Free Astronomy Magazine November-December 2019

NOVEMBER-DECEMBER 2019 T he Hertzsprung- Russell diagram. Terrestrial life can evolve for billions of years only in the lower part of the main sequence. the inner limit – any closer and a runaway greenhouse effect would be activated. Such an inner position in the habitable zone will result in our planet becoming unlivable long before the Sun completes its transit through the main sequence. Indeed, our star will gradually become hotter, and researchers estimate that within 1-2 billion years the oceans will evaporate. Another Earth, a little less close to another Sun, would already be superhabitable. It would be even more so if it had a larger sur- face area than our planet, with even more surface water collected in a greater number