Astronomers have confirmed the existence of a new class of planet:
a waterworld with a thick, steamy atmosphere.
The exoplanet GJ 1214b is a so-called "Super Earth" - bigger than
our planet, but smaller than gas giants such as Jupiter.
Observations using the Hubble telescope now seem to confirm that
a large fraction of its mass is water.
The planet's high temperatures suggest exotic materials might exist there.
"GJ 1214b is like no planet we know of," said lead author Zachory Berta,
from the Harvard Smithsonian Center for Astrophysics.
The planet was discovered in 2009 by ground-based telescopes. It is
about 2.7 times the Earth's diameter, but weighs almost seven times
as much. It orbits its red-dwarf star at a distance of just two million km,
meaning temperatures on GJ 1214b probably reach above 200C.
In 2010, astronomers released measurements of its atmosphere. These
suggested that GJ 1214b's atmosphere was probably made up of
water, but there was another possibility - that the planet was covered
in a haze, of the type that envelopes Saturn's moon Titan.Hot ice
Mr Berta and his colleagues used the Hubble Space Telescope's wide-field
camera to study the planet as it crossed in front of its star - a transit.
During these transits, the star's light is filtered through the planet's
atmosphere, giving clues to the mixture of gases present.
can give clues to gases in the planet's atmosphere
The researchers said their results are more consistent with a dense
atmosphere of water vapour, than one with a haze.
Calculations of the planet's density also suggest that GJ 1214b has
more water than Earth. This means the internal structure of this world
would be very different to that of our own.
"The high temperatures and pressures would form exotic materials like
'hot ice' or 'superfluid water', substances that are completely alien to
our everyday experience," said Dr Berta.
The planet's short distance from Earth makes it a likely candidate for
follow-up observations with the James Webb Space Telescope, which
may launch by the end of this decade.
The study has been accepted for publication by the Astrophysical Journal.