Watery exoplanets are more common than we thought, but not very accommodating

A new study suggests that watery exoplanets might be more common than we thought, but this doesn't necessarily mean they are habitable.
By Tyler MacDonald | Aug 21, 2018
New research suggests that watery planets outside of the Milky Way might be more common than we thought, comprising approximately 35 percent of exoplanets that are two to four times the size of the Earth. The finding stems from data gained from the Kepler Space Telescope and Gaia mission that suggest that plenty of planet masses are half water, compared to the 0.02 percent water that Earth contains.

Water is a necessary ingredient for extraterrestrial life and a basic component of biology, which is why it is so high on the list of exoplanet properties that astrobiologists look for when searching for life on other planets.

The new study found that over 4,000 candidate or confirmed exoplanets are 1.5 to 2.5 times the radius of the Earth. But when the team attempted to model the insides of these Earthlike planets, they found something interesting.

"We have looked at how mass relates to radius, and developed a model which might explain the relationship," says Li Zeng, who led the research team. "The model indicates that those exoplanets which have a radius of around x1.5 Earth radius tend to be rocky planets (of typically x5 the mass of the Earth), while those with a radius of x2.5 Earth radius (with a mass around x10 that of the Earth) are probably water worlds."

This means that roughly 35 percent of exoplanets larger than Earth are water worlds. However, most of these environments are more akin to pressure cookers than ideal places for life.

"This is water, but not as commonly found here on Earth," Zeng said. "Their surface temperature is expected to be in the 200 to 500 degree Celsius (392 to 932 F) 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 we reaching the solid rocky core. The beauty of the model is that it explains just how composition relates to the known facts about these planets.

"Our data indicate that about 35 percent of all known exoplanets which are bigger than Earth should be water-rich. These water worlds likely formed in similar ways to the giant planet cores (Jupiter, Saturn, Uranus, Neptune), which we find in our own solar system. The newly-launched TESS mission will find many more of them, with the help of ground-based spectroscopic follow-up. The next generation space telescope, the James Webb Space Telescope, will hopefully characterize the atmosphere of some of them. This is an exciting time for those interested in these remote worlds."

The findings were presented at the Goldschmidt Conference.

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