Since 2016, astronomers have known that the solar system next door to ours—a triple-sun system—has one planet, Proxima b, located in the so-called habitable zone. Now a group of researchers led by Mario Damasso from the National Institute for Astrophysics in Italy claims to have discovered a second planet called Proxima c, a super-Earth with nearly two times the mass of Earth.
The newly detected planet completes one orbit around its host star Proxima Centauri every 5.2 years, and is located well beyond the snow line—the distance from the star where it is cold enough for gaseous compounds such as water, ammonia, carbon dioxide and methane to condense into solid ice grains.
The discovery, which is subject to further confirmation, is remarkable on many grounds. First, it leads us to question our traditional thoughts about planet formation, particularly on how a super-Earth can form so far away from its host star, and how planets can have stable orbits in a triple-star system. Second, detecting a planet just a bit larger than Earth so far away from its host star is an impressive technical achievement, requiring very sensitive measurements. And third, the authors—laudably—did not hype their discovery by raising the possibility of life on Proxima c.
So often in the past, discoverers of exoplanets have been quick to point out that the newly found world is in the “habitable zone.” Even when technically accurate, it means only that the expected temperature is within a range that would allow liquid water on the planet’s surface. True habitability requires much more, including the presence of an atmosphere (or an ice crust) so that liquids on the surface cannot escape into space, a magnetic field to protect from cosmic radiation, organic building blocks and other elements essential for life, and other conditions. On the other hand, worlds not located in the “habitable zone” might still harbor life, as in the (possible) case of Jupiter’s ice-covered moon Europa.
The previously discovered planet around Proxima Centauri, Proxima b, illustrates some of this complexity. The planet is in the habitable zone around its host star, but because that star is a less luminous red dwarf star with only one-eighth the mass of our Sun, the habitable zone is very close to Proxima Centauri. Red dwarf stars flare a lot after their formation, and given how much radiation Proxima b still receives today from the star, it is very unlikely that life exists, or ever existed, on this planet. Same with our Moon. It’s located right next to Earth, in the Sun’s habitable zone. But it’s devoid of all life today, lacking an atmosphere and liquid water on its surface.
We have to be careful, and not get carried away in our eagerness to find a second Earth. The discovery of Proxima c in a triple-sun system, in a location where nobody expected it to be, demonstrates how little we still understand about exoplanets. Who knows, we may yet find an inhabited planet in an even more unexpected location somewhere in the galaxy. But maybe not in the solar system nearest to us.