Discovery of an Earth-like planet

Astronomers have confirmed the existence of an Earth-like planet in the "habitable zone" around a star not unlike our own.

The planet, Kepler 22-b, lies about 600 light-years away and is about 2.4 times the size of Earth, and has a temperature of about 22C. It is the closest confirmed planet yet to one like ours - an "Earth 2.0". However, the team does not yet know if Kepler 22-b is made mostly of rock, gas or liquid.

Kepler 22-b was one of 54 exoplanet candidates in habitable zones reported by the Kepler team in February, and is just the first to be formally confirmed using other telescopes.

More of these "Earth 2.0" candidates are likely to be confirmed in the near future, though a redefinition of the habitable zone's boundaries has brought that number down to 48. Ten of those are Earth-sized.

The Kepler space telescope was designed to look at a fixed swathe of the night sky, staring intently at about 150,000 stars. The telescope is sensitive enough to see when a planet passes in front of its host star, dimming the star's light by a minuscule amount.

Kepler identifies these slight changes in starlight as candidate planets, which are then confirmed by further observations by Kepler and other telescopes in orbit and on Earth.

Kepler 22-b lies 15% closer to its sun than the Earth is to our Sun, and its year takes about 290 days. However, the planet's host star puts out about 25% less light, keeping the planet at its balmy temperature that would support the existence of liquid water.

The Kepler team had to wait for three passes of the planet before upping its status from "candidate" to "confirmed". As candidates for planets similar to Earth are confirmed, the Search for Extraterrestrial Intelligence (Seti) has a narrower focus for its ongoing hunt.

 Kepler space Telescope

Stares fixedly at a patch corresponding to 1/400th of the sky

Looks at more than 155,000 stars

Has so far found 2,326 candidate planets

Among them are 207 Earth-sized planets, 10 of which are in the "habitable zone" where liquid water can exist