A research team led by Harvard University astronomer Li Zeng has shown that water is likely to be a major component of extrasolar planets which are between 2 to 4 times the size of Earth. The scientists presented their results August 17 at the Goldschmidt 2018 conference in Boston, Massachusetts.
An artist’s impression of a water-world exoplanet. Image credit: Sci-News.com.
The 1992 discovery of exoplanets orbiting other stars has sparked interest in understanding the composition of these planets to determine, among other goals, whether they are suitable for the development of life.
Now a new analysis of data from NASA’s Kepler Space Telescope and ESA’s Gaia satellite indicates that many of the known exoplanets may contain as much as 50% water. This is much more than the Earth’s 0.02% (by weight) water content.
“It was a huge surprise to realize that there must be so many water-worlds,” Dr. Zeng said.
Many of the 4,000 confirmed or candidate exoplanets discovered so far fall into two size categories: those with the planetary radius averaging around 1.5 that of the Earth, and those averaging around 2.5 times the radius of the Earth.
“We have looked at how mass relates to radius, and developed a model which might explain the relationship,” Dr. Zeng said.
The team’s model indicates that those exoplanets which have a radius of around 1.5 Earth radii tend to be rocky planets (typically 5 times the mass of the Earth), while those with a radius of 2.5 Earth radii (with a mass around 10 times that of the Earth) are probably water worlds.
“This is water, but not as commonly found here on Earth,” Dr. Zeng said.
“The surface temperature of these planets 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 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% of all known exoplanets which are bigger than Earth should be water-rich,” Dr. Zeng said.
“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.”
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Li Zeng et al. Growth Model Interpretation of Planet Size Distribution. Goldschmidt 2018 Abstracts, 01f: 309
