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Carbon-containing molecules have been detected in the atmosphere of exoplanet K2-18 b in the habitable zone by an international team of astronomers using data from the James Webb Space Telescope of NASA. These results are consistent with an exoplanet possibly containing an ocean-covered surface in a hydrogen-rich atmosphere. This discovery offers a fascinating glimpse of a planet unlike any other in our solar system and opens up exciting perspectives on habitable worlds elsewhere else in the universe.
A new investigation with NASA’s James Webb Space Telescope of K2-18 b, an exoplanet 8.6 times more massive than Earth, has revealed the presence of carbon-containing molecules, including includes methane and carbon dioxide.Webb’s findings add to recent studies suggesting K2-18 b may be a Hycean exoplanet, likely with a hydrogen-rich atmosphere and a surface covered with water and oceans.
The first insights into the properties of this exoplanet’s atmosphere in the habitable zone came from observations with NASA’s Hubble Space Telescope, leading to further studies that have since changed our understanding of this system.
K2-18 b orbits the cool dwarf star K2-18 in the habitable zone and is 120 light-years from Earth in the constellation Leo. Exoplanets like K2-18 b, which range in size from Earth to Neptune, are unlike any other planet in our solar system. The lack of equivalent nearby planets means that these “sub-Neptunes” are poorly understood, and the nature of their atmospheres is the subject of active debate among astronomers.
The suggestion that asteroid K2-18 b may be a Hycean exoplanet is intriguing, as some astronomers believe that these worlds are promising environments in which to search for evidence of live on exoplanets. “Our results highlight the importance of taking into account the diversity of habitable environments,” explains Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing the results. in the process of searching for life elsewhere.”“Traditionally, the search for life on exoplanets has focused primarily on smaller rocky planets, but larger Hycean worlds are significantly easier to observe atmospherically.”
The abundance of methane and carbon dioxide, as well as the lack of ammonia, support the hypothesis that there may be a water ocean beneath the hydrogen-rich atmosphere in K2-18 b. Webb’s initial observations also discovered a molecule called dimethyl sulfide (DMS). On Earth, this is only created by life. Most of the DMS present in Earth’s atmosphere is emitted by phytoplankton in marine environments.
DMS inference is less robust and requires further validation.”Further observations by Webb will be able to confirm whether DMS is indeed present in K2-18 b’s atmosphere at significant levels,” Madhusudhan explained.
Although K2-18 b is in the habitable zone and is currently known to contain carbon molecules, this does not necessarily mean that the planet can support life. The planet’s large size – with a radius 2.6 times that of Earth – means its interior could contain a huge layer of high-pressure ice, like Neptune, but with an atmosphere thinner, hydrogen-rich, and ocean surface. Hycean worlds should have oceans of water.However, it is also possible that the ocean is too warm to be habitable or that there is no liquid at all.
“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.”
