Aliens could be hiding on almost any of the Milky Way’s roughly 100 billion planets, but so far, we haven’t been able to find them (dubious claims to the contrary notwithstanding). Part of the problem is that astronomers don’t know exactly where to look or what to look for. To have a chance of locating alien life-forms — which is like searching for a needle that may not exist in an infinitely large haystack — they’ll have to narrow the search.
Astronomers hoping to find extraterrestrial life are looking largely for exoplanets (planets outside Earth’s solar system) in the so-called “Goldilocks zone” around each star: a distance range in which a planet is not too hot and not too cold, making it possible for liquid water to exist on the surface. But after studying our own world and many other planetary systems, scientists have come to believe that many factors other than distance are key to the development of life. These include the mix of gases in the atmosphere, the age of the planet and host star, whether the host star often puts out harmful radiation, and how fast the planet rotates — some planets rotate at a rate that leaves the same side always facing their star, so one hemisphere is stuck in perpetual night while the other is locked into scorching day. This makes it a complex problem that scientists can start to tackle with powerful computers, data and statistics. These tools — and new telescope technology — could make the discovery of life beyond Earth more likely.
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