Centauri Dreams has often discussed red dwarf stars and the question of habitability. This time let’s focus in on a nearby candidate called AU Mic, an M-class dwarf some 10 parsecs (roughly 32 light years) from Earth. At twelve million years old, this is one of the nearest young dwarfs, and it’s known to possess a dusty debris disk. In fact, what we see around AU Mic looks like the late stages of planet formation, with planet-sized objects disturbing smaller disk materials and creating something like our own Kuiper Belt.
The lesson of AU Mic seems clear: planet formation around M-class dwarfs is probably common, even though we see few debris disks around older stars of this class. That may simply be the result of the sensitivity of our search technologies, and in any case the smallest exoplanets we’ve yet found orbiting main sequence stars orbit red dwarfs (consider the rocky world around Gliese 876).
Let’s be clear on this: we’re likely to get solid confirmation of smaller, terrestrial-sized worlds using the transit method within a fairly short time, conceivably even before the 2008 launch of the Kepler mission. There is every reason to suspect they are common.
In that light (dim and red though it be), red dwarfs are very much on the table for future terrestrial planet finder missions as well as SETI searches. That was the conclusion of a 2005 interdisciplinary workshop that reappraised M-stars in terms of habitability. The session was organized by the SETI Institute and sponsored by NASA. And the newly available paper on the proceedings contains heartening news for the astrobiology community, for the increasing focus on these stars greatly expands the range of possible life-bearing environments. By some measures, M-class stars comprise fully 75 percent of all stars (not counting brown dwarfs), and half of the stellar mass in the Milky Way.
Detecting terrestrial planets around such stars is obviously a challenge, but even more so is the analysis of such a planet once found. We know that worlds in the tight orbits required (.1 to .35 AU for the larger M stars and even closer for the smaller) will become tidally locked, yet numerous recent studies have shown that habitable environments are possible. Seen from Earth, an active atmosphere should show an interesting infrared signature. From the paper reporting on the workshop’s findings:
On a dry world, or a planet whose atmosphere has collapsed, IR emission mostly comes from the surface, so that the dark side will appear far colder than the dayside when viewed at these frequencies. However, with a hydrological cycle comes the presence of water vapor and clouds – the SP will have clouds present for most of the time, and so IR emission here will come mostly from the cold cloud tops. Indeed, the dayside of such a planet might actually appear as cold in the IR as the darkside, in the same way as the Western Pacific region, which has the hottest sea surface temperature, appears very cold when seen in the IR. Such observations might actually provide evidence for a planet with an active hydrological cycle.
‘SP’ in the passage above refers to ‘substellar point,’ the location where the planet’s star is directly overhead. Here again we run into interesting variables. You would think the terminator — the boundary between the day and night sides of the planet — would be the most likely place for life because red dwarfs are frequently prone to dangerous flare activity. But you also get less sunlight there, whereas thick clouds could change the effects of solar radiation everywhere on the day side. So far there are no ‘showstoppers.’ It continues to appear that if liquid water and the needed constituents for life can be found in this environment, M-stars should become high value astrobiological targets.
The paper is Tarter et al., “A Re-appraisal of the Habitability of Planets Around M Dwarf Stars,” accepted for publication in Astrobiology, with preprint available here. It’s a long study that belongs on the shelf or disk of anyone following planet habitability and/or SETI studies, and a second workshop slated for 2007 should refine its conclusions.
Hi Paul
Saw that one. It’s a decent summary of the current discussion. The “methane runaway” mentioned is a bit worrying though. I wonder just how much accumulates and how rapidly. A methanogenic biosphere might choke any hope of an oxic biosphere arising because the excess methane will continually scavenge the oxygen released. Ammonia will accumulated also, though that might not be so incompatible with oxygen breathing life – there are cave bats that breathe 6% ammonia quite happily. It’s from all their guano.
Adam
Eeww!
Hi Eric
David Attenborough had to climb a huge mound of the stuff in one documentary – even worse cause it’s covered in cave bugs like giant centipedes.
Adam
Double eeww!
Astrophysics, abstract
astro-ph/0610179
From: John Johnson [view email]
Date: Fri, 6 Oct 2006 01:15:04 GMT (68kb)
A Long-Period Jupiter-Mass Planet Orbiting the Nearby M Dwarf GJ849
Authors: R. Paul Butler, John A. Johnson, Geoffrey W. Marcy, Jason T. Wright, Steven S. Vogt, Debra A. Fischer
Comments: 12 pages, 3 figues, 2 tables, PASP Accepted
We report precise Doppler measurements of GJ849 (M3.5V) that reveal the presence of a planet with a minimum mass of 0.82 Mjup in a 5.16 year orbit. At a = 2.35 AU, GJ849b is the first planet discovered around an M dwarf to orbit beyond 0.21 AU, and is only the second Jupiter mass planet discovered around a star less massive than 0.5 Msun.
This detection brings to 4 the number of M stars known to harbor planets. Based on the results of our survey of 1300 FGKM main–sequence stars we find that giant planets within 2.5 AU are ~3 times more common around GK stars than around M stars. Due to the GJ849’s proximity of 8.8 pc, the planet’s angular separation is 0.”27, making this system a prime target for high–resolution imaging using adaptive optics and future space–borne missions such as the Space Interferometry Mission. We also find evidence of a linear trend in the velocity time series, which may be indicative of an additional planetary companion.
http://arxiv.org/abs/astro-ph/0610179
É possível viajar-se entre as estrelas, sim. Ou melhor: será possível se a humanidade conseguir manter seu desnvolvimento tecnológico, o que é pouco provável, já que nosso desenvolvimento é medido, principalmente, pelo poderio bélico de cada nação ou grupo enquanto os líderes destes mesmos grupos carregam patuás, crucifixos,…coisas do gênero, e guerreiam entre si em nome de deuses hipotéticos, pondo em risco toda a humanidade a uma volta à estaca zero, como já deve ter acontecido muitas vezes durante todos estes milhões de anos hà que a humanidade vem existindo. É, há muita coisa para ser contada, que não o foi, sòmente pelo simples fato de que nada restou para ser contado: imagine quanto tempo uma espécie regredida à condição de coletores nômades, em um planêta hostil que tenta se regenerar, levará para conseguir reunir-se em grupos, aldeias, desenvolver idiomas comuns para os mais próximos e chegar a alcançar o nível de tecnologia que temos agora. Vinte mil anos? cinqüenta? cem? É difícil dizer, pois, há seis milênios atrás não dispunhamos nem de uma escrita decente para contar nossa história e tudo o que nos chega hoje não passa de lendas sem fundamento, salvo o fato que nosso história é contada por assassinos, grandes conquistadores…heróis, assim como por lendas sobre deuses por nós criados à nossa imagem e semelhança em benefício de seus criadores…chega! É o seguinte: a velocidade de escape para sair-se do sistema solar(como um todo)deve ser equivalente à que um neutrino ou um fóton desenvolve para escapar de um àtomo, proporcionalmente, e tem que ser alcançada ainda dentro do sistema. Muito mais preocupante é o material de revestimento da nave, pois à esta velocidade, dentro do sistema, haverá super aquecimento. Uma vez lá fora, será o inverso. Quanto a desenvolver tal velocidade, meios é que não faltam; apenas não desenvolvemos ainda tal tecnologia, mas, teorias é que não faltam; e a aceleração é plenamente suportável: Quanto tempo você acha necessário para atingir-se tal velocidade em aceleração constante de 1G na imponderabilidade do espaço sideral?
Gostaria de discutir sobre o assunto: caso não fosse abuso de minha parte, pediria que a matéria viesse traduzida para o português, ou de forma a poder ser traduzida. Obrigado, David-Nefas.
A note to David Quirino dos Santos (I’m translating using Babelfish) — David asked if we could make some of the content here available in Portuguese. I’m not able to do it, but if there are any Portuguese/English linguists out there who want to attempt it, by all means give it a go. To David:
Eu sou pesaroso, David, mim desejo que eu poderia traduzir o trabalho aqui no português, mas mim estou receoso que eu não tenho nenhum conhecimento trabalhando da língua. Eu estou usando Babelfish traduzir este (http://babelfish.altavista.com/tr). Eu ajudaria se eu poderia! David, você pode querer tentar Babelfish você mesmo ver se for de alguma ajuda.
Astrophysics, abstract
astro-ph/0703576
From: Jack Lissauer [view email]
Date: Thu, 22 Mar 2007 00:19:15 GMT (63kb)
Planets Formed in Habitable Zones of M Dwarf Stars Probably are Deficient in Volatiles
Authors: Jack J. Lissauer
Comments: 11 pages, 1 figure, Astrophysical Journal Letters, in press
Dynamical considerations, presented herein via analytic scalings and numerical experiments, imply that Earth-mass planets accreting in regions that become habitable zones of M dwarf stars form within several million years. Temperatures in these regions during planetary accretion are higher than those encountered by the material that formed the Earth. Collision velocities during and after the prime accretionary epoch are larger than for Earth.
These factors suggest that planets orbiting low mass main sequence stars are likely to be either too distant (and thus too cold) for carbon/water based life on their surfaces or have abundances of the required volatiles that are substantially less than on Earth.
http://arxiv.org/abs/astro-ph/0703576