Exoplanets covered in ocean may not be able to survive long enough to support alien life, if the stars they orbit strip away their atmospheres. CC0 Creative Commons

Alien worlds covered in water might not be comfortable for living things because some will have their atmospheres stripped from them in solar storms.

Scientists consider an atmosphere and liquid water to be ingredients for life, but finding them existing together on a planet outside of Earth has proven challenging. A team of researchers used computer simulations to test how atmospheres composed mostly of water vapor would hold up in the face of stellar wind — the gas and charged particles blown out from a star toward the planets — and other space events, finding that certain star-planet matchups could kill off an atmosphere before life has a chance to form.

“Ocean planets, also referred to as water worlds … are anticipated to be volatile-rich, and possess oceans that are conceivably hundreds of kilometers deep,” according to a study in the Astrophysical Journal.

That’s compared to rocky planets in the habitable zones of their stars, the distance at which the temperature is just right to support water as a liquid, rather than a solid or gas. Those planets may also contain water.

In examining worlds that are completely covered in ocean, the researchers found that the ones closely orbiting certain stars could be in trouble. Although their simulations suggested that planets with “Earth-like oceans” would not have their atmospheres stripped and their water evaporated over timeframes of billions of years, exoplanets in the habitable zones of M-dwarfs, a class of red dwarf stars, would see a much more rapid change — “the escape rate (of water ions) will be 2-3 orders of magnitude higher than on Earth even without considering space weather effects,” the study says. Part of that has to do with how close in a red dwarf’s habitable zone is, putting an exoplanet in the line of fire from stellar material despite their comfortable temperatures. “Thus, planets orbiting these stars could have their oceans depleted over [billion-year] timescales, especially when extreme space weather events are taken into account.”

A billion years sounds like a long time, but it is not long at all in the grand scheme of life developing and evolving. Compared to the universe’s nearly 14 billion years, Earth is only estimated to be about 4.5 billion years old. And life is thought to have appeared on Earth about 3.8 billion years ago.

“Rapid desiccation would have important consequences for the evolution of life on such planets, especially with regard to oceanic and coastal biodiversity, productivity, and food webs,” the study says.

Red dwarf stars are the most common ones in the universe, so these simulations could potentially apply to many exoplanets and rule them out as worlds that are capable of hosting extraterrestrial life.

“One of the critical factors in determining if a planet could really be habitable is the presence of an enduring atmosphere,” the Harvard-Smithsonian Center for Astrophysics explained in a statement. “The deep oceans on a water world offer a reservoir for water vapor for its atmosphere, and so scientists have been trying to calculate how stable an exoplanet’s ocean and atmosphere are, especially to effects like evaporation by winds from the star.”

The information from these new simulations could help scientists narrow down their search for alien life and their understanding of how planets and the water bodies on them evolve. As the study points out, water is crucial to Earth’s plate tectonic system, through which our planet’s surface and interior changes and recycles. There is also evidence that Earth was once much more covered in ocean than it is today.

That may not be a rare case. Research has suggested that exoplanets with water may be up to 90 percent covered in ocean, compared to the 70 percent of Earth’s surface. That has to do with how much space is available in its ocean basins to store water before it floods and submerges any landmasses.

The researchers also point out that Mars and Venus are believed to have once supported oceans.