Free Astronomy Magazine November-December 2019

42 NOVEMBER-DECEMBER 2019 EXOPLANETS Therefore, even if a star just 1.5 times more massive than the Sun (the F-type upper mass limit) can remain stable for about 4 billion years, that still may not be sufficient to develop an Earth- type biosphere. On the contrary, signifi- cantly less massive stars, such as K-type stars (from 0.45 to 0.8 solar masses), can guarantee a habitable zone for tens of bil- lions of years, allowing the development of life forms perhaps unimaginable. Planets and large moons orbiting these stars are likely to fall into the category of so-called “superhabitable worlds,” an expression coined in 2013 by René Heller (Max Planck Institute for Solar System Research) and John Armstrong (Weber State University), to indi- cate those planetary bodies that, due to their physical and dynamic features, can be even more habitable than the Earth. Because of our inveterate anthropocentrism, we take for granted that Earth is the best place in the universe to live. This is true only for the living species on our planet, which over billions of years have evolved by adapt- R ené Heller and John Armstrong, the first astronomers to propose the concept of “su- perhabitable worlds.” Below, a comparison of the average sizes of stars of differ- ent spectral classes. The Sun is a G-type star, while K-type stars are those believed to offer the best condi- tions for habit- ability. ing to any available habitats, sometimes even transforming entire ecosystems to their own advantage. It is therefore unlikely to find another planet that is equally well- suited to us. But this does not mean that the Earth is the “best” possible planet for other potential forms of life: if Earth had had other physical and dynamic features, per- haps life would have been even more luxu- riant and diversified. Still, in 2013, a team of researchers led by as- trobiologist Ravi Kumar Kopparapu (NASA Goddard Space Flight Center) calculated that the habitable zone around stars very similar to the Sun extends, on average, between 0.99 and 1.7 astronomical units (in literature, we also find markedly different values). This is to say that the Earth orbits very close to