16 ASTRONOMY • JANUARY 2020
QUANTUM GRAVITY
Astronomers have uncovered
water vapor in the atmosphere of
a super-Earth exoplanet, called K2-18 b,
orbiting within the habitable zone of its
star. That position means temperatures
may be warm enough for liquid water to
also exist on the rocky world’s surface,
potentially even forming a global ocean.
The discovery, published September 11
in Nature Astronomy, was made by ana-
lyzing data taken with NASA’s Hubble
Space Telescope in 2016 and 2017. It
is the first detection of water vapor in
the atmosphere of a planet in its star’s
habitable zone. Thanks to a sophisticated
algorithm, the researchers were able to
tease out the undeniable fingerprint of
water vapor in the atmosphere of K2-18 b,
but they couldn’t determine exactly how
much is there. In their paper, they give
a broad estimate for the abundance of
water in the atmosphere — somewhere
between 0.01 and 50 percent.
Based on the find, the team says the
planet is a prime target for follow-up
studies on habitability. “This is the only
planet right now that we know outsidethe solar system that has the correct
temperature to support water. It has
an atmosphere, and it has water in it,
making this planet the best candidate for
habitability that we know right now,” said
lead author Angelos Tsiaras of University
College London in a press conference.
K2-18 b sits some 110 light-years away
in the constellation Leo. It orbits a red
dwarf star roughly one-third the mass of
our own Sun. Red dwarfs are infamous
for emitting powerful f lares, but the
researchers point out that this particular
star appears surprisingly docile.
This bodes well for the water-bearing
planet, as its 33-day orbit brings it about
twice as close to its star as Mercury is
to the Sun. “Given that the star is much
cooler than the Sun, in the end, the
planet is receiving similar radiation to
the Earth,” said Tsiaras. “And based
on calculations, the temperature of the
planet is also similar to the temperature
of the Earth.”
Although K2-18 b f launts some of the
most Earth-like features observed in an
exoplanet so far — water vapor, habitableWater vapor found on a potentially
habitable exoplanet
temperatures, and a rocky surface — the
researchers point out the world is still vastly
different. K2-18 b is roughly twice the
diameter of Earth and about eight times as
massive. This puts K2-18 b near the upper
limit of what we call a super-Earth —
planets ranging from a little more massive
than Earth to about 10 Earth masses.
With a density about twice that of
Neptune, K2-18 b has a composition most
similar to Mars or the Moon. Researchers
say it’s feasible that K2-18 b could be a
water world with a global ocean covering
its entire surface. However, they cannot
say for sure. It is also feasible that the
world is more Neptune-like than Earth-
like, with an extended atmosphere and
a limited solid surface.
To pin down exactly how much water
is really on K2-18 b, the researchers say,
we’ll have to wait for the next generation
of advanced space telescopes to come
online. That includes instruments like
NASA’s James Webb Space Telescope
and the European Space Agency’s
Atmospheric Remote-sensing Infrared
Exoplanet Large-survey Telescope. — J.P.WET WORLD? Exoplanet K2-18 b orbits a red
dwarf star and has an extended atmosphere
containing at least some water vapor, as seen
in this artist’s concept. NASA/ESA/HUBBLE/M. KORNMESSER