Really Old Stars Perhaps Ideal for Advanced Civilizations

The idea that other, less-massive, dimmer stars than the Sun could also host habitable worlds has long been debated. A particular class, M-Stars, are of interest simply because there are so many of them—they are the most common star in the galaxy. They’re the cool stars that inhabit our neighborhood.

There’s considerable interest in the question of whether M-Stars could host habitable planets. Would the planets be tidally locked with one face always directed toward the M-Star? Would flares wipe out life on the local planet? If M-Stars could host habitable planets, life may be much more widespread that we’ve previously thought. Thus, M-Stars are of interest to astrobiologists including SETI scientists who are searching for life beyond Earth.

http://bcast1.imaginova.com/t?r=2&ctl=EA50:4A48D

M stars, or red dwarfs, are by far the most common stars in any galaxy. I forget the exact figures, but they are about 10 times more common than G class stars such as our Sun.

On the other hand, they are also about 10 times dimmer, which means the "habilitability zone" for earth-like life is 10 times smaller. That means you would have only 1/10th the chance to find a planet at the just right distance. So, on average, you might expect just as many life-bearing worlds around M stars as around G stars--but you would have to search 10x as many M stars to find them. Needle in a haystack.

Some SF writers point out--I haven't checked this--that the planets of an M-class star would be so close to their primary that they would be tidally locked, with only one side facing the star. That surely would have consequences for life, possibly negative.

Still, an intriguing idea.

Cool topic! I followed your link one more step to:
http://www.seti.org/site/apps/nl/conten ... ct=1232463

A few things occurred to me as additional problems to the ones they list. In the process of solar system formation in a small environment like one that would produce a class M, I wonder if the combination of the right materials and the possibility of them condensing in the correct orbit for a decent planetary magnetic field is possible/probable.

Given that being really close to a star would likely involve a lot more high energy particles hitting the planet, flare or not I think a planet with a class M primary would need a pretty decent magnetic field. Then again - if there was enough CO2 in the atmosphere to distribute heat to the dark side, perhaps life would form only on the there, shielded by the rock. Also, depending on the atmosphere and the amount of solar wind interacting with it, light could be produced in an aurora-like way (visually - not physically.) Sounds like a cool setting for a scifi.

PS> A really advanced civilization could perhaps take a larger planet and gravitationally balance it with a gas giant or collection of objects around a class M to create a stable, non-tidally locked orbit. Could be a good set up for a long run at things once the local class G got too old.

SC

I sniffed around myself, since this is an absolutely fascinating question. As the previous posts mentioned, we have flares and tidal locking on the negative side, an essentially immortal primary on the positive.

The newer models seem to indicate that convection would solve the dark/light side dichotomy and allow retention of atmosphere and of water, plus maintain a low temperature differential. The astrophysicists additionally mention that flares subside after the star's early life. Also, if the mostly infrared emission of the primary makes chlorophyll-based photosynthesis impossible, life on such a planet could draw on geothermal energy (which should be plentiful). At any rate, photosynthesis occurs with pigments besides chlorophyll which give red or yellow foliage, the same ones that we see on earth when fall comes.

Finally, it may be easier to discover planets around M type stars rather than F/G/K. Since they are much smaller, planets would create disproportionately large wobbles in their orbit.

Here are some more links that discuss the issue from several angles:

http://www.exoplaneten.de/uvceti/english.html
http://www.centauri-dreams.org/?p=1149
http://www.centauri-dreams.org/?p=1154

“M Stars as Targets for Terrestrial Exoplanet Searches and Biosignature Detection,” appearing in the latest issue of Astrobiology:

http://www.liebertonline.com/doi/pdfplu ... ookieSet=1

Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants

Authors: John A. Johnson, Debra A. Fischer, Geoffrey W. Marcy, Jason T. Wright, Peter Driscoll, R. P. Butler, Saskia Hekker, Sabine Reffert, Steven S. Vogt

(Submitted on 19 Apr 2007 (v1), last revised 19 Apr 2007 (this version, v2))

Abstract: We report precision Doppler measurements of three intermediate-mass subgiants from Lick and Keck Observatories. All three stars show variability in their radial velocities consistent with planet-mass companions in Keplerian orbits. We find a planet with a minimum mass of 2.5 Mjup in a 351.5 day orbit around HD 192699, a planet with a minimum mass of 2.0 Mjup in a 341.1 day orbit around HD 210702, and a planet with a minimum mass of 0.61 Mjup in a 297.3 day orbit around HD 175541. Stellar mass estimates from evolutionary models indicate that all of these stars were formerly A-type dwarfs with masses ranging from 1.65 to 1.85 Msun. These three long-period planets would not have been detectable during their stars' main-sequence phases due to the large rotational velocities and stellar jitter exhibited by early-type dwarfs. There are now 9 "retired" (evolved) A-type stars (Mstar > 1.6 Msun) with known planets. All 9 planets orbit at distances a \geq 0.78 AU, which is significantly different than the semimajor axis distribution of planets around lower-mass stars. We examine the possibility that the observed lack of close-in planets is due to engulfment by their expanding host stars, but we find that this explanation is inadequate given the relatively small stellar radii of K giants (Rstar < 32 Rsun = 0.15 AU) and subgiants (Rstar < 7 Rsun = 0.03 AU). Instead, we conclude that planets around intermediate-mass stars reside preferentially beyond ~0.8 AU, which may be a reflection of different formation and migration histories of planets around A-type stars.

Comments:

31 pages, 9 figures, 6 tables, ApJ accepted, corrected minor typos

Subjects:

Astrophysics (astro-ph)

Cite as:

arXiv:0704.2455v2 [astro-ph]

Submission history

From: John Johnson [view email]

[v1] Thu, 19 Apr 2007 03:48:52 GMT (118kb)

[v2] Thu, 19 Apr 2007 20:26:19 GMT (118kb)

http://arxiv.org/abs/0704.2455

Of relevance to this thread:

http://www.france24.com/france24Public/ ... earth.html

This finding about an Earth-like planet circling Gliese 581, coupled to recent conclusions about stable planetary orbits around binary systems and habitable planets around red dwarves make the Alpha Centauri system an even more alluring target than before, given its relative nearness and the possibility that all three stars might harbor planets.

The most famous rendering of a Proxima planet is David Hardy's. Besides Proxima itself hovering at the horizon, you can see the Alpha Centauri A/B binary in the far right. Proxima's Planet 1

Another image, by John Whatmough, which shows Proxima's prominent flares. Proxima's Planet 2