Kepler's K2 mission confirms 104 Earth-like planets

An international team of astronomers has confirmed a treasure trove of new exoplanets spotted by NASA's Kepler spacecraft during its K2 mission.

Kepler was launched in 2009 to search for Earth-like planets around Sun-like stars. Scientists describe the region where an exoplanet might be the right distance away from a star to harbour water as the Habitable Zone or the Goldilocks Zone.

The spacecraft spotted 197 exoplanet candidates over a year, and astronomers have confirmed the status of 104 planets - the largest haul of planets obtained during the K2 mission.

The results published in the Astrophysical Journal Supplement Series highlight 37 planets that are of particular interest for being similar to Earth in size. Five of those planets receive Earth-like irradiation levels and are in multi-planet systems like the Solar System.

The planets orbit the M dwarf K2-72 that is found by looking in the direction of the Aquarius constellation 181 light years away.

As a dwarf star, it is less bright and half the size of the Sun. The planets are all 20 to 50 per cent larger than Earth in diameter and have a range of orbital periods. The shortest being 24 days and the longest is five-and-a-half years.

Exoplanet candidates are flagged by Kepler when it detects in a dip in a star's brightness during a partial eclipse as a passing planet blocks out the star's light in the Habitable Zone. Information such as the size of the exoplanet relative to the star can be estimated by measuring the difference in brightness as the exoplanet transits.

The Kepler mission originally had a lifetime of three years but was extended for another year. The K2 stage began in 2014 after two of Kepler's reaction wheels malfunctioned. Scientists failed to reactivate the crippled wheels. The spacecraft could no longer rely on its reaction wheels to control its attitude, and scientists feared it could no longer focus on a target without drifting off course. The original Kepler mission was terminated.

But, scientists managed to find another way to keep the spacecraft steady using solar pressure. The stream of photons emitted from a star strikes the surface of Kepler's solar panels and exerts a pressure on the spacecraft. If Kepler is properly positioned, it can be gradually nudged by solar pressure and maintain trajectory.

The space telescope aboard the spacecraft still works and sends pictures back to Earth. The high-resolution images allow astronomers to validate the exoplanets. Further properties of an exoplanet can be gleaned by splitting the starlight through a prism.

Physical properties such as a star's mass, radius and temperature allow properties of the any orbiting planets to be inferred. Astronomers believe that from extrapolation of the current planetary yield, K2 will discover between 500 - 1000 in its planned four-year mission.

"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," said Steve Howell, project scientist for Kepler and K2 at NASA's Ames Research Center.

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