Proxima Centauri

Before dealing specifically with Proxima Centauri, let’s ask: how likely is it that a red dwarf could have a habitable planet?

The conventional wisdom among SF writers had long been that if a planet were the right distance a red dwarf for Earthlike temperatures, it would be close enough to be “tidally locked” - it would keep the same face to the star at all times, much as we used to think Mercury did to the sun. As a result, half of the planet would be a baked wasteland, and the other half a frozen wasteland, with (at best) only a narrow habitable band where it was perpetual twilight. Or, worse, the constant freezing on the night side would cause the planet to lose its atmosphere completely.

Another problem with red dwarfs as usable suns is that many of them are flare stars. Even if an ecosystem developed under these conditions, wouldn’t the constant solar flares mean doom for any settlers from Earth?

However, in an article in the November 2005 Scientific American, Mark Alpert raises the possibility that if a planet with a red-dwarf primary had an atmosphere containing modest amounts of carbon dioxide, enough heat to prevent freezing could spread to the night side. It would be cold, but not frozen solid.

And a strong enough magnetic field could deflect any particles from the solar wind.

In an interview with Astrobiology Magazine, astronomer Todd Henry further points out that if such a planet had an orbital companion (say, if it were a moon of a gas giant), it could have a day-night cycle not unfamiliar to Earth people.

Of course, the life zone of an M star is narrower than that of a G or even K star, so any given one would not be particularly likely to have a planet where we could live. But Henry also observes that there are about 12 times as many M as G stars (at least 240 within 10 parsecs of Earth versus 21 Gs), so there may be about the same number of usable planets around each type.

So although Alpha Centauri A is the nearby star that gets all the attention from SF writers, don’t count Proxima out! It’s a fairly typical M5eV flare star, but none of that seems to count it out as a possibility. According to Solstation.com, it appears to be over 5 billion years old, too… lots of time for an ecosystem to develop, given the right conditions on the planet. I’ve always thought it would be a nice irony if the flashier Alpha Centauri stars came up empty for us, but neglected Proxima had a perfectly good world.