Two phenomena known to inhibit the potential habitability of planets – tidal forces and vigorous stellar activity – could combine to transform uninhabitable gas giants into potentially habitable terrestrial planets, says a group of scientists led by Rodrigo Luger from the University of Washington, Seattle.
Terrestrial exoplanet and its moons. Image credit: Luciano Mendez / CC BY-SA 3.0.
Most of the stars in the Milky Way Galaxy are so called M-dwarfs. Smaller and dimmer than our Sun, they make good targets for finding and studying potentially habitable planets.
Astronomers expect to find many Earth-size and super-Earth planets in the habitable zones of these stars in coming years, so it’s important to know if they might indeed support life.
“There are many processes that are negligible on Earth but can affect the habitability of planets orbiting M dwarfs. Two important ones are strong tidal effects and vigorous stellar activity,” said Luger, who is the lead author on the study published in the journal Astrobiology.
A tidal force is a star’s gravitational tug on an orbiting planet, and is stronger on the near side of the planet, facing the host star, than on the far side, since gravity weakens with distance. This pulling can stretch a planet into an ellipsoidal or egg-like shape as well as possibly causing it to migrate closer to its 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,” Luger said.
This stretching causes friction in a planet’s interior that gives off huge amounts of energy. This can drive surface volcanism and in some cases even heat the planet into a runaway greenhouse state, boiling away its oceans, and all chance of habitability.
Vigorous stellar activity also can destroy any chance for life on planets orbiting low-mass stars.
M dwarfs are very bright when young and emit lots of high-energy X-rays and UV radiation that can heat a planet’s upper atmosphere, spawning strong winds that can erode the atmosphere away entirely.
Using computer models, Luger and his colleagues found that tidal forces and atmospheric escape can sometimes shape planets that start out as mini-Neptunes into potentially habitable rocky worlds.
Mini-Neptunes “are initially freezing cold, inhospitable worlds. 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 UV radiation,” Luger said.
This can in turn lead to rapid loss of the atmospheric gases to space, sometimes leaving behind a hydrogen-free, rocky world smack dab in the habitable zone.
The scientists call such planets ‘habitable evaporated cores.’
_____
Luger R et al. 2015. Habitable Evaporated Cores: Transforming Mini-Neptunes into Super-Earths in the Habitable Zones of M Dwarfs. Astrobiology 15 (1): 57-88; doi: 10.1089/ast.2014.1215
