Free Astronomy Magazine July-August 2014

T he field of view of the Kepler space tele- scope is just a tiny section of sky, but where al- ready more than 3,000 extrasolar planets have been discovered. [NOAO/ NASA] Below: a video summary of the discovery of the first mega-Earth and related key data. [NASA/ CNN] peared to have a mass 4.56 times that of Earth, a diameter of 18,100 km and a den- sity of 8.8 g/cm 3 , but a new study conduct- ed by a team led by Xavier Dumusque (Harvard-Smithsonian Center for Astrophys- ics), which too was presented at the last AAS meeting, has significantly lowered the value of the density of Kepler-10b to about 5.8 g/cm 3 , which is very similar to that of the Earth. Dumusque and colleagues have however turned their attention to the sec- ond and last known planet in the Kepler- 10 system, the “c”, and it is precisely from this that came the most sensational disco- very, i.e. the existence of a new class of pla- nets, the mega-Earths. So far we knew of the existence of Earth- like planets (with sizes sometimes smaller and sometimes larger than ours), of super- Earths (from 5 to 10 times more massive than Earth), of mini-Neptunes (in some ways similar to the super-Earths but with a thick atmosphere of hydrogen and helium), and of giant planets (from tens to hun- dreds of Earth masses). These groupings seemed sufficient to enclose every type of planet, but as soon as Dumusque and colleagues pulled the sum of 148 high-resolution measurements of the ra- dial velocity of Kepler-10, it became clear that the star’s displacements attri- butable to Kepler-10c could be justified only by admitting that the planet had a mass equivalent to 17.2 Earths. Nothing un- usual, except that Kepler-