Free Astronomy Magazine September-October 2019

7 ASTROBIOLOGY number of “candidate Earths” have been discovered around this type of dwarf. Among the 18 Earth-size exoplanets known today, one was discovered around a G dwarf, two around K dwarfs, and 15 around M dwarfs. At first glance, it doesn’t matter if host stars are solar-type dwarfs or red dwarfs. What is important is that the planets are in the habitable zone, although in the case of M dwarfs, this means being very close to the star surface. Conversely, especially in recent years and through mostly theo- retical studies, researchers have realized that things are not as obvious as they ap- peared up-to a few years ago. If the goal is to discover potentially habitable or in- habited planets, the worst places to look for them might just be M dwarf systems. They have evolutionary characteristics that conflict with the possibility of hosting life. During the first billion years of existence, in their pre-main sequence phase, these stars are much brighter and warmer than they are after reaching the main sequence. Moreover, for a few billions of years after F amiliar bright stars and se- lected exoplane- tary host stars within 65 light- years of our Sun. Names of host stars appear in turquoise; names of stars without known planets appear in violet. [Backalley- astronomy]