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Astronomers have been announcing on a regular basis for some time now major discoveries from NASA's exoplanet hunting Kepler space telescope. On Tuesday 20th December they revealed the latest find made by the orbiting telescope, the detection of the first planets outside our Solar System that are about the same size as the Earth and orbit a star similar to the Sun.

The two new planets have been named Kepler-20e and Kepler-20f. Of these, Kepler-20f is almost exactly the same size as the Earth while Kepler-20e is slightly smaller at 0.87 times the radius of the Earth. Both planets are substantially closer to their parent star than the Earth is to the Sun, hence orbit much faster than the Earth does. Kepler-20f completes one orbit around the host star in 20 days, while Kepler-20e requires just six days. The Earth of course takes one year to orbit the Sun. Since they are much closer, they are far too hot to support life, although scientists say that Kepler-20f could have a thick water-vapour atmosphere. Kepler-20f has a surface temperature of 425C (approx. 800F), which is similar to the planet Mercury. The surface temperature of Kepler-20e is hotter still at more than 760C (1400F), high enough to melt glass.

Artist's impression showing the comparative sizes of Kepler-20e, Kepler-20f, Earth and Venus (NASA/Ames/JPL-Caltech)

A stated condition for life to exist on a planet is the presence of liquid water on the surface. In the distant past it is thought that both planets were further from their star than today, much cooler and therefore better suited for liquid water to exist. In particular Kepler-20f, the large of the two, might well have been an Earth twin at some time in its history.

Dr Francois Fressin, one of the astronomers from the Harvard-Smithsonian Centre for Astrophysics in Cambridge, US, said the discovery herald a new "era". He added, "It is the first time humanity has been able to discover an object similar to the Earth around a star, so maybe we will be able to find others. This could be an important milestone. I think 10 years or maybe even 100 years from now people will look back and ask when was the first Earth-sized planet found. It is very exciting."

In addition, the Kepler-20 system includes three other planets that are larger than Earth but smaller than Neptune. The host star belongs to the same G-type class as our Sun with the system located about 1,000 light-years from Earth in the constellation Lyra.

Artist's impression of how Kepler-20f may look (NASA/Ames/JPL-Caltech)

Artist's impression of how Kepler-20e may look (NASA/Ames/JPL-Caltech)

The Kepler space telescope detects planets and planet candidates by measuring dips in the brightness of more than 150,000 stars. 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. "With every new discovery we're getting closer to the 'holy grail' of an Earth-like planet around a Sun-like star" he said.

See also

Astronomers announce discovery of 50 new exoplanets
18 Scorpii - The nearest Solar twin
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Earth like planet Kepler 22b is confirmed