Two factors that are believed to prevent life from forming could actually do the opposite on certain planets that orbit low-mass stars, and these potentially-habitable planets could be discovered in upcoming years.
New research suggests tidal forces and vigorous stellar activity could combine to transform uninhabitable gaseous "mini-Neptunes" into potentially-habitable gas-free planets, the University of Washington reported.
These planets orbit low-mass stars, also called M dwarfs that are much smaller and dimmer than our own Sun; this also means they have closer habitable zones.
"There are many processes that are negligible on Earth but can affect the habitability of M dwarf planets," said UW doctoral student Rodrigo Luger. "Two important ones are strong tidal effects and vigorous stellar activity."
A tidal force is the gravitational tug on a planet imposed by its host star, which can influence the planet's shape.
"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," Luger said. "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."
The stretching caused by the tidal forces generates an enormous amount of energy that can spur surface volcanism and heat the planet into a "runaway greenhouse" state, preventing life from prospering. Vigorous stellar activity can also inhibit chances of life on a planet because when M dwarfs are young they emit harmful X-rays.
Using computer models the researchers determined tidal forces and atmospheric escape can shape planets that are born as mini-Neptunes into gas-free and potentially-habitable worlds. Mini-Neptunes tend to form far away from their host star where ice molecules combine with hydrogen and helium to form the planet's icy rock core surrounded by an immensely gaseous atmosphere.
"They are initially freezing cold, inhospitable worlds," Luger said. "But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration." This process can bring mini-Neptunes into their host star's habitable zone, where they are exposed to much higher levels of X-ray and ultraviolet radiation"
The phenomenon can lead to a rapid escaping of gas, leaving behind a hydrogen-free planet within its star's habitable zone.
"Such a planet is likely to have abundant surface water, since its core is rich in water ice," Luger said. "Once in the habitable zone, this ice can melt and form oceans."
The researchers noted a number of other conditions would need to be present in order for life to form under these conditions, such as the development of an atmosphere that can recycle nutrients across the planet.
"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," Luger said. "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."
The findings wer published in a recent edition of the journal Astrobiology.
