European astronomers have discovered an Earth-sized planet right next door in the Alpha Centauri system.
It's the lightest exoplanet ever discovered around a star like the sun, and was detected using the HARPS instrument on the 3.6-metre telescope at ESO's La Silla Observatory in Chile.
Just 4.3 light years away, Alpha Centauri is the nearest star system to our own, and is actually a triple star, with two stars similar to our sun - Alpha Centauri A and B - orbiting close to each other, plus a more distant and faint red star known as Proxima Centauri.
Astronomers and science fiction authors alike have speculated about the possibility of planets, but there's been no indication of one until now.
To find it, the team looked for the tiny wobbles in the motion of Alpha Centauri B created by the gravitational pull of the planet. The effect is minute, with the star moving back and forth at a little over one mile per hour.
"Our observations extended over more than four years using the HARPS instrument and have revealed a tiny, but real, signal from a planet orbiting Alpha Centauri B every 3.2 days," says Xavier Dumusque of the Geneva Observatory. "It's an extraordinary discovery and it has pushed our technique to the limit!"
Alpha Centauri B is very similar to the sun but slightly smaller and less bright. The newly discovered planet, with a mass of a little more than that of Earth, orbits about six million kilometres away, much closer than Mercury is to our own sun.
"This is the first planet with a mass similar to Earth ever found around a star like the sun," says Stéphane Udry of the Geneva Observatory. "Its orbit is very close to its star and it must be much too hot for life as we know it, but it may well be just one planet in a system of several."
Other HARPS data, as well as recent findings from Kepler, strongly indicate that the majority of low-mass planets are found in such systems.
"This result represents a major step towards the detection of a twin Earth in the immediate vicinity of the sun," says Dumusque. "We live in exciting times!"