When I was an astronomy-obsessed kid, I learned that most of the stars in our galaxy and beyond are very similar to our Sun. No less an authority than Carl Sagan wrote that “the Sun is an ordinary, even a mediocre star.” If that insight diminished the importance of our place in the universe, it also made it seem likely that there must be many other living worlds around us. If the Sun is a typical star, couldn’t the Earth be a typical planet?
Except that Sagan was wrong, or at least misleading. More than 90 percent of the stars in the Milky Way are cooler and dimmer than the Sun. A full three-fourths are red dwarfs, the smallest of the small. Furthermore, dwarf stars seem to be especially likely to have rocky planets. Together, those statistics indicate that there are a trillion planets around red dwarfs in our galaxy, including at least 100 billion potentially habitable Earth-size worlds. The big unknown is whether those planets are actually habitable — that is, if the genuinely typical Earth-size worlds out there are really anything at all like our own.
The exciting news is that we’re not going to have to wait long to find out. Red dwarf stars are exceedingly dim, but now that astronomers are targeting them in earnest, all kinds of intriguing planets are popping into view. Probably the most famous of these is Proxima b: an approximately Earth-mass planet orbiting in the habitable zone around Proxima Centauri, the very nearest star beyond the Sun. Less than a year later, a team led by Michaël Gillon at the University of Liege announced seven Earth-size planets around the red dwarf star TRAPPIST-1, located 40 light years away — nearly 10 times as distant as Proxima b, but still among the Sun’s closest neighbors.
Astronomers have revealed two more major red dwarf discoveries just in the past month. The minuscule red star known as Teegarden’s Star turns out to have two planets, each a minimum of 1.1 times the mass of the Earth, quite possibly part of a larger system that includes other, as-yet-undetected worlds. Both of the newfound planets lie within the star’s habitable zone, meaning that their surface temperatures could potentially fall in the range that allows liquid water. (If you are wondering about the unusual name, Teegarden’s Star is named after Bonnard Teegarden, who discovered the star only in 2003. There’s still a lot to explore right in our galactic back yard.)
Meanwhile, researchers working with NASA’s new TESS space telescope have just released a paper describing an intriguing new red dwarf planet called LTT 1445Ab. Yes, these planets need more user-friendly names, but bear with me here. It’s not just one star, it’s a triplet — a fairly tight pair and a third more distant one, all gravitationally tied together. A planet orbits that third outlier, meaning that in its sky it sees the two other stars dancing about, each a brilliant red dot about as bright as the full Moon on Earth.
There are several enticing details about LTT 1445Ab. Its diameter is just 1.35 times that of Earth, so it is probably a rocky planet like ours. It is too hot for life as we know it, with a nominal temperature of around 150 degrees C (300 degrees F), but it’s not all that far off the mark. Best of all, LTT 1445Ab–should we just call it “Abby” for now? — is the nearest transiting red dwarf planet, which makes it a great target for further scientific investigation.
Transiting meaning that the planet appears to pass in front of its star from our perspective. During a transit, starlight floods past the planet and shines through whatever atmosphere it might possess. On the other side of its orbit, the planet briefly disappears behind its star. There are no existing telescopes good enough to view the planet separately from its star, but by analyzing the light astronomers can separate the two and begin to reveal intimate details about the planet. This is how we will know if it has an atmosphere, if it has water, if it has clouds, what other kind of gases are present, what the actual temperatures are like down there.
All of these insights are crucial for making sense of red dwarf planets and, by extension, for determining how common life may be in the universe. We already know that most of the stars out there are not like the Sun, and whatever red dwarf planets may be like, they are certainly not exactly like Earth. What they are like is the subject of much scientific speculation. Despite the many confident-looking illustrations you can find floating around on the internet (many of them from reputable sources like NASA and the European Southern Observatory), the only honest answer is that nobody knows.
Because red dwarf stars are so dim and cool, any habitable planets will have to be in very close orbits. At such short range, gravity from the parent star would tend to slow the rotation of a planet until it is tidally locked, with one hemisphere always facing toward the star and the other perpetually turned away.
That sounds like a nasty situation for life–one side baking, the other freezing–but the presence of a significant atmosphere could prevent that from happening. And even if tidal locking does occur, atmosphere and oceans could efficiently redistribute heat. Verdict: red dwarf planets might or might not be habitable.
Red dwarf stars themselves present some environmental challenges. When the stars are very young, they are much brighter than they are once they settle down. Any planet close enough to be comfortable in the star’s middle age might have been thoroughly cooked early on. Red dwarfs are also prone to pop off powerful flares of radiation, which over time might strip planets of their atmospheres.
Grim news, but here, too, there are other factors to consider. The closely-spaced planets in red dwarf systems would also raise tides on each other, keeping their interiors hot. A hot, geologically active planet could sustain a strong magnetic field, which would act as a shield against radiation from the star. Geologic activity could also help replenish any loss of atmosphere. Dwarf planetary systems might be born with a lot of water and with a lot of comets that could also help replenish what is lost. Verdict (again): red dwarf planets might or might not be habitable.
The only way to get meaningful answers about red-dwarf planets is to closely examine some real-life examples and see what they are like. Probably the environmental conditions vary significantly from planet to planet and system to system. Almost certainly these planets are systematically quite different than Earth. As we learn more, we will find out about the true prospects for life among the dwarfs. And we will finally find out what a typical planet around a typical star is really like.
