Astronomers from the European Southern Observatory (ESO) have announced the discovery of a new exoplanet. What’s the big deal? Scientists have found thousands of those already, after all. But this time it’s different — and historic. The newly discovered exoplanet is at least somewhat Earth-like, and more importantly, it orbits the star closest to our own, known as Proxima Centauri. It’s a mere 4.2 light years away, which is really in our own astronomical backyard. For exoplanets, it doesn’t get any closer.
The Centauri system consists of three stars. There is the Alpha Centauri binary made up of two stars similar to our sun, and then the much smaller Proxima Centauri that orbits them. The newly discovered planet is known as Proxima Centauri b, or just Proxima b. It orbits its parent star every 11.2 Earth days and has a mass at least 1.3 times that of Earth. Given the size, it’s likely that Proxima b has a solid rocky surface, but the truly fascinating thing is that it is in the habitable zone of its star.
Astronomers consider the habitable zone of a star to be anyplace where liquid water can exist on the surface. That’s not a guarantee that it does have water, but Proxima b could be quite wet. It orbits a mere 4.5 million miles from Proxima Centauri, whereas Earth is 93 million miles from the sun. Proxima Centauri is what’s called a red dwarf, though. It’s much smaller and cooler than the sun. If Proxima b was that far away from its star, it wouldn’t be in the habitable zone.
Because Proxima Centauri is so close, astronomers have been looking at it for years in hope of finding an exoplanet. There have been some hints of a planet, but none of them have panned out until now. This time, the ESO really got all its ducks in a row before making an official announcement. It tracked the activity of Proxima Centauri using a network of telescopes around the world to make sure the subtle signal from Proxima b wasn’t a glitch.
This planet was discovered with the Doppler shift method, which differs from the transit method used by the Kepler satellite. The latter only works when we have an edge-on view of the star and the planet blocks a detectable portion of its light. Instead, the Doppler method watches for small counter movements in the star caused by a massive object orbiting around it. The faster and larger the planet, the larger the Doppler shift. However, Proxima b isn’t big, or moving particularly fast.
The likelihood of finding liquid water (or even life) on Proxima b depend on the atmospheric composition. It could be vaguely Earth-like or more like Mars. The effect of radiation from being so near to its star is also a concern, but it’s going to take time to get a more accurate picture of Proxima b. At just 4.2 light years away, there’s a good chance that astronomers can characterize the atmosphere and surface of Proxima b in a way that we haven’t been able to with exoplanets that are dozens or hundreds of light years away. All in all, a tremendous discovery.