Free Astronomy Magazine September-October 2016

PLANETOLOGY P hotometry of K2-72. From the overall signal, Ian Crossfield and colleagues ex- tracted the light curves of the transits of the 4 planets, whose minima are shown in differ- ent colours. [ApJ, Crossfield et al.] Below, an artist’s rendition of the K2-72 planetary system. All 4 plan- ets composing it are Earth-sized and two of them might be habit- able. [NASA/JPL] later subjected to intensive photometric, spectroscopic and high-resolution imaging analysis using several powerful instruments, including the W. M. Keck telescopes in Ha- waii, the Gemini telescopes (Hawaii and Chile), the Large Binocular Telescope (Ari- zona) and the Automated Planet Finder (California). The data collected were then processed using statistical validation models. The result from all this was that 104 of the initial 197 candidates were validated and that 57 of them are located in multi-planet systems. 30 other candidates have instead turned out to be false positives, while for the remaining 63 some uncertainties exist and further study will be needed to deter- mine their nature. Among all the newly val- idated planets, 37 have diameters less than twice that of the Earth and 5 of them re- ceive from their star the same amount of energy that our planet receives from the Sun. The most interesting finding emerging from the work of Crossfield’s team involves the planetary system of a red dwarf star of spectral type M, called K2-72, 181 light-years away, in the constellation Aquarius. Around this star rotate 4 planets, arranged on much smaller orbits than that of Mercury. All 4 have sizes comparable to that of the Earth (from 1.2 to 1.5 Earth diameters) and almost