TRAPPIST-1 (also known as 2MASS J23062928-0502285, which handily contains its location on the sky) is about 12 parsecs away, or 39.5 light years. That means it’s one of the closest 500 stars to the Sun, and we now know it has THREE planets. And if that’s not enough, the planets are all in the habitable zone.
For one thing, TRAPPIST-1 is tiny: its spectral type is M8V, where “M” means it’s the smallest, coldest, and reddest type of star; and “8” means it’s on the small side even for an M dwarf. It’s about 1/14th the mass of the Sun, or about 75 times the mass of Jupiter, barely big enough to fuse hydrogen into helium. Our current understanding of star formation says the gas cloud from which it formed must have been quite small, which raises the question of whether there’d be enough material left over to form planets. The discovery of planets – and not just a planet, but THREE close-in planets – confirms that the mechanisms that make planets still operate in situations with that little mass.
Technically speaking, this isn’t the lowest-mass object orbited by a planet; that honor goes to 2MASS J12073346-3932539, which has a 5 Jupiter Mass planet orbiting a 25 Jupiter mass brown dwarf… but in that case, the 5:1 mass ratio makes it more likely they formed like a binary star system. TRAPPIST-1’s planets are much, much smaller than the star – they appear to be close to the diameter of the Earth, and simulations of planet formation suggest that planets like that are probably made of silicate rock, like the Earth. If that’s true, they probably have masses close to the Earth’s. Incidentally, thanks to a quirk in the density of objects made of hydrogen and helium, TRAPPIST-1 is only slightly larger than Jupiter in diameter. This is one of the reasons why planetary systems around M dwarfs are so interesting – you could have a planet with the same diameter as the star. That would be cool, but unfortunately that’s not true here.
These planets are also in the habitable zone where light from the star would keep water on the surface liquid. For a star this feeble, that means VERY short orbits. Planet b has a 1.5 day orbit. Planet c has a 2.4 day orbit. Planet d… it’s not clear yet, but it’s probably an 18 day orbit. That means Planet b is about 50 times further from the star than the Moon is from the Earth. Jupiter has moons more distant than that.
If you think about it, life on any of those planets would be very, very strange: Because the planets are so close to the star, they pretty much have to be tidally locked. That means one side will always face the star – a huge red orb, because planet b is only 10 stellar diameters away from the Sun, and planet c is only 14 diameters away.
The other side will NEVER face the star. On the dark side, the stars would seem to wheel around like crazy; the constellations we wait six months to see would appear every 16 hours. However, if you were standing on the dark side of the inner planet (b), every five-ish “years” (which, for a 1.5 day orbit would be 7.5 days), planet c would swing by – 60% farther away than the Moon, but more than twice the diameter… so it would loom larger than our Moon does. I’d imagine any alien inhabitants of that planet would have some complex mythology surrounding that event. And probably something for the passage of d, the smaller orb. And rumors of the giant red star that lurks on the other side of the planet.
The last reason why TRAPPIST-1 is so interesting is somewhat less obvious: M dwarfs are incredibly common. Some of my friends have done studies that show that despite the fact that the habitable zone area around each M dwarf is very small, when you add up all the M dwarfs, they account for most of the habitable real estate in the Galaxy. Now that we’ve proven even the truly tiny stars have planets, that means a lot more strange vistas to explore.