First Earth-sized planets spotted

• Astronomers have detected the first Earth-sized planets, which are orbiting a star similar to our own Sun.
• In the distant past they may have been able to support life and one of them may have had conditions similar to our own planet – a so-called Earth-twin – according to the research team.
• They have described their findings as the most important planets ever discovered outside our Solar System.
• Both planets are now thought to be too hot to be capable of supporting life.
• But according to Dr Fressin, the planets were once further from their star and cool enough for liquid water to exist on their surface, which is a necessary condition for life.
• One of the planets, named Kepler 20f, is almost exactly the size of the Earth. Kepler 20e is slightly smaller at 0.87 times the radius of Earth and is closer to its star than 20f.
• They are both much closer to their star than the Earth is to the Sun and so they complete an orbit much more quickly: 20e circles its star in just six days, 20f completes an orbit in 20 days whereas the Earth takes a year.
• The discovery is important because it is the first confirmation that planets the size of Earth and smaller exist outside our Solar System. It also shows that the Kepler Space Telescope is capable of detecting relatively small planets around stars that are thousands of light-years away.
• The telescope has discovered 35 planets so far. Apart from 20e and f, they have all been larger than the Earth.
• Up until now, the most significant discovery, also by a group including Dr Fressin, was of a planet nearly two-and-a-half times the size of Earth that lay in the so-called "Goldilocks zone". This is the region around a star where it is neither too hot, nor too cold, but just right for liquid water and therefore life to exist on the planet.
• The telescope is scanning 150,000 stars and Professor Andrew Coates of the Mullard Space Science Laboratory in Surrey believes that they will soon find a planet the size of Earth in the Goldilocks Zone.

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