exoplanet alien world
Artist's impression of a solar system discovered with four rocky planets, two of which have potential to host alien lifeNASA/JPL

Two potentially habitable rocky planets have been discovered with Nasa's Kepler telescope during a 'round-up' of exoplanets in which more than 100 new worlds were found. While current technology would not allow scientists to probe the planets any further, they could be shortlisted as targets for the James Webb telescope, which would be able to detect signs that a planet is potentially suitable to host alien life.

Of the 104 exoplanets, four are believed to be rocky. They are 20-50% bigger than Earth and orbit a star around half the size and of the Sun. They all orbit the M dwarf star K2-72, which is 181 light years away, taking 5.5-24 days to complete their elliptical course.

While two of these planets would be far too hot to support life as we know it, the other two appear to have irritation levels from their star similar to what Earth receives from the Sun, and are located in the solar system's habitable zone – where liquid water could exist on the surface.

The planet closer to its star, K2-72c, has an orbit of 15 days and would be around 10% hotter than Earth, while the other, K2-72e, has an orbital period of 24 days and would be 6% cooler than our planet.

The haul of planets was part of Kepler's K2 mission, where the spacecraft searches out new planets by looking for dips in a star's brightness. This indicates that something large – such as a planet – is passing in front of it. The latest findings are published in the Astrophysical Journal Supplement Series.

When the James Webb telescope launches in 2018, exoplanets discovered by Kepler can be re-examined in far greater detail, potentially revealing signatures of alien life. Steve Howell, project scientist for Kepler and K2 at NASA's Ames Research Centre, said: "This bountiful list of validated exoplanets from the K2 mission highlights the fact that the targeted examination of bright stars and nearby stars along the ecliptic is providing many interesting new planets.

"This allows the astronomical community ease of follow-up and characterisation, and picks out a few gems for first study by the James Webb Space Telescope, which could perhaps provide information about their atmospheres."