#Planets #Life – Old planets always get too hot or cold for life in the end : Age matters. Searching for alien life on planets orbiting older stars may be fruitless because they always become prohibitively hot or cold.
The search for life on other worlds has focused on planets in what’s known as the habitable zone – the ring around stars where it’s the right temperature for liquid water. That has led some to target planets orbiting red dwarf stars, as their smaller size and cooler temperatures mean planets in the habitable zone are closer in, and so easier to spot.
But we should also look for planets whose stars are the right age, regardless of their size, sayShintaro Kadoya and Eiichi Tajika at the University of Tokyo, Japan. Because stars grow brighter with age, planets at the inner edge of the habitable zone eventually enter a “runaway greenhouse mode”, in which their oceans boil away.
Meanwhile, planets at the outer edge lose heat-trapping gases from their atmospheres over time as volcanic activity decreases, so they enter an ice-covered “snowball state”. Kadoya and Tajika built a model of how planets heat or cool over time, and found that both fates set in after about 3 billion years.
Smaller stars don’t actually provide life-friendly environments for any longer than sun-sized stars, says Kadoya. This has implications for planned exoplanet missions like the Transiting Exoplanet Survey Satellite due to launch in 2017, and the Giant Magellan Telescope under construction in Chile, both of which will target smaller red dwarf stars.