by Mike Wall
August 22, 2018
This artist's conception
similarities to Earth.
left: Kepler-22b, Kepler-69c,
Kepler-452b, Kepler-62f and Kepler-186f.
line is Earth itself.
worlds" are incredibly common throughout the Milky Way
galaxy, a new study suggests.
Midsize alien planets - those two to four times larger than Earth -
tend to harbor huge amounts of water, according to the research.
Indeed, some of these exotic worlds are probably up to 50 percent
water by weight.
(Our seemingly wet Earth,
by contrast, is just 0.02 percent water by weight.)
"Our data indicate
that about 35 percent of all known
exoplanets which are bigger than Earth should be
water-rich," study leader Li Zeng, a postdoctoral fellow in the
Department of Earth and Planetary Sciences at Harvard
University, said in a statement.
"It was a huge
surprise to realize that there must be so many water worlds."
Li Zeng and his
colleagues analyzed data gathered by NASA's
Kepler space telescope, which has
discovered about 70 percent of the 3,800 known exoplanets to date,
and the European Space Agency's
The study's (Growth
Model Interpretation of Planet Size Distribution) researchers
used this information to develop a model explaining the relationship
between an exoplanet's mass and its radius.
"The beauty of the
model is that it explains just how composition relates to the
known facts about these planets," said Zeng, who presented the
results in Boston Friday (Aug. 17) at the Goldschmidt
Conference, a high-profile annual geochemistry meeting.
The team's model suggests
that alien worlds about 1.5 times the size of Earth or smaller tend
to be rocky, whereas those that are a bit bigger are generally
(The planets in the next
size class up are primarily gaseous. For example, Neptune,
the smallest gas giant in our solar system, is about four times
wider than Earth.)
But these alien water worlds are not just flooded versions of a
"This is water, but
not as [it is] commonly found here on Earth," Zeng said.
temperature is expected to be in the 200 to 500º Celsius [390 to
930º Fahrenheit] range. Their surface may be shrouded in a
water-vapor-dominated atmosphere, with a liquid water layer
Moving deeper, one
would expect to find this water transforms into high-pressure
ices before reaching the solid, rocky core."
Exoplanet Survey Satellite (TESS),
which launched in April, will likely find lots of these water
worlds, Zeng added.
And the agency's $8.9
billion James Webb Space Telescope (JWST),
which is scheduled to lift off in 2021, may be able to characterize
the atmospheres of some of these worlds, he said.
"It's amazing to
think that the enigmatic, intermediate-size exoplanets could be
water worlds with vast amounts of water," TESS Deputy Science
Director Sara Seager, a planetary scientist at the Massachusetts
Institute of Technology, said in the same statement.
"Hopefully, atmosphere observations in the future - of thick
steam atmospheres - can support or refute the new findings,"
added Seager, who was not involved in the study.