Free Astronomy Magazine November-December 2019

45 NOVEMBER-DECEMBER 2019 EXOPLANETS eration of substances essential to life. A study carried out a few years ago (Noack and Breuer, 2011) suggests that plate tectonics is active within planets between 1 and 5 Earth masses, and is particularly efficient near 2 Earth masses; beyond this value, the crustal movements start to slow down. A planet twice as massive as Earth, orbiting in the hab- itable zone of a star like our own or smaller, would probably be superhabitable. More- over, the superficial gravity could be identical to what we experience daily if that planet had a diameter 1.4 times that of the Earth. According to Heller and Armstrong, the rotat- ing core of a planet of that size would also A hypothetical ter- restrial-type landscape on a gas giant's moon. These planets have their own habitable zone, which in particular cases can be comple- mentary with or inde- pendent of the stellar one. In the video to the side, the process of plate tectonics, con- sidered essential to the evolution of life.