James Webb and the Ocean Covered Planet

Un team di ricerca usando il telescopio spaziale James Webb ha scovato un pianeta ricoperto di oceani d'acqua che potrebbe ospitare la vita
This illustration shows how the exoplanet K2-18b might appear based on scientific data. K2-18b, an exoplanet 8.6 times more massive than Earth, orbits the cold dwarf star K2-18 in the habitable zone and is located 120 light-years from Earth. Credit:

James Webb Space Telescope has discovered evidence of carbon-based molecules in the atmosphere of an ocean-covered planet. The exoplanet known as K2-18b is an enticing target for astronomers searching for life beyond the Solar System. Previous research and observations with the Hubble Space Telescope indicated that the planet could be covered in water, a fundamental ingredient for life. K2-18b has a radius between two and three times that of Earth and is located 120 light-years away from us.

Sub-Neptune Planets

Results have shown traces of carbon dioxide (CO2) and methane (CH4) in K2-18b’s atmosphere without detecting ammonia. This likely indicates a water ocean beneath a hydrogen-rich atmosphere. “Our results emphasize the importance of considering different habitable environments in the search for life elsewhere” said Nikku Madhusudhan, the lead author of the research (ref.).

“Traditionally, the search for life on exoplanets has primarily focused on smaller rocky planets. But larger oceanic worlds are significantly more favorable for atmospheric observations.” With a mass approximately 8.6 times that of Earth and situated in the habitable zone of its star, the region that is neither too hot nor too cold to support liquid water.

K2-18b is an example of a planet with a size between Earth and Neptune. These worlds are called “sub-Neptunes” and are a sort of mystery for astronomers. The scientific community is currently debating the nature of their atmospheres. This research should help unravel doubts surrounding the atmospheres and environmental conditions of sub-Neptune and oceanic worlds.

Is it evidence of life beyond Earth?

In addition to the discovery of carbon molecules, the James Webb ‘s results have also shown the possible presence of something potentially more exciting on the ocean-covered planet. The space telescope appears to have detected dimethylsulfide (DMS), which on Earth is produced by phytoplankton. The team is cautious about this detection, which is much less certain than the presence of carbon molecules. “Future Webb observations should be able to confirm whether DMS is actually present in the atmosphere of K2-18b at significant levels” explained Madhusudhan.

This sense of caution should be applied to the results in general when it comes to speculating about alien life. Even if the planet has a liquid water ocean and an atmosphere containing carbon molecules, it does not necessarily mean it hosts life or that the exoplanet can even support living beings. With a width of about 2.6 times that of Earth, it may contain high-pressure ice inside. This means the planet could have oceans too hot to support life.

Assessing the composition of the atmospheres of distant worlds like K2-18b is not an easy task because the light reflected from their atmospheres is much weaker compared to the light from their parent stars. Madhusudhan and the team used the transit technique. The light from the parent star passes directly through K2-18b’s atmosphere.

Atmospheric composition of K2-18b

Elements and chemical compounds absorb and emit light at specific and characteristic wavelengths. When they are in a planet’s atmosphere, they leave a distinctive “fingerprint” called an absorption spectrum. “This result was only made possible by the extensive wavelength range and unprecedented sensitivity of the James Webb. We obtained a reliable detection of spectral features with just two transits” explained Madhusudhan. “To put it in perspective, one Webb transit observation provided precision comparable to eight observations with Hubble and in a relatively narrow wavelength range”.

Spectra of K2-18b, obtained with the James Webb's Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec), show an abundance of methane and carbon dioxide in the exoplanet's atmosphere. There is also a potential detection of dimethylsulfide (DMS).
Spectra of K2-18b, obtained with the James Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) and Near-Infrared Spectrograph (NIRSpec), show an abundance of methane and carbon dioxide in the exoplanet’s atmosphere. There is also a potential detection of dimethylsulfide (DMS). Credit: NASA

Further observations of the exoplanet are on the way, but the team behind these discoveries believes that what they have seen so far is yet another testament to the power of the James Webb. The team will now continue to observe K2-18b with James Webb and its Mid-Infrared Instrument (MIRI). “Our ultimate goal is the identification of life on a habitable extrasolar planet, which would transform our understanding of our place in the universe” concluded Madhusudhan. “Our findings represent a promising step towards a deeper understanding of oceanic worlds in this quest”.

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