How Space Telescopes Spotted an Exoplanet With a Possible Hydrogen-Rich Atmosphere

In September NASA's James Webb Space Telescope investigated an exoplanet 8.6 times as massive as Earth, and 'revealed the presence of carbon-bearing molecules including methane and carbon dioxide.'

Webb's discovery adds to recent studies suggesting that [planet] 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 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.

NASA's announcement included some additional context:
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...

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.

Long-time Slashdot reader Baron_Yam noticed some sites resurfacing the news from September this week with more spectacular headlines, like 'NASA discovered a planet twice as big as Earth with a gas that is 'only produced by life'' and 'Discovery... sparks huge excitement among astronomers.



NASA's announcement? It's early Webb observations 'provided a possible detection of a molecule called dimethyl sulfide. On Earth, this is only produced by life.'

The bulk of the dimethyl sulfide in Earth's atmosphere is emitted from phytoplankton in marine environments.
The inference of dimethyl sulfide is less robust and requires further validation. 'Upcoming Webb observations should be able to confirm if dimethyl sulfide is indeed present in the atmosphere of K2-18 b at significant levels,' explained Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of the paper announcing these results.

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.

But it's all a validation of the new discoveries being made possible by space telescopes — new and old. '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...'

'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... '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 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.'

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