European astronomers have discovered a planet with a mass similar to that of Earth orbiting a star in the Alpha Centauri – the closest to Earth. It is also the lightest exoplanet yet discovered around a Sun-like star the planet was detected using the HARPS instrument installed at the 3.6-meter telescope at ESO’s La Silla Observatory in Chile. The results appeared online in the journal Nature, in its issue of October 17, 2012.

planet orbiting the star alpha centauri b

Alpha Centauri is one of the brightest stars in the southern sky and the closest star system to our Solar System – is just 4.3 light years away. Actually it is a triple star system, consisting of two orbiting Sun-like stars near each other, designated as Alpha Centauri A and B, and a more distant faint red star known as Proxima Centauri. Since the nineteenth century, astronomers speculated on the possibility of the existence of planets orbiting these bodies, as it would be the nearest place where you find a host that could harbor life beyond the solar system, but highly accurate searches revealed no nothing until now.

“Our observations lasted more than four years, using the HARPS instrument, and have relieved a signal tiny, but real, that shows a planet orbiting Alpha Centauri B every 3.2 days,” says Xavier Dumusque (Observatory of Geneva, Switzerland and Center for Astrophysics, University of Porto, Portugal), author of the paper. “It is an extraordinary discovery that has led our technology to its limits!”

The European team detected the planet by capturing the small wobbles in the motion of the star Alpha Centauri B generated by the gravitational pull of the orbiting planet. The effect is tiny – makes the star move back and forth not more than 51 centimeters per second (1.8 km / hour or less velocity when it reaches a baby crawls). The highest precision is never achieved with this technique.

Alpha Centauri B is very similar to the Sun, but slightly smaller and less bright. The new planet discovered, with a mass slightly larger than that of Earth, orbiting the star is about six million kilometers away, much smaller than Mercury to the Sun in our Solar System. The orbit of the other component of this bright double star, Alpha Centauri A, is holding hundreds of times that distance, but would still be a very bright object in the skies of this planet.

The first exoplanet around a Sun-like star was discovered by the same team in 1995 and since then, there have been more than 800 confirmed discoveries, but most are much larger than planets Earth, Jupiter-like planets abound. The challenge now facing astronomers is to detect and characterize a planet with mass similar to Earth orbiting in the habitable zone of another star. We have already taken this first step.

“This is the first planet with a mass similar to Earth found around a sun-like star orbits very close to its star and be too hot to support life as we know it,” says Stéphane Udry (Observatory of Geneva), co-author and team member, “but it may be part of a system in which there are more planets. Other findings from HARPS and Kepler’s new discoveries clearly show that the majority of low-mass planets are found in this type of system.”

“This represents a major step towards the detection of a twin planet to Earth in the immediate vicinity of the Sun that we live in exciting times!” concludes Xavier Dumusque.