The nearest known exoplanet circles its star in just eleven days
Proxima Centauri b orbits the closest star to the Sun, about 4.2 light-years away, and sits in that star's habitable zone. It is probably rocky and a little heavier than Earth. But its year lasts only about 11.2 days, and its small red sun erupts in flares that may have stripped any air away.
The discovery came from patience. Earlier searches in 2008 and 2009 had ruled out planets bigger than Earth in the habitable zone, yet instruments of the European Southern Observatory in Chile kept picking up small wobbles in the star that flares could not explain. In January 2016 astronomers launched the Pale Red Dot campaign to test the idea, and on 24 August 2016 a team led by Guillem Anglada-Escudé announced a planet. It is now probably the exoplanet best known to the public.
Its orbit is tiny, around 0.048 astronomical units, more than 20 times closer to its star than Earth is to the Sun. That is survivable only because Proxima is a dim red dwarf, the most common kind of star; roughly 20 to 40 percent of red dwarfs host a planet in their habitable zone. The measured minimum mass is about 1.06 Earths, and if the orbit tilts as expected the true figure could be around 1.44. Its radius is poorly known, so it could be anything from a dense, iron-heavy world like Mercury to one drenched in water.
It probably did not form where it is now, since the disc there held too little material; it more likely migrated inward. It is likely tidally locked, keeping one face permanently toward the star, which would produce extreme contrasts between a scorching day side and frozen night side. If instead it spins in a resonance like Mercury's, tidal heating could drive volcanoes and might shut down any magnetic field, and its oceans, if any, would see tides far stronger than ours.
Proxima b has company. A second planet, Proxima d, orbiting even closer, was confirmed in 2022, while a candidate called Proxima c, reported in 2020, remains disputed. The system's age is uncertain, estimated at about 5 billion years or perhaps 7 to 8 billion.
Source: Proxima Centauri b
