Webb detects methane and carbon dioxide in the atmosphere of K2-18b

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A new investigation by NASA’s James Webb Space Telescope of K2-18 b, an exoplanet 8.6 times Earth’s mass, 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 an exoplanet, one with the potential to have a hydrogen-rich atmosphere and a surface covered in a water ocean.

The first insight into the atmospheric properties of this planet in the habitable zone came from NASA’s Hubble Space Telescope observations, prompting 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 is located 120 light-years from Earth in the constellation Leo. Exoplanets like K2-18 b, which are between the size of Earth and Neptune, are unlike anything in our solar system. This lack of equivalent nearby planets means that these “sub-Neptunian planets” are not well understood, and the nature of their atmospheres is the subject of active debate among astronomers.

The suggestion that the sub-Neptunian planet K2-18 b could be an exoplanet is intriguing, as some astronomers believe these worlds are promising environments for searching for evidence of life on exoplanets.

“Our findings underscore the importance of considering diverse habitable environments when searching for life elsewhere,” explained Nico Madhusudan, an astronomer at the University of Cambridge and lead author of the paper announcing these findings. “Traditionally, the search for life on exoplanets has focused primarily on smaller, rocky planets, but larger Hessian worlds are more suitable for atmospheric observations.”

The abundance of methane and carbon dioxide, and the lack of ammonia, support the hypothesis that there may be a water ocean under a hydrogen-rich atmosphere in K2-18 b. These initial Webb observations also provided the potential discovery of a molecule called dimethyl sulfide (DMS). On Earth, this is only produced by life. The bulk of DMS in the Earth’s atmosphere is emitted by phytoplankton in marine environments.

The DMS conclusion is less robust and requires further validation.

“Upcoming Webb observations should be able to confirm whether DMS is indeed present in the atmosphere of K2-18 b at significant levels,” Madhusudan explained.

While K2-18 b is located in the habitable zone, and is now known to harbor carbon-bearing molecules, this does not necessarily mean the planet could support life. The planet’s large size — its radius is 2.6 times that of Earth — means the planet’s interior likely contains a large mantle of high-pressure ice, like Neptune’s, but with a thinner hydrogen-rich atmosphere and an oceanic surface. Hessian worlds are expected to contain oceans of water. However, it is also possible that the ocean is too hot to be habitable or liquid.

“Although this type of planet does not exist in our solar system, sub-Neptunian planets are the most common type of planet known so far in the galaxy,” explained team member Subhajit Sarkar from Cardiff University. “We obtained the most detailed spectrum of Neptune’s habitable subzone to date, and this allowed us to identify molecules in its atmosphere.”

Characterizing the atmospheres of exoplanets like K2-18 b – which means determining their gases and physical conditions – is a very active field in astronomy. However, these planets are literally outshined by the glare of their larger stars, making exploration of exoplanetary atmospheres particularly difficult.

The team avoided 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 we can detect a drop in brightness as it passes through the face of its host star. This is how the exoplanet was first discovered in 2015 by NASA’s K2 mission. This means that during the transit, a small portion of starlight will pass through the exoplanet’s atmosphere before reaching telescopes like Webb. The passage of starlight through an exoplanet’s atmosphere leaves traces that astronomers can piece together to identify gases in the exoplanet’s atmosphere.

“This result was only possible due to the extended wavelength range and unprecedented sensitivity of the Webb, which enabled robust detection of spectral features with just two transits,” Madhusudan said. “For comparison, one of the transit observations using Webb provided a resolution comparable to eight Hubble observations made over a few years and in a relatively narrow wavelength range.”

“These results are the result of just two observations of K2-18 b, and there are more on the way,” explained team member Savvas Constantinou from the University of Cambridge. “This means that our work here is only an early demonstration of what Webb can do. Observe exoplanets in the habitable zone.

The team’s results have been accepted for publication in The Astrophysical Journal Letters.

The team now intends to conduct follow-up research using the telescope’s MIRI (mid-infrared instrument) spectrometer, which they hope will further validate their findings and provide new insights into environmental conditions on K2-18 b.

“Our ultimate goal is to identify life on a habitable exoplanet, which would change our understanding of our place in the universe,” Madhusudan concluded. “Our findings are a promising step toward a deeper understanding of the Hessian worlds in this endeavor.”

The James Webb Space Telescope is the world’s leading space science observatory. Webb solves the mysteries of our solar system, looks beyond the distant worlds around other stars, and explores the mysterious structures and origins of our universe and our place in it. WEB is an international program led by NASA with its partners, ESA (European Space Agency) and the Canadian Space Agency.

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