Most of our galaxy’s stars are M dwarfs or low mass stars. These are smaller and dimmer than our sun and their habitable zones are closer in. This makes them good targets in the search for potentially habitable planets.

As the next round of exoplanet searching missions begins, scientists expect to find many Earth-like planets around these stars. However, researchers would never consider a planet like Neptune to be potentially habitable. That may be about to change.

Neptune is “mostly made of a very thick, very hot combination of water (H2O), ammonia (NH3), and methane (CH4) over a possible heavier, approximately Earth-sized, solid core,” according to NASA.

A recent paper in the journal Astrobiology shows that “mini-Neptunes” could be pulled into the habitable zone and made gas free and potentially habitable given the right solar conditions.

“There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets. Two important ones are strong tidal effects and vigorous stellar activity,” said University of Washington doctoral student and co-author of the paper Rodrigo Luger in a statement.

The two things that are known to damage the habitability of planets are tidal forces and vigorous stellar activity. According to Luger and his co-author , research assistant professor, Rory Barnes it could be those very forces which could combine to make mini-Neptunes habitable.

Tidal forces are gravitational tugs on planets from their host stars. These forces are stronger on the side of the planet facing the star. Over time this uneven pull can stretch a world into an egg like shape and can pull it closer to the star.

“This is the reason we have ocean tides on Earth, as tidal forces from both the moon and the sun can tug on the oceans, creating a bulge that we experience as a high tide. Luckily, on Earth it’s really only the water in the oceans that gets distorted, and only by a few feet. But close-in planets, like those in the habitable zones of M dwarfs, experience much stronger tidal forces,” said Luger.

In addition to stretching and ocean tides, the friction can cause a planet’s interior to give off a large amount of energy. This energy can drive volcanic activity, which can warm a planet, thawing ice and causing water to flow. In extreme cases, excessive energy can boil away a planets ocean and cause runaway greenhouse effects, ending a planets potential for habitability.

Planets can also lost their potential for habitability through intense stellar activity. When M dwarfs are young they emit large amounts of ultraviolet radiation and X-rays. That radiation can heat a planet’s upper atmosphere to the point where strong winds erode the atmosphere away entirely.

Using computer models however, the researchers found that these same forces can cause mini-Neptunes to vent gas into space, leaving them rocky and Earth-like and potentially habitable.

“The bottom line is that this process — the transformation of a mini-Neptune into an Earthlike world — could be a pathway to the formation of habitable worlds around M dwarf stars. Either way, these evaporated cores are probably lurking out there in the habitable zones of these stars, and many may be discovered in the coming years,” said Luger.

Over the next several years the TESS mission, the James Webb Space Telescope, the European Extremely Large Telescope and other missions and projects are likely to reveal thousands of new planets. It will be important to researchers to know which planets might be habitable.

The mini-Neptune idea is one possible path to habitability that could have easily been overlooked.