Free Astronomy Magazine November-December 2019

NOVEMBER-DECEMBER 2019 I n this infographic, the most significant stages of the evo- lution of life on our planet are highlighted. Under similar initial conditions, the first stages may have also occurred on other planets. Below, the astrophysi- cist Su-Shu Huang, who coined the term “habitable zone.” face of planets within it. Huang considered a fundamental factor to establish the dura- tion of the habitability of planets, namely, the time of permanence of the associated stars in the main sequence. Indeed, the more this time is extended, the greater the chance that life may appear and evolve. The fossil and chemical indicators tell us that life on Earth appeared about one billion years after the formation of the planet, and that, to reach an important biodiversity comparable to the current one, terrestrial life took another 3 billion years to evolve. It is possible that this timing is also valid for other planetary systems, but the clear de- pendency is on the type of star, which may not necessarily be identical to the Sun. Ap- preciably more massive stars have a perma- nence time in the main sequence shorter than that necessary for life to transform the environment that hosts them, at least up to being detectable by other worlds. Even at that time, it was clear that the natural starting point for characteriz- ing the habitability of a world is the cal- culation of the en- ergy received from its star. A little over a century later, an American astrophysi- cist of Chinese de- scent, Su-Shu Huang, proposed at a con- ference of the Astro- nomical Society of the Pacific the term “habitable zone” to define the region around the star where liquid water can exist on the sur-