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