Red suns

Artist's conception of planets orbiting a red dwarf. Credit: NASA/JPL-Caltech.

Artist’s conception of planets orbiting a red dwarf. Credit: NASA/JPL-Caltech.

The most common type of stars in the universe are Type-M dwarfs, better known as red dwarfs. These are stars that are less than about half the mass of our Sun, less than a tenth as bright, and much cooler, hence the reddish color. Red dwarfs make up about two thirds of the stars in our galaxy.

Red dwarfs aren’t really red. They put out most of their light in the red part of the spectrum, but they also make some green and blue light. To the eye, they would look either pale orange, like the picture, or perhaps even pink. But the important thing is that red dwarfs are so faint that planets have to orbit very close to them to keep warm.

Could a red dwarf have an Earth-like planet? For a while, astronomers said probably not, but now we’re starting to change our minds.

The first reason to worry about red dwarfs having Earth-like planets is that they’re little. It’s a good guess that little stars have little planets, and for once, that turned out to be right! There are very few big, Jupiter-sized planets orbiting red dwarf stars. At first, we guessed that there wouldn’t be many Earth-sized planets either, be once we started looking, we found that there might actually be more of them than around Sun-like stars.

The second problem is that young red dwarf stars are very fussy. Red dwarfs have solar flares, just like the Sun, and these flares are just as big, even though the stars are much smaller. That’s bad news for a planet orbiting close by. Red dwarfs tend to settle down when they mature, but unlike Sun-like stars, which take 100 million years to do so, the longer-lived red dwarfs take over a billion years. If there was an Earth-like planet nearby, the flares could have burned off its atmosphere by then.

Or maybe not. Atmospheric loss is still poorly understood, and it’s entirely possible that a planet could hold on to its atmosphere against these giant flares. But there’s another problem: a planet orbiting a star that close would become tidally locked, always keeping one side facing its star, like the Moon does toward Earth. On that kind of planet, one side would always be boiling hot, while the other would be freezing cold, and the expected weather patterns would end up with all the water frozen solid on the night side.

Or maybe not. Atmospheric circulation is even more poorly understood that atmospheric loss. After all, we can’t even predict the weather on Earth more than a few days out. With a thick enough atmosphere, even a tidally-locked planet could be warm all the way around.

Noted physicist Freeman Dyson says that we should look for life where it’s easy to look, not where it’s likely to live, an idea her expresses in this TED talk. This makes sense because even if life is not likely to be there, we’re much more likely to actually find it. In many cases, red dwarfs are easy places to look: they produce less light, so planets don’t get quite as lost in the glare. Hence the great scrutiny given to the red dwarf planet GJ 1214b. If Earth-like planets are as common around red dwarfs as we’re starting to think, this is good news for planet hunters.

About Alex R. Howe

I'm a full-time astrophysicist and a part-time science fiction writer.
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