In the hunt for planets outside the Solar System that could be habitable, astronomers often look for exact Earth clones, which makes sense as we know our planet supports life. But what if some planets deemed inhospitable are actually friendlier than they appear?

That’s what one team of scientists from KU Leuven in Belgium are proposing in a paper published in the Monthly Notices of the Royal Astronomical Society. They said that some planets with rotations of less than 12 days may have unique climate systems that funnel heat around the planet, providing unexpectedly habitable environments.

In the research, the team ran 165 climate simulations for planets that were tidally locked to red dwarf stars – meaning one of their hemispheres always faces the star. This means the length of the year matches the length of a day, so one hemisphere is in perpetual sunlight, and the other endless darkness.

Until 1997, astronomers thought that tidally locked planets would be inhospitable, as the temperatures on the day side would be scorching hot, although this was later shown to be wrong. In this study, the astronomers expanded upon previous work and identified a climate system akin to an “air conditioning system,” where heat from the day side is channelled to the night side, keeping surface temperatures in the habitable range.

They identified three climate models for exoplanets with orbital periods less than 12 days. The first describes an eastward equatorial wind jet known as a superrotation, which interferes with atmospheric circulation and stops heat from spreading. In the other two climate models, though, for planets approaching Earth in size and orbiting between six and 10 days, the superrotation can be weakened by westward-moving wind jets at higher altitudes. “The day side [of these planets] would be… a good place for life as we know it,” lead author Ludmila Carone told IFLScience.

^{The two climate systems modeled center and right are thought to be habitable. KU Leuven/Ludmila Carone.}

He said that some climates on these planets could bring temperatures on the day side down to about 340 Kelvin (70°C, 150°F), which crucially is below the boiling point of water. If the orbital period is too fast, or too slow, temperatures can exceed 380K (105°C, 225°F), which would not permit liquid water on the surface. But with the right orbital period and planet size, the air conditioning system could take effect. 

“I think we should definitely consider planets with tight orbits with orbital periods of 10 days and maybe down to six days [to be habitable],” Carone said. “My work also appears to suggest that smaller planets are more habitable than bigger ones.”

As is always the case with these sorts of studies, four words are key: More research is needed. But upcoming telescopes like the James Webb Space Telescope (JWST), due to launch in 2018, could help reclassify planets that were previously thought to be uninhabitable, by studying their atmospheres.

“I'm definitely excited, because we learn so much, just by ‘playing’ around and going to extreme scenarios,” said Carone.