A team of ESO (European Southern Observatory) astronomers announced that they found a planet slightly larger than the Earth orbiting the star Alpha Centauri B, only 4.3 light years away from our solar system.

Alpha Centauri is a system composed of three stars consisting of a yellow dwarf (Alpha Centauri A), an orange dwarf (Alpha Centauri B) and a red dwarf star (Alpha Centauri C, also known as Proxima Centauri). The two stars A and B are very close to each other and to the naked eye they appear to be a single star, Proxima is farther away and orbits around its two companions.

These three are the closest stars to our solar system so they’ve always been studied by astronomers. They’ve also been a target for the search for planets but so far with no results. Now, however, the use of the instrument called HARPS (High Accuracy Radial velocity Planet Searcher), a high precision spectrograph at ESO’s La Silla Observatory in Chile, allowed the detection of traces of a planet orbiting Alpha Centauri B.

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The team of astronomers has detected fluctuations in the motion of Alpha Centauri B caused by planet’s gravity. This effect is very small so the star moves at no more than 1.8 km/h (a little more than 1 mph) requiring a remarkable precision in the observations to detect it. It’s in fact the highest accuracy achieved so far using this method to detect extrasolar planets.

The star Alpha Centauri B is a bit smaller than the Sun and its spectral type is K1. The planet discovered by ESO, called Alpha Centauri Bb, has a mass which was estimated in 1.13 times the Earth’s. Unfortunately, its distance from the star is about 6 million kilometers (about 3.6 million miles) with an orbital period of about 3.2 days. This means that it’s much closer to its star than Mercury is to the Sun and probably its surface is composed of molten rock.

Obviously it would be fantastic to find a planet in the Alpha Centauri system in the habitable zone of its parent star so what’s the great news considering the fact that extrasolar planets are now found all the time? The Alpha Centauri stars are the closest to the Sun and to find at least one planet there proves even more that the existence of planets is very common. The search for really Earth-like planets, even in the Alpha Centauri system, continues.