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 a lot left to discover 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.
Most of the potential Earths out there are therefore not Earthlike at all, since they huddle closely to their cool, ruddy stars.