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JWST spots methane and carbon dioxide in K2-18b atmosphere

12 Sep 2023

Results from optical instruments suggest exoplanet may have a water ocean.

A new investigation by NASA's James Webb Space Telescope (JWST) into the exoplanet named K2-18b has revealed the presence of carbon-bearing molecules including methane and carbon dioxide in its atmosphere.

The detection of these molecules, along with a shortage of ammonia, is consistent with the presence of an ocean underneath a hydrogen-rich atmosphere, which would make K2-18b the kind of exoplanet termed a Hycean world. Some astronomers believe that these are promising environments to search for evidence of life on exoplanets.

"Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere," commented Nikku Madhusudhan from the University of Cambridge, lead author of a paper submitted to Astrophysical Journal Letters about the findings.

"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."

JWST's science payload includes a suite of four integrated optical instruments designed to allow imaging and spectroscopy over a range of 0.6 to 28.3 microns, from visible to mid-infrared wavelengths.

The telescope's Near Infrared Camera (NIRCam) acts as its primary imager, employing ten mercury-cadmium-telluride detector arrays. NIRCam also acts as a wavefront sensor for JWST's Optical Telescope Element, based around the telescope's 6.5-meter primary mirror.

The other instruments are the Near-Infrared Spectrograph (MIRSpec) gathering spectra in a 9-square-arcminute field of view; the cryogenically cooled Mid-Infrared Instrument (MIRI) for broadband imagery and spectroscopy; and the Fine Guidance Sensor/Near Infrared Imager and Slitless Spectrograph (FGS/NIRISS) used in particular for exoplanet detection and characterization, and exoplanet transit spectroscopy.

One JWST observation comparable to eight with Hubble

Described as a "technical and scientific triumph" at the SPIE Astro conference shortly after it delivered the first NIRCam deep-field images of distant galaxies in 2022, JWST should now assist astronomers in characterizing the atmospheres of exoplanets like K2-18b, previously a tricky proposition.

In this case researchers analyzed light from K2-18b's parent star as it passed through the exoplanet's atmosphere during a transit of the planet, allowing them to detect the presence of gases in that atmosphere and identify its constituents.

"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."

Next steps will include using the telescope's MIRI spectrograph to validate the findings and investigate the environmental conditions on K2-18b. The ultimate goal, of course, is to see if life can be identified on a habitable exoplanet, a quest in which the JWST's optical instruments will be key.

"These results are the product of just two observations of K2-18b, with many more on the way,” said team member Savvas Constantinou from the University of Cambridge. “This means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.”

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