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These Earth-Like Planets Aren't That Earth-Like After All

The search for habitable planets is complicated when inhospitable worlds look friendly from afar.
​Image: ​Flickr

​We're closer to finding another Earth than we've ever been. In the next decade, there are multiple​ planned missions specifically searching for nearby planets capable of supporting life. Finding these worlds would be a big leap forward in the search for extraterrestrial life.

But some of our nearby planets most likely to be capable of supporting life are actually just barren deathtraps masquerading as hospitable worlds, according to a paper to be​ publishedin Astrobiology.

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See, the universe is really damn big, so if we want to find extraterrestrial life, we have to find a way to narrow our search. One way of doing this is by focusing on "M dwarf" stars; these are low-mass stars, smaller than our sun.

About 40 percent of M dwarfs are expected to have Earth-sized planets within what's called the habitable zone—think Goldilocks, not too hot and not too cold. Since they're lower mass, these stars are a bit dim, which makes that perfect-temperature zone closer in. Planets closer to their star are easier for us to detect, because we often find them by measuring how light changes as they pass in front of their star.

Logically, this seems like a great place to go hunting for habitable planets, and we've already found a few, like the rocky ​planet that orbits an M-class dwarf star 22 lightyears from Earth called GJ 667C (catchy name).

But researchers at the University of Washington have been running computer simulations to study how these planets evolve and they've discovered some of them might be deceiving us.

They could look a lot like Earth from afar — but if you look more closely you'll find that they're really a mirage

Lower mass means lower gravity which means M dwarfs take a longer time than other stars to fully collapse. As planets start to form around these stars, the dwarfs are still super bright: poor timing when you're a planet trying to cool and form oceans.

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"These planets are going to initially be very hot, with surface temperatures in excess of a thousand degrees. When this happens, your oceans boil and your entire atmosphere becomes steam," explained author and doctoral student Rodrigo Luger in a press ​release.

He also explained that M dwarf stars emit high levels of X-ray and ultraviolet light, further turning up the thermostat on their fledgling planets and causing gas in the planets' upper atmospheres to escape into space.

"Many of the planets in the habitable zones of M dwarfs could have been dried up by this process early on, severely decreasing their chance of actually being habitable."

And we might be able to tell these planets were dead ends, except for another process that takes place. Luger discovered the ultraviolet radiation also causes the boiling water on the planet's surface to split up into hydrogen and oxygen. The hydrogen escapes while the oxygen builds up, which can lead us astray when searching for Earth-like planets.

"Searches for life often rely on oxygen as a tracer of extraterrestrial life—so the abiological production of such huge quantities of oxygen could confound our search for life on exoplanets," explained Rory Barnes, an assisted professor who co-authored the paper.

The good news is that computer simulations like this will help us adjust what we're looking for well before we actually start sending probes up on the hunt for new life.

But it's a little troubling to think we may be duped by a planet that's little more than a hunk of rock. Luger said they have given their paper the appropriate working title "Mirage Earths."

"Because of the oxygen they build up, they could look a lot like Earth from afar — but if you look more closely you'll find that they're really a mirage; there's just no water there."