Up to 35 percent of exoplanets may be ocean worlds

Temperatures and pressures of these worlds may not be conducive to harboring life.

Somewhere between 30 and 35 percent of all exoplanets may be ocean worlds two to four times the size of Earth, according to a team of scientists who presented their findings at the 2018 Goldschmidt Conference of the Geochemical Society.

The researchers analyzed data returned by NASA’s Kepler Space Telescope and Gaia mission and determined that many super-Earths, planets several times larger than our own, likely have compositions that are up to 50 percent water. In contrast, Earth’s composition is just 0.02 percent water.

Two types of super-Earths were identified in the study. Those with 1.5 times Earth’s radius or less are likely to be rocky while those with 2.5 times Earth’s radius are likely to be icy like Uranus and Neptune in our solar system.

Large super-Earths likely have water vapor atmospheres and surface oceans with extreme pressures, whose temperatures may range between 390 and 930 degrees Fahrenheit (200 and 500 degrees Celsius). Structurally, they may resemble gas giants, with cores far beneath their dense atmospheres.

In spite of having high amounts of water, the latter group of planets are not likely to be habitable.

“It was a huge surprise to realize that there must be so many water worlds, said Li Zeng of Harvard University, who led the study.

“This is water, but not as commonly found here on Earth. Their surface temperature is expected to be in the 200 to 500 degree Celsius range. Their surface may be shrouded in a water-vapor-dominated atmosphere, with a liquid water layer underneath. Moving deeper, one would expect to find this water transforms into high-pressure ices before reaching the solid, rocky core,” Zeng said.

He acknowledged that life could develop in layers close to these planets’ surfaces if their pressures, temperatures, and chemical conditions are just right.

The larger water worlds likely formed in processes similar to those that formed the cores of Uranus and Neptune, Zeng added.

NASA’s Transiting Exoplanet Survey Satellite (TESS), which launched earlier this year, and the James Webb Space Telescope (JWST), now scheduled for launch in 2021, will likely find many more water worlds, he noted. Discoveries by space observatories will be followed by ground-based spectroscopic observations.

JWST will enable scientists to identify the components in exoplanets’ atmospheres.

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