Scientists have discovered what they say is the closest temperate planet to Earth around a quiet star – one that could host liquid water. And it may be one of the best places to look for life outside the Solar System.
The planet has been called Ross 128 b, discovered by the European Southern Observatory’s (ESO) HARPS instrument at the La Silla Observatory in Chile, and it’s located 11 light-years from Earth. That makes it the second closest temperate planet we know of, after Proxima b, which is 4.2 light-years away.
However, Proxima b orbits a relatively fiery red dwarf star, Proxima Centauri. Ross 128 b, by comparison, orbits the star Ross 128, which is said to be one of the most sedate stars nearby. The discovery is published in the journal Astronomy and Astrophysics.
“In a habitability context, this makes survival of its atmosphere against erosion more likely,” the researchers wrote in their paper. “Ross 128 b is the second closest known exo-Earth, after Proxima Centauri b, and the closest temperate planet known around a quiet star.”
The planet orbits its star every 9.86 Earth days, at a distance of 0.049 AU (astronomical units, 1 AU is the Earth-Sun distance). That’s 20 times closer than Earth is to the Sun, but the star is 280 times less luminous than our Sun. This means Ross 128 b receives just 1.38 times more energy than our planet.
However, we don’t quite know its equilibrium temperature, estimated to be between -60°C and 20°C (-76°F and 68°F). For this reason, the researchers are calling the planet “temperate” for now, rather than habitable, as we don’t know if the planet is in its star’s habitable zone where liquid water can exist.
A planet at this distance “either having liquid water or just shy of having some makes an extremely appealing characterization target,” the team write. Unfortunately, Ross 128 b does not transit its host star from our point of view, so follow-up observations will not be so easy.
While it’s a bit more tricky to study than Proxima b, the researchers note that upcoming telescopes like the European Extremely Large Telescope (E-ELT), due to come online in 2024, could provide us with valuable data and search for biosignatures.
It’s the star itself, though, that makes this world so tantalizing. Ross 128 is an extremely quiet red dwarf, with fewer flaring events than others, meaning it is less likely to have stripped away the atmosphere of Ross 128 b if it has one.
“Ross 128 is one of the quietest stars of our sample and, although it is a little further away from us [than Proxima b], it makes for an excellent alternative target,” Xavier Bonfils from the University of Grenoble in France, the study's lead author, told IFLScience.
Interestingly, Ross 128 is moving towards us and in 79,000 years it will surpass Proxima Centauri as our closest neighbor. Hopefully, by then, we’ll know if it has life or not.