Traditionally, anyone seriously researching extraterrestrial life has tended to ignore red dwarfs. Typically a disproportionately low number of them have been included in SETI searches, for instance. They’re troublesome, ill-tempered little things which like to randomly flare up for seemingly no reason. Small, cool and difficult to spot, relatively few red dwarfs can even be seen from Earth because they’re so faint. Their violent tantrums cause huge flares far more powerful than anything the Sun could put out, and they have such powerful magnetic activity that starspots can significantly reduce their brightness, sometimes by up to 40%.
This might not sound like a nice kind of place to live, but red dwarfs do, however, have a number of things going for them. For one, they’re extremely populous. They make up a tentatively estimated 75% of all stars (this is probably the case for the whole Universe). What’s more, they’re incredibly long lived. Depending upon their size (and thus, how quickly they burn) red dwarfs are expected to live for anywhere between 50 billion years to several trillion years. Let me put that into perspective. Barring any (literally) disasterous events, no red dwarf has ever died in our Universe. The Universe itself simply isn’t old enough. There could well be some truly ancient red dwarfs out there, slowly burning away since shortly after the dawn of time itself.
But we don’t have to look that far to find one. At a mere 4 light years away, Proxima Centauri is our closest stellar neighbour (shown to scale in this image, between The Sun and Jupiter). Maybe not as dramatic as many of the stars in the sky, but could it serve as a cradle for life? A star of Proxima’s diminutive size is expected to live for around 4 trillion years. If anything could evolve, it could do so at a leisurely rate!
In fact, a red dwarf like Proxima will live for so long, that the length of habitable time becomes more a matter of how long a planet could support life, as opposed to how long the star will live. Assuming, as we like to do, that habitability needs liquid water on it’s surface causes us to run into a problem though. Because red dwarfs are so small, their habitable zones tend to be proportionately smaller. A relatively narrow habitable band encircles these stars, reducing the likelihood of a planet actually existing in the right place. In turn, a planet close enough to be Earth-like would be tidally locked. One face would be constantly roasted by the hot star, while the other side would be frozen and silent. The planet would be doomed because of the vast temperature gradient between its day and night sides. Or so we’ve always thought.
As it happens, studies have shown that red dwarfs might be quite good hosts for planets. Enough have been found with planet-forming disks that it’s safe to conclude that they’re quite common, and models have shown that terrestrial planets are just as likely to form around red dwarfs as they are around yellow dwarfs like The Sun. The possible existence of gas giants could even help to shepherd fledgling planets into the star’s habitable zone; so much so that some wonder if Jupiter analogs might, in fact, be prerequisite to habitable planets forming. Planets have already been found around red dwarfs, ranging from 7.5 Earth masses to Jupiter mass and above. While this wouldn’t look good for Proxima which, to our knowledge, hosts no giant planets, it wouldn’t rule out the multitudes of unexplored red dwarfs out there.
An interesting thought is that red dwarfs, perhaps even Proxima, could host miniature versions of our solar system. It’s not an unreasonable thought, given that Jupiter and Saturn each host a contingent of moons. Could a tiny star do the same?
Assuming these planets form, and their atmospheres don’t collapse due to that big temperature gradient I mentioned above, tidal locking isn’t actually any barrier to habitability. For one thing, a planet’s geothermal energy would do a lot to help keep any oceans liquid, at least below the surface. Given the vast lifespan of a red dwarf though, an interesting (and open) question is how long a planet could remain geothermally active. Futhermore, the oceans themselves could help effectively transport heat around the planet, as could a significant Earth-like atmosphere. While this may make a planet Earth-like in temperature, that atmosphere would likely be very different though.
Planetary atmospheres in the solar system are driven largely by sunlight. It affects their chemistries and powers their weather. Around a star like Proxima though, the light would be very different. Less ultraviolet would mean different photochemistry. Molecules could accumulate in planetary atmospheres, which would be destroyed swiftly by sunlight. A planet bathed in Proxima light could accumulate things like CH4 and N2O in its atmosphere. The fact that CH4, in particular, is such a potent greenhouse gas, means there’s a good chance that this could extend Proxima’s habitable zone. On the planet however, that dreaded temperature gradient could once again cause problems. It would drive powerful winds between the day and night sides, perhaps causing life on the planet’s surface to evolve very differently to life on Earth.
In closing, as our only example of life itself is based on water and carbon, it’s not difficult to make the following deduction: If a planet is within a star’s habitable zone, and contains both liquid water and the relevant necessary chemicals, there’s no reason why it shouldn’t be habitable. Of course, there’s still no hard evidence for any of this, but by searching closely around red dwarfs for terrestrial sized planets, and by aiming SETI searches at such stars, we could find some interesting answers.
So could Proxima be a tiny outpost of alien life? Frankly, as far as I know, it’s currently impossible to tell. No planets have been found around our tiny neighbour, but our tecniques simply aren’t good enough to detect terrestrial sized bodies (though this is a problem which people are working to fix). Perhaps there’s a tiny Europa analogue, barely 0.1 AU from Proxima’s surface, which could have oceans, ecosystems and even civillisations. Or perhaps not, who knows? It’s certainly worth bearing in mind that if moons can form around Jupiter, planets could have formed around Proxima the same way.
For now, I guess we’ll all just have to wait and see…
Jill C. Tarter, Peter R. Backus, Rocco L. Mancinelli, Jonathan M. Aurnou, Dana E. Backman, Gibor S. Basri, Alan P. Boss, Andrew Clarke, Drake Deming, Laurance R. Doyle, Eric D. Feigelson, Friedmann Freund, David H. Grinspoon, Robert M. Haberle, Steven A. Hauck, Martin J. Heath, Todd J. Henry, Jeffery L. Hollingsworth, Manoj M. Joshi, Steven Kilston, Michael C. Liu, Eric Meikle, I. Neill Reid, Lynn J. Rothschild, John Scalo, Antigona Segura, Carol M. Tang, James M. Tiedje, Margaret C. Turnbull, Lucianne M. Walkowicz, Arthur L. Weber, Richard E. Young (2007). A Reappraisal of The Habitability of Planets around M Dwarf Stars Astrobiology, 7 (1), 30-65 DOI: 10.1089/ast.2006.0124