What planets should we search to find alien life? – Astronomy Magazine

Just add starlightWhen contemplating what types of planets might harbor life, scientists first consider their host stars. High-mass stars live fast and die young, blazing with tremendous energy but burning out in just a few million to a few hundred million years. Thats probably not long enough for life to really get going on any orbiting planets.Stars with just a few solar masses or lower produce less radiant energy and thus have smaller habitable zones, defined as the region around a star where a planet can sustain temperatures conducive to hosting liquid water on its surface. But these stars longer lifespans give their planets billions of years for any primitive life to evolve into complex forms. The Sun, for example, has given terrestrial life plenty of time, and life has needed that time to express its full potential. Multicellular life didnt emerge until Earth had been around for roughly 3.5 billion years. And advanced plants and animals didnt arise until Earth was 4 billion years old, after oxygen had built up in the atmosphere. This highly reactive element provided the required energy for more dynamic metabolism. A major question mark centers around the lowest-mass stars, red dwarfs. They comprise three-fourths of our galaxys stars and live for trillions of years. But their feeble energy outputs ensure small habitable zones nestled very close in to the star. This proximity subjects planets to bombardment from powerful flares, which can erode their atmospheres. But red dwarfs calm down over time, potentially giving planets time to regenerate a gaseous envelope. Most of these stars habitable-zone planets are tidally locked, with one side always facing the star and the other in permanent night. But thick greenhouse-gas atmospheres and raging winds could smooth out the hemispheric temperature differences that result from tidal locking.Even the concept of the habitable zone is overly simplistic. As traditionally conceived, the Suns habitable zone currently encompasses Earth and extends to roughly the distance of Mars. And in fact, NASA missions have returned overwhelming evidence that Mars once had rivers, lakes, and oceans of liquid water.

Whether a planet has liquid water on its surface is a function of both surface temperature and atmospheric pressure. For example, water boils on Mars today despite the cold temperatures because of its very low atmospheric pressure. Mars originally had a thicker atmosphere, which allowed it to have liquid water on its surface. But the Red Planet lost most of its atmosphere long ago, partially because it is a low-mass planet. If Mars had higher mass, it could have hung on to its atmosphere for a much longer period of time, perhaps all the way to the present.

That raises intriguing questions about whether planets of certain sizes and masses are best suited for life. Ren Heller of the Max Planck Institute for Solar System Research has argued that Earth is not necessarily the most habitable type of planet. After all, large tracts of Earths surface are nearly devoid of life, such as deserts, the poles, and nutrient-poor oceanic regions. Everybody is crazy about the most Earth-like planet, says Heller. But from a more general, more reflective perspective on the question of life in the universe, I dont see any reason why Earth should be the optimal place for life to form and evolve. There might even exist planets that have more benign environments for life to develop and to diversify.