Recent estimates are that 6 percent of red dwarf stars in our galaxy should have Earth-sized worlds. Six percent! That means that our galaxy could potentially be overflowing with terrestrial planets.
Six percent may not sound like a lot, but red dwarfs are astonishingly populous. Approximately 75 percent of all stars are red dwarfs. Assuming that there are around 300 billion stars in the Milky Way, that means there should be over 13.5 billion exo-Earths orbiting the tiny red suns strewn across our galaxy.
Red dwarfs have always been a source of controversy for planet hunters and astrobiologists. They’ve been frequently ignored in exoplanet searches, largely because they’re noisy little beasts that makes finding anything out about them rather taxing. That is, until Courtney Dressing and her colleagues looked at data from Kepler in an attempt to settle the arguments. As it happens, their study found that around 60 percent of red dwarfs should have planets smaller than Neptune, which means — as some astronomers have long suspected — these tiny stars are likely to be good hunting grounds for exoplanets.
So what exactly is the deal with red dwarfs? Are they good homes for life or not?
Well, a big problem with red dwarfs is that many of them are ill-tempered little things; prone to violent and unpredictable outbursts.
These little stars are fully convective, meaning that material circulates all the way from the core up to their surfaces, unlike the sun that only has convection currents in its exterior layers. This gives rise to immense magnetic fields, which are responsible for Goliath temper tantrums.
Many red dwarfs are flare stars, occasionally increasing significantly in brightness. Flares seen on these little red stars can put the solar flares we see on the sun to shame. They’re also correspondingly more lethal, brightening the star’s electromagnetic output from radio waves all the way up to x rays!
Paradoxically, the same magnetic activity powering these flares also causes dramatic starspot activity — just like sunspots, only much much more so.
Because a red dwarf is so much smaller than the sun, and its magnetic fields are so much stronger, starspots can cause the brightness of these grouchy little stars to drop by up to 40 percent. I hasten to add that not all red dwarfs are quite so volatile. But many of them are.
The other problem with life under a red sun is that red dwarf stars are cool. And I don’t mean cool like Neil deGrasse Tyson, I mean literally cool. The surface temperature of an average red dwarf is around 2000-4000 Kelvin. Compared with the sun’s surface temperature of nearly 6000 Kelvin, that’s only lukewarm. At this lower temperature, most red dwarfs struggle to put out even 1 percent the luminosity of the sun.
This means that habitable planets around red dwarfs have to lie in very tight orbits to stay warm. Practically hugging their tiny red suns for warmth, they’re very much in danger from the titanic flares that these stars can belch out.
This isn’t to say, however, that life isn’t possible under a red sun — but it would have to be very different to what we’re familiar with.
For one thing, the habitable zone around a red dwarf is so close that any of these watery Earth-like worlds would surely be tidally locked. One side would constantly be facing the warmth of their parent star, while the other side would be freezing cold and in constant night.
This may not be as bad as it sounds. Some have hypothesized that the star-facing side of such planets may have a perpetual storm, and the night side would likely be frozen solid. In between the extremes though, such planets may have a belt of warm and potentially life sustaining surface.
Geothermal energy locked in these planets could help to stabilize their temperatures, oceans would certainly help to transport warmth around a planet. A dense Earth-like atmosphere would help too, and even more of that precious red sunlight could be trapped by greenhouse gasses which might accumulate in that atmosphere. Red dwarf stars don’t emit a lot of ultraviolet light.
With less ultraviolet to break molecules apart, potent greenhouse gasses like methane could accumulate and act like planetary blankets. The chemistry of planets around red dwarfs stars is likely to be very different to anything we see in our own solar system.
Red dwarfs have one more trick up their sleeves. They’re very very long lived. So long lived, in fact, that no red dwarf has ever died in our universe, because the universe isn’t old enough yet!
The sun formed roughly 4.6 billion years, and will keep on burning just as it is today for another 5.4 billion years. By contrast, a red dwarf born at the same time would barely be a teenager. For example, Barnard’s star — a friendly neighborhood red dwarf, just under 6 light years away — has a life expectancy of 2.5 trillion years. Over 200 times that of the sun. No one even knows if a planet can support life for that long!
While we know that life evolved here on Earth quite soon after our planet formed, we have no idea how long it would normally take for this to happen. If it’s even remotely possible for the same thing to happen on a red dwarf planet, it would have literally all the time in the universe in which to do so!
Whether or not life exists is still an unanswered question, and one that we won’t be able to answer until our telescopes are powerful enough to actually take a detailed look at planets around red dwarf stars. That said, with 13.5 billion planets to choose from, probability may well be on the side of the astrobiologists!
This is a huge subject, and I’ve barely been able to scratch the surface in this article. If you’d like some further reading, including some more technical details, information for this article was compiled from Tarter et al (2007), Pascucci et al (2009), and the TV series Alien Worlds.
Image: An image manipulation created from public domain NASA images, showing a size comparison of (from left to right) Jupiter, Proxima Centauri, Gliese 581, and the sun.