The search for potentially habitable exoplanets similar to Earth, as well as the search for extraterrestrial, life, probably just got more difficult.

New research suggests that planets orbiting the habitable zones of M dwarf stars, the most common type of star in the universe, probably don't have any water left on them after it all burned up early on. And as water is the primary requirement for sustaining life, that means those planets aren't really habitable.

These exoplanets, or planets outside our solar system, seem a lot like Earth when viewed from afar. They have similar masses and reside in their star's habitable zones, that sweet spot where a planet sits in orbit around its star where it can sustain liquid water on its surface.

However, researchers at the University of Washington think that these planets aren't really like Earth at all, their water long having evaporated and their atmospheres depleted.

This is due to the fact that many of these exoplanets orbit stars called M dwarfs, or stars with low mass. In computer simulations, researchers discovered that planets orbiting these stars probably had both their water and atmospheres burned up early in their formation.

M dwarf stars don't fully collapse into gas as quickly as stars with higher masses because they have less gravity. It often takes them hundreds of millions of years to collapse. This means that planets form around them when they are still extremely bright and giving off massive amounts of heat.

"That's not good for habitability, since these planets are going to initially be very hot, with surface temperatures in excess of a thousand degrees," says Rodrigo Luger, a doctoral student at the University of Washington. "When this happens, your oceans boil and your entire atmosphere becomes steam."

M dwarf stars also give off a lot of X-ray and ultraviolet light which heats up planets' atmospheres, causing gas to expand quickly, forcing it away from the planet. This also causes water to split off into its basic elements, hydrogen and oxygen. Hydrogen, which is lighter, flies off into space. That leaves entirely too much oxygen near the surface of the planet, which is not conducive for life forming there.

The working title of the paper based on the research is "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," says Luger.

[Photo Credit: NASA/JPL-Caltech]