Or is that the Goldilocks Variations?

The new planet found orbiting in the Habitable Zone of Proxima Centuri is an unexpected, but delightful, surprise. The more luminous a star is, the wider is its Habitable Zone, the region where water can be found in liquid form. On our sun, that HZ stretches from Venus to Mars, with Earth smack in the middle of it. On a giant star, the habitable zone could be easily larger than our entire 8 planet orbital plane, and it would be located much further out from the primary. You could conceivably have hundreds of habitable worlds in a system like that. Unfortunately, bright giants don’t last long enough for life to evolve around them, and they are very rare.

Faint, cool stars have very narrow Habitable Zones. If planetary orbit location selection is a purely random process, then you would expect there would be very few planets in the HZ of red dwarfs. The chances of a world forming in that thin zone of optimal temperature would be very small. This is sad, because red dwarfs are extremely common, highly stable, and often extremely old–perfect places to look for life.

But as the Proxima discovery underscores, quite a few planets HAVE been discovered in the habzones of red dwarfs.

Granted, this may be just a selection effect. Red dwarfs are very common, a majority of all stars fall in that category. Not only are they the most likely to form in the first place; once formed, they have extremely long, stable lifetimes. This no doubt partially makes up for the fact that they have extremely narrow habzones. Further, these habzones tend to be much nearer to red dwarfs than is the case of larger, heavier, brighter suns. In addition, all of our techniques for detecting exoplanets work much better at finding planets orbiting very near light stars. Our search for exoplanets is going to turn up a disproportionately large number of them around red dwarfs, even taking into account that these faint stars are much harder to detect at long distances, whether they have planets or not.

There are contraindications to life forming around even suitable planets orbiting red dwarf candidates. Planets orbiting in the habzone would be very near their star, and their rotation would be tidally locked to the primary. It would keep one face toward the sun throughout its entire orbit, with negative consequences to the planetary weather and climate. It is possible to conceive of circumstances that would partially mitigate this, such as seasonal life cycles for hibernating organisms, a highly inclined, rapidly precessing rotational axis, extreme libration, leading to wide and temperate (albeit fluctuating) polar and twilight zones.

Red dwarfs are also prone to flares and other photospheric eruptions which would bathe nearby planets with radiation and strong solar winds, and would certainly play havoc with atmospheric formation. Still, life could still thrive underground or under water.

Although the surface of a planet closely orbiting a red dwarf might not be an ideal location for life to arise and prosper, it opens a whole new frontier for us to search for life. It is another reason to think that the Galaxy may be crawling with living things. I tend to find this a very comforting thought.