Earth-size planet found just 4.37 lightyears away

A planet with a mass probably not much greater than that of the Earth has been discovered orbiting a star in the closest stellar system to our own.

The discovery was announced yesterday by the European Southern Observatory in Chile. The planet orbits Alpha Centauri B, one of pair of Sun-like stars (the other one is Alpha Centauri A) that orbit each other about once every 80 years in elongated orbits that sometimes bring them as close to one another as the distance between the Sun and Saturn.

The planet orbits so close to Alpha Centauri B that its year last just 3.2 days. (Mercury, the closest planet to our Sun, takes 88 days to complete an orbit.) At that distance the planet must be incredibly hot, and it's very unlikely life as we know it could live there.

There's a third star, called Alpha Centauri C or Proxima Centauri, that's a bit closer to us than the Alpha Centauri A-B pair. It's thought to orbit the other two, but at a huge distance, nearly 1/4 of a lightyear.

The planet was discovered by the so-called radial velocity method, which can be used to estimate the minimum mass of the planet but not the maximum. As a practical matter, however, the actual mass is unlikely to be more than a small multiple of the minimum mass.

(Updated for clarity.)

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