Free Astronomy Magazine July-August 2016

JULY-AUGUST 2016 PLANETOLOGY sique et de Géophysique, Université de Liè- ge) has discovered the first planetary sys- tem of an ultracool dwarf. The star, at first classified as 2MASS J23062928-0502285 but later renamed TRAPPIST-1, is located in the constellation Aquarius, a little less than 40 light-years from Earth, is of spectral type M8, is 2000 times less luminous than the Sun, has a mass that is only 8% that of the Sun and a diameter of just 160,000 km, thus not much greater than that of Jupiter (140,000 km). Constantly monitored by TRAPPIST every 1.2 minutes, for 245 hours over 62 nights between September and December 2015, the star showed 11 dips in light close to 1%, consistent with the transit of some planets on its disk. A second phase of observations made with HAWK-I (a cryo- genic wide field imager) on ESO’s Very Large Telescope in Chile, with the Himala- yan Chandra Telescope in India and with the United Kingdom Infra-Red Telescope in Ha- waii, confirmed the existence of 3 planets with sizes very similar to those of Earth. The two inner ones, TRAPPIST-1b and TRAP- PIST-1c, orbit around their star in just 1.51 days and 2.42 days respectively, at medium distances of 0.011 and 0.015 AU, and are