How different could life be under a red dwarf sun?

I’ve been mulling over this paper for a few days now. Last week, NASA JPL put out a press release about cool stars having a different mix of life forming chemicals to sun-like stars. The release was immediately picked up by news sites and bloggers alike. With good reason too — the findings could have a lot of implications for future astrobiology searches. With my interest piqued, I thought I’d get hold of a copy of the paper and find out more…

The furore centres around a couple of familiar molecules — acetylene (C2H2) and Hydrogen Cyanide (HCN). These are both well known in astrophysical environments, and both widely believed to be the building blocks of larger, more complex, molecules. In particular, HCN is known to be able to form adenine (one of the four nucleobases that make up DNA). The whole process was actually the subject of the first paper I ever reviewed here on Supernova Condensate. (Ah, nostalgia).

Red dwarfs have previously been considered as possible crucibles for life and, despite a few difficulties, there are quite a few reasons to believe they may host habitable planets. But this work by Pascucci et al gave an interesting result. Cool stars (i.e. red dwarfs and brown dwarfs) seem to be deficient in HCN. By inference, no HCN means no adenine. Some people, it seems, have even considered this a death knell for astrobiology research around red dwarf stars. Personally, I’m not convinced such a drastic conclusion is entirely warranted…

So let’s take a step back for a moment. The objective behind this work was essentially to investigate the differences between sun-like stars and cool stars. The stars in question are all young stars, nestled in star forming regions. Low mass star forming regions without any troublesome O or B-type massive stars nearby. Massive stars can affect other nearby stars through their prodigous stellar winds and ultraviolet output. So this study considers the intrinsic properties of sun-like and cool stars, without any external influence. The two types of star are actually quite radically different.

As anyone who’s ever spent too long sunbathing will know, sun-like stars still have a considerable UV output. Cool stars, however, don’t give out much UV. They also have weaker stellar winds, so that their planet forming disks last much longer. Chemically, there are a few differences as a result. Silicate dust around cool stars forms larger, more crystalline grains. Silicates aside though, the most interesting result, is in what molecules the respectve stars show. C2H2 seems to be prevalent around cool stars, being found much more frequently around the tiny dwarfs than it is around sun-like stars. On the other hand, red dwarfs mostly show no HCN at all. Curious.

So what exactly does all of this mean? Well, it almost certainly has something to do with the UV output of the stars. If, as Pascucci et al believe, HCN is formed when interstellar N2 molecules get broken apart by UV, then it could well be possible that red dwarfs have trouble forming HCN. This would certainly prove a hindrance to any possible Earth-like biochemistry.

On the other hand, there are some possibilities the authors of this paper don’t consider. As Ehrenfreund et al noted in The Aromatic World Hypothesis paper, adenine and other large carbon-nitrogen molecules are readily broken down by UV. In the relative absence of UV, it’s not inconceivable that there’s an absence of HCN because once it’s combined into larger molecules, those molecules are more persistent. Nitrogen around these cool stars may already be present in larger molecules. Or, like in the atmospheres of red giants, it might combine into longer chains such as HC3N and other nitriles. Until these possibilities are considered, I don’t see any reason to write off the idea of life forming around cool stars just yet. Though ones thing’s a definite — as this paper states, “If exogenous HCN has played a key role in the synthesis of prebiotic molecules on Earth as proposed, then prebiotic chemistry may unfold differently on planets around cool stars.”

In any case, it’s noteworthy that the work in this paper actually presents the first ever detection of organic molecules around cool, low mass stars. Certainly an achievement to be applauded! I hope they investigate these stars further. It would be interesting to see what else they might find!

That fantastic artist’s impression of a red dwarf planet, incidentally, has been featured on apod and was created by the lovely Inga Nielsen. I’d recommend anyone to go and visit her gallery. It’s simply breathtaking! I. Pascucci, D. Apai, K. Luhman, Th. Hemming, J. Bouwman, M. R. Meyer, F. Lahuis, A. Natta (2009). The Different Evolution of Gas and Dust in Disks around Sun-like and Cool Stars Astrophysical Journal (submitted), arXiv:0810.2552v2

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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10 Responses to How different could life be under a red dwarf sun?

  1. invaderxan says:

    In that case, art is probably an excellent outlet for you… Lots of artists have been inspired by science and maths (including Salvador Dali!).
    I say go for it! Painting is fun. :)

  2. That picture is amazing. I have actually been thinking about art as a way to channel my scientific creativity! I want to have it in my life without actually becoming a scientist. I used to paint. Maybe I will get back into it. :O

  3. invaderxan says:

    Admittedly, they are rather dangerous for exactly that reason. That’s still a chink in the armour of those theories… How life might adapt (if, indeed, it can) is still not entirely clear. Mind you, we still don’t entirely understand red dwarfs. It seems some are rather quieter than others.
    And adenine… Well, it’s essential in all terrestrial biochemistry, though whether any other molecule could substitute for it has yet to be seen. While those amino acids I was writing about before seem to be common in the universe, there’s no indication that DNA or RNA are universal. We just don’t know — life elsewhere may even utilise an entirely different polymer!

  4. invaderxan says:

    Hmmmm… If you have access to it, you may find this journal article interesting. Dirk Chulze-Makuch is probably one of the world experts on such matters…
    The prospect of alien life in exotic forms on other worlds
    Schulze-Makuch, Dirk; Irwin, Louis N.
    Naturwissenschaften, 2006, Volume 93, Issue 4, pp.155-172

  5. 6_bleen_7 says:

    Very interesting! I was of the impression that red dwarfs were rather dangerous to budding life on account of their variability.
    Is adenine that crucial a molecule in itself, or is it a kind of bellwether for nucleic acid biochemistry? I’m pretty sure that adenine is not a prerequisite for RNA-like (or DNA-like) polymers, as long as there exist “bases” of the right size and hydrogen-bonding properties to form Watson-Crick base pairs or the equivalent.

  6. Anonymous says:

    Hmm… I didn’t know about the fact that silicon structures could break up with water. Personally, I’ve been curious about different solvents for alien microbes:

  7. invaderxan says:

    Very true. The authors themselves note in places that their measurements weren’t good enough to determine certain things (such as the temperature of the excited gas, for instance). The case is, IMHO, by no means closed.
    Though I must say, chemically speaking, I’m not sure silicon’s really suitable as a major player in life — though no doubt life somewhere might use it in ways we aren’t anticipating. Actually, there are a couple of outside bets I’d put my money on first, such as sulfur. But that’s a post for another day. :)

  8. Anonymous says:

    It seems like a small group of scientists took a quick glance and made a conclusion. I would rather wait until we have enough powerful instruments that can make more detailed and accurate measurements. Plus, let’s not forget that silicon can actually be a base for some living things given the right solvents and amino acids. It would be fairly static and unexciting life, but life nonetheless…

  9. invaderxan says:

    I read that somewhere… For some, she uses a program called Terragen, and then finishes them with Photoshop. They impressed me, anyway!

  10. nimblenimbus says:

    Wow, thanks for the link to the gallery. That picture is amazing and so is everything else. I wonder what medium(s) the artist uses. The use of color and everything is great.

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