Webb’s study reveals methane and carbon dioxide in the atmosphere of K2-18 b.


Webb's study reveals methane and carbon dioxide in the atmosphere of K2-18 b.
This artist’s concept shows what K2-18 b could look like, 120 light-years away from Earth in the habitable zone around the cool dwarf star K2-18. Data collected by the telescope reveals the presence of carbon-bearing molecules such as methane and carbon dioxide, suggesting K2-18 b may have a water ocean underneath a hydrogen-rich atmosphere.

NASA’s James Webb Space Telescope has revealed a new investigation into K2-18 b, an exoplanet 8.6 times as massive as Earth. It has discovered the presence of carbon-bearing molecules including methane and carbon dioxide. This adds to recent studies that suggest K2-18 b could be a Hycean exoplanet, meaning it has the potential to possess a hydrogen-rich atmosphere and a water ocean-covered surface. Previously, this knowledge was gained from observations with Hubble Space Telescope, and further studies have since changed understanding of the system.

K2-18 b is an exoplanet located 120 light-years away in the constellation Leo that orbits a cool dwarf star. This exoplanet is unlike any other in our solar system, being between the size of Earth and Neptune. Because of this, the nature of its atmosphere is unknown and hotly debated among astronomers. The suggestion that this sub-Neptune could be Hycean has raised some interesting possibilities, as these planets are thought of as potential habitats for extraterrestrial life.

Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results, has pointed out the importance of considering diverse habitable environments in the search for life elsewhere. Traditionally, the search for life has focused on smaller rocky planets, but Hycean worlds are much more conducive to atmospheric observations.

In the case of K2-18 b, Webb observations have revealed an abundance of methane and carbon dioxide and a shortage of ammonia, which suggests that there may be a water ocean underneath a hydrogen-rich atmosphere. Moreover, there is a possible detection of dimethyl sulfide (DMS), a molecule that is produced only by life on Earth. The bulk of the atmospheric DMS on Earth is emitted from phytoplankton in marine environments. Thus, these initial observations support the hypothesis that there is potential for life on K2-18 b.

Spectra of K2-18 b, obtained with Webb’s NIRISS and NIRSpec.

Webb's study reveals methane and carbon dioxide in the atmosphere of K2-18 b.
K2-18 b, a planet 8.6 times as massive as Earth which orbits the cool dwarf star K2-18 in the habitable zone, Spectra of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph). Analysis of these spectra show an abundance of methane and carbon dioxide in the exoplanet’s atmosphere, along with a possible detection of a molecule called dimethyl sulfide (DMS). Not only does this support the hypothesis that there may be a water ocean underneath a hydrogen-rich atmosphere in K2-18 b, but also suggests that the lack of detectible ammonia could be an indication of life existing on K2-18 b.

The team led by professor Nikku Madhusudhan of used data from the Hubble and Spitzer telescopes to infer the presence of DMS (dimethyl sulfide) in the atmosphere of K2-18 b, a planet located in the habitable zone of its star. This inference is less robust and requires further validation, however, upcoming Webb observations should be able to confirm if it is indeed present at significant levels. The planet’s large size, with a radius 2.6 times that of Earth, suggests that it likely contains a mantle of high-pressure ice like Neptune, but with a thinner hydrogen-rich atmosphere and an ocean surface.

Sub-Neptunes are the most common type of planet known in the galaxy and this discovery provides the most detailed spectrum of such a planet in the habitable zone. This allowed the team to work out the molecules that exist in its atmosphere. Whether or not this kind of planet can support life is yet to be determined and further investigations are needed in order to validate their findings.

Characterizing the atmospheres of exoplanets like K2-18 b is a very active area in astronomy, however these exoplanets are often difficult to explore due to the glare given off from their much larger parent stars. To get around this, astronomers analyze the light from the planet’s parent star as it passes through the exoplanet’s atmosphere. This is possible because of K2-18 b’s transiting nature, meaning that as it passes across the face of its host star, a drop in brightness is detectable.

A tiny fraction of starlight will pass through the exoplanet’s atmosphere before reaching telescopes like Webb, and the starlight’s passage through the atmosphere leaves traces that astronomers can piece together to determine the gases present. By studying these traces, astronomers can gain insight into the physical conditions of exoplanets like K2-18 b, allowing them to further their understanding of exoplanetary atmospheres.

Led by Nikku Madhusudhan of the University of Cambridge, the team achieved these results through two transits of the exoplanet K2-18 b by the James Webb Space Telescope. Madhusudhan explains that this was only possible due to the extended wavelength range and unprecedented sensitivity of Webb, which enabled robust detection of spectral features with just two transits. 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.

Savvas Constantinou of the University of Cambridge adds that this is just an early demonstration of what Webb can observe in habitable-zone exoplanets and that further findings can be expected with follow-up research with the telescope’s MIRI (Mid-Infrared Instrument) spectrograph. The team’s results were accepted for publication in The Astrophysical Journal Letters and their findings will continue to provide new insights into the environmental conditions on K2-18 b.

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