and 1.5 times that of Earth. A little bigger than Earth, then, with gravity enough to make a human a bit out of breath after climbing the stairs. There is one thing, though, which makes this brave new world very un-earthlike indeed.

Gliese 581, you see, is a red dwarf. Red dwarfs are still believed to be capable of supporting life, although others believe that a different chemistry around red dwarfs may render them incapable of evolving life in the first place. Perhaps the most unusual thing about Gliese 581, however, is the fact that it’s tidally locked. Gliese 581 is only a red dwarf, putting out a relatively feeble 0.2% of the light and heat that the Sun produces. The habitable zone around Gliese 581, then, is much closer than the zone Earth sits in around the Sun. So close to it’s parent star (only around 0.15 AU away), the planet Gliese 581g experiences gravitational forces so different on its nearside and its farside, that it simply cannot rotate. Just like the Moon orbiting Earth, this planet has once side always facing away from its parent. This also means that the dayside of this planet will be scorchingly hot, while the night side will be perpetually frozen solid. Let’s face it, the idea of a planet which is frozen solid on one side, but with boiling oceans on the other side does sound like it may be a problem for life to exist on it.** Mind you, through a combination of geothermal energy and a nice dense atmosphere, that may not be as much of a problem as it sounds.

Though the jury is still out on whether it’s even possible, the idea of life on such troublesome tidally locked worlds like these has been explored before in some detail. For instance, in 2005, National Geographic aired a short two episode series called Alien Worlds. The series was scientific in some ways, and fanciful in others, but proved overall to be quite insightful. Actually, I was rather disappointed that they never made any more episodes.

One of the possible planets they explored in the series was a tidally locked planet around a red dwarf star, dubbed “Aurelia.” With a frozen darkside and a perpetual storm on the dayside, Aurelia seems like a fairly good idea for what Gliese 581g could potentially be like. If the planet is capable of supporting life, it would be tied to the region near where the terminator — the region where day meets night. Could life adapt to such an unusual environment? In all honesty, it’s not as if life hasn’t adapted to more unusual environments here on Earth.

On a final note, Gliese 581 is likely to be a little too massive to be plagued by the intense stellar flares that many red dwarfs produce. An absence of such flares would be a major plus to any potential of life existing there. For habitability, red dwarfs have a tendancy to be their own worst enemies. Life, however, has shown on Earth that once it has a foothold, it can be remarkably tenacious and adaptable. Perhaps the real question isn’t whether life could survive under a red dwarf sun. Perhaps the real question should be whether life could evolve there in the first place!

*Some people may remember that Gliese 581c a planet that a Ukrainian radio telescope transmitted a message to a couple of years ago. A message from users of Bebo on the internet!

**Well ok, maybe not boiling. But that temperature gradient’s got to be nasty, right?

Images:
Top — Orbital paths of planets around Gliese 581 – Wikimedia Commons User:Icalanise
Bottom — “Aurelia” – Alien Worlds, National Geographic

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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8 Responses to A world between fire and ice

  1. davidnm says:

    Re metallicity, it’s the standard red dwarf issue. The wikipedia value glibly says +/- 0.1 which is, umm, rather more accurately then you can measure it there. Too many complex molecular lines, basically, and they’re all nice and hot and excited, so M dwarfs are a bit of a mess, really. Basically if they’d said something like +/- (say) 0.3 or 0.4, I’d have believed them.
    (On one of the T dwarf papers I’m a coauthor on, we got pulled up on this – we naivley applied a colour-absolute magnitude relation to an M dwarf companion to try and get a metallicity. It was a nice idea, but various people pointed out that ugly facts defeated it!)

  2. invaderxan says:

    Good old Larry Niven! I should’ve known that he’d have written something on the subject — if anyone would, it would be him! Thanks a lot — I’m going to have to look for that one! :D

  3. invaderxan says:

    Oh, the achievements in exoplanet science are amazing, aren’t they? It’s been rather nice being able to watch the entire field blossom since the first planets were discovered (when I was back in school!).
    And I’m not sure, I kinda get the impression that the 100% certainty thing may have been an offbeat comment he made, which news reporters pounced on and overemphasised. At least I hope he wasn’t gauche enough to state that as his central point! Whatever the intent, it seems to act as a reminder to be careful of your wording when giving information to the press…

  4. invaderxan says:

    I’ll admit that I don’t know an awful lot about red dwarfs, but it seems intuitive to believe that older stars have fewer flares. Given their rotation would slow with time and they’d undergo less internal mixing to twist up those magnetic field lines.
    As concerns metallicity, I guess I’d have to spend a while reading how they inferred their Z value before I believe or disbelieve it… But it’s certainly true that I wouldn’t expect so many planets around a metal-poor star. And you make an interesting point about the effects on any biochemistry… Hmmm… Thought provoking! :)

  5. 6_bleen_7 says:

    Very interesting stuff! I recommend Larry Niven’s story (novelette, I think) “Flare Time”, about a tidally locked, life-bearing world orbiting a flare star.

  6. While I think it’s a pretty nifty thing that we’ve found that Gliese 581 has a reasonable planet in the habitable zone. It shows just how much progress has been made by that field– it was laughable but 10-15 short years ago!
    I am very concerned by this astronomer’s liberal use of “certainty” about hosting life. It’s a little gauche to me. Sure, I’d be super excited if I found something like this. And sure, there’s a good chance that life could survive even on a tidally locked planet. But to say 100% certain? Comeon, isn’t this sending mixed messages to citizens about the nature of science????
    Sigh. Ahh well. Cool discovered, albeit a bit marred in my mind with this whole “100% certain” thing. Ugh.

  7. davidnm says:

    As a further thought, of course, I suppose it also hinges on how much you want to read into the metallicity, given that the primary’s an M dwarf and they’re notoriously hard to measure :)

  8. davidnm says:

    Re flares, I think (if – if! – I’m remembering correctly) they’re more generally associated with younger M dwarfs rather than older ones, and the age estimates I’ve seen for Gl 581 are 7-11 Gyr, which would presumably count in its favour as well.
    The other thing that caught my eye is that apparently the star has a subsolar metallicity (m/H ~ -0.3), which is interesting given the exoplanet-metallicity correlation. And I do wonder what low Z is going to mean for the availability of chemical elements in the protostellar disc, and thus the planets it’s formed? And that’s going to have an effect on anything that evolves there, as a subsequent issue. (If there’s not much potassium, for instance, can a putative Gliesian have an ion-exchange nervous system quite like we do?)
    Although that said, -0.3 is hardly stunningly low Z. Gl 581 isn’t a subdwarf ;)
    But it’s fascinating to watch all of this. And interesting that this star in particular keeps refusing to go away!

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