Exoplanets that once resembled the gas giant Neptune could see their oceans blown away by their local stars, leaving behind a potentially-habitable core, a new study from the University of Washington suggests.

Class M dwarf stars make up the majority of stellar bodies in the Milky Way galaxy. These suns are smaller and cooler than our own local star. Alien life is thought to be most likely to form on planets orbiting at distances from their stars where liquid water could form on their surfaces. This range of distances is known as habitable zone, or Goldilocks zone, around an alien star. Planets circling around class M dwarfs would need to orbit close to their stars in order to receive enough warmth for liquid water to be present on the surface of the world.  

Mini-Neptunes could form with thick atmospheres, far from the forces streaming from their parent star, but these planets can spiral inward toward their suns, driven by tidal forces.

"They are initially freezing cold, inhospitable worlds. But planets need not always remain in place. Alongside other processes, tidal forces can induce inward planet migration," Rodrigo Luger from the University of Washington said.

As planets migrate closer to their stellar parents, powerful streams of radiation can push away the thick gases, leaving behind a "habitable evaporated cores."

Tidal forces are felt on Earth, as gravitation from both the Sun and Moon raised both land and sea levels.

"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.

These tidal forces result in flexing of the bodies involved, with each adopting a shape similar to an egg, with the two pointy ends facing one another. This process results in friction, producing heat which can warm the world, the study found.

Tidal forces can also result in increased vulcanism, which can result in the release of vast quantities of material into the atmosphere of the alien world. This could produce a greenhouse effect, further warming the planet. However, if this process continues unabated, it could lead to a runaway greenhouse effect, boiling away any oceans in which life appeared.

Many conditions would have to be met for even primitive life to form and thrive on other worlds, at any time in the past, much less thrive to this day. However, these worlds could prove to be a possible home for alien life.

Investigation of the possible habitability of mini-Neptunes was profiled in the journal Astrobiology.

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