This artist’s concept shows what exoplanet K2-18 b could look like based on science data. K2-18 b, an exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light-years from Earth. A new investigation with NASA’s James Webb Space Telescope into K2-18 b has revealed the presence of carbon-bearing molecules including methane and carbon dioxide. The abundance of methane and carbon dioxide, and shortage of ammonia, support the hypothesis that there may be a water ocean underneath a hydrogen-rich atmosphere in K2-18 b. Credits: Illustration: NASA, CSA, ESA, J. Olmsted (STScI), Science: N. Madhusudhan (Cambridge University)
A new investigation with NASA’s
James Webb Space Telescope into K2-18 b, an exoplanet 8.6 times as massive as
Earth, has revealed the presence of carbon-bearing molecules including methane
and carbon dioxide. Webb’s discovery adds to recent studies suggesting that
K2-18 b could be a Hycean exoplanet, one which has the potential to possess a
hydrogen-rich atmosphere and a water ocean-covered surface.
The first insight into the
atmospheric properties of this habitable-zone exoplanet came from observations with
NASA’s Hubble Space Telescope, which prompted further studies that have since
changed our understanding of the system.
K2-18 b orbits the cool dwarf star
K2-18 in the habitable zone and lies 120 light-years from
Earth in the constellation Leo. Exoplanets such as K2-18 b, which have sizes
between those of Earth and Neptune, are unlike anything in our solar system.
This lack of equivalent nearby planets means that these ‘sub-Neptunes’ are
poorly understood, and the nature of their atmospheres is a matter of active
debate among astronomers.
The suggestion that the sub-Neptune
K2-18 b could be a Hycean exoplanet is intriguing, as some astronomers believe
that these worlds are promising environments to search for evidence for life on
exoplanets.
"Our findings underscore the
importance of considering diverse habitable environments in the search for life
elsewhere," explained Nikku Madhusudhan, an astronomer at the University
of Cambridge and lead author of the paper announcing these results.
"Traditionally, the search for life on exoplanets has focused primarily on
smaller rocky planets, but the larger Hycean worlds are significantly more
conducive to atmospheric observations."
The abundance of methane and carbon
dioxide, and shortage of ammonia, support the hypothesis that there may be a
water ocean underneath a hydrogen-rich atmosphere in K2-18 b. These initial
Webb observations also provided a possible detection of a molecule called
dimethyl sulfide (DMS). On Earth, this is only produced by life. The bulk of
the DMS in Earth’s atmosphere is emitted from phytoplankton in marine
environments.
Spectra
of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless
Spectrograph) and NIRSpec (Near-Infrared Spectrograph), display an abundance of
methane and carbon dioxide in the exoplanet’s atmosphere, as well as a possible
detection of a molecule called dimethyl sulfide (DMS). The detection of methane
and carbon dioxide, and shortage of ammonia, support the hypothesis that there
may be a water ocean underneath a hydrogen-rich atmosphere in K2-18 b. K2-18 b,
8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the
habitable zone and lies 120 light-years from Earth.
Credits: Illustration: NASA, CSA, ESA, R. Crawford
(STScI), J. Olmsted (STScI), Science: N. Madhusudhan (Cambridge University)
The inference of DMS is less robust and
requires further validation. “Upcoming Webb observations should be able to
confirm if DMS is indeed present in the atmosphere of K2-18 b at significant
levels,” explained Madhusudhan.
While K2-18 b lies in the habitable zone,
and is now known to harbor carbon-bearing molecules, this does not necessarily
mean that the planet can support life. The planet's large size — with a radius
2.6 times the radius of Earth — means that the planet’s interior likely
contains a large mantle of high-pressure ice, like Neptune, but with a thinner
hydrogen-rich atmosphere and an ocean surface. Hycean worlds are predicted to
have oceans of water. However, it is also possible that the ocean is too hot to
be habitable or be liquid.
"Although this kind of planet does not exist in our solar system,
sub-Neptunes are the most common type of planet known so far in the
galaxy," explained team member Subhajit Sarkar of Cardiff University. “We
have obtained the most detailed spectrum of a habitable-zone sub-Neptune to
date, and this allowed us to work out the molecules that exist in its
atmosphere.”
Characterizing the atmospheres of
exoplanets like K2-18 b — meaning identifying their gases and physical
conditions — is a very active area in astronomy. However, these planets are
outshone — literally — by the glare of their much larger parent stars, which
makes exploring exoplanet atmospheres particularly challenging.
The team sidestepped this challenge by
analyzing light from K2-18 b's parent star as it passed through the exoplanet's
atmosphere. K2-18 b is a transiting exoplanet, meaning that we can detect a
drop in brightness as it passes across the face of its host star. This is how
the exoplanet was first discovered in 2015 with NASA’s K2 mission. This means
that during transits a tiny fraction of starlight will pass through the
exoplanet's atmosphere before reaching telescopes like Webb. The starlight's
passage through the exoplanet atmosphere leaves traces that astronomers can
piece together to determine the gases of the exoplanet's atmosphere.
"This result was only possible
because of the extended wavelength range and unprecedented sensitivity of Webb,
which enabled robust detection of spectral features with just two
transits," said Madhusudhan. "For comparison, one transit observation
with Webb provided comparable precision to eight observations with Hubble
conducted over a few years and in a relatively narrow wavelength range."
"These results are the product of
just two observations of K2-18 b, with many more on the way,” explained team
member Savvas Constantinou of the University of Cambridge. “This means our work
here is but an early demonstration of what Webb can observe in habitable-zone
exoplanets.”
The team’s results were accepted for
publication in The Astrophysical Journal Letters.
The team now intends to conduct follow-up
research with the telescope's MIRI (Mid-Infrared Instrument) spectrograph that
they hope will further validate their findings and provide new insights into
the environmental conditions on K2-18 b.
"Our ultimate goal is the
identification of life on a habitable exoplanet, which would transform our
understanding of our place in the universe," concluded Madhusudhan.
"Our findings are a promising step towards a deeper understanding of Hycean
worlds in this quest."
The James Webb Space Telescope is the world's premier space science observatory. Webb is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe and our place in it. Webb is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.
Source: Webb Discovers Methane, Carbon Dioxide in Atmosphere of K2-18 b | NASA
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