A planet’s lifetime is finite. Even our own cosmic utopia Earth will die out when the Sun reaches later stages of its life. As a star ages, it expands, heating up and shifting its habitable zone further than the orbit of Earth-like planets. As a result, liquid water ceases to exist on these planets, killing any chance of surface life on the planet. In our case, we have about 1.7 billion years left before Earth turns into a terrestrial hot-house like Venus is today. The Sun will be about 118% brighter and hotter than it is today, evaporating out oceans. Considering Earth has been around for about 4.5 billion years, about 70% of its lifetime in the Sun’s habitable zone has already passed.
Scientists believe that the best place to look for intelligent aliens is around small stars called red dwarf stars. These stars are only about 1/5 the size of the Sun, but this results in a habitable lifetime for planets that well exceeds Earth’s. These stars, scientists believe, should be the main targets of SETI (Search for Extraterrestrial Intelligence) missions, since any civilization there doesn’t have the same time constraints as those around large stars. Additionally, NASA’s Kepler mission has turned up evidence of many Earth-like planets possibly located around red dwarfs.
A visual guide to a star’s habitable zone based on its size. Source: Discovery
Kepler gives us information about distance from Earth, orbital period, and size. It does not, however, give us any information about the atmospheric composition, tectonic activity, or tilt of the planet. It also doesn’t help us determine the age of the host star, meaning we do not the evolutionary status of any hypothetical life that exists on these worlds. Using this information, researchers have determined that the exoplanet Gilese 518g is “the most habitable exoplanet found to date”. It orbits a red dwarf star, which means that its habitable conditions are believed to exist for another 5 billion years longer than Earth’s.
There is, of course, the threat of tidal locking occurring quickly on planets that are close to the red dwarf, because its smaller size actually accelerates the process. If tidal locking occurs, life on planet will become impossible.
