Planets in the habitable zone may contain water oceans.

Planet K2-18 b is twice the size of Earth and orbits in the habitable zone of its host star 120 light years from the Solar System.

Simulation of exoplanet K2-18 b in the habitable zone of its host star. Photo: NASA — Simulation of exoplanet K2-18 b in the habitable zone of its host star. Photo: *NASA*

The Webb Space Telescope (JWST) has detected evidence of carbon-containing molecules in the atmosphere of a planet suspected to have an ocean. The exoplanet K2-18 b is an attractive target for astronomers as they search for life beyond our solar system. Previous studies and observations with the Hubble Space Telescope have suggested that the planet could be a world with an ocean of liquid water, an essential ingredient for life, Space reported on September 11.

K2-18 b has a radius two to three times that of Earth and is 120 light years away from the Solar System. The new study found traces of carbon dioxide and methane in K2-18 b's atmosphere, but no ammonia, suggesting a water ocean beneath the hydrogen-rich atmosphere. The finding underscores the importance of considering diverse environments when searching for alien life, said lead author Nikku Madhusudhan of the University of Cambridge.

With a mass about 8.6 times that of Earth and located in the habitable zone of its cool star, the region that is neither too hot nor too cold to harbor liquid water, K2-18 b is an example of a planet that falls between Earth and Neptune in size. Such planets are known as “mini-Neptunes” and are unlike any other planet in the solar system. They are a mystery to astronomers, who are still debating the nature of their atmospheres. This research will help to explore the atmospheres and environmental conditions of both sub-Neptunian planets and Hycean planets (worlds with vast oceans of liquid water beneath hydrogen-rich atmospheres).

JWST also appears to have detected dimethyl sulfide (DMS) in K2-18 b’s atmosphere. The compound is primarily produced by plankton on Earth. However, the team is cautious. Madhusudhan says further observations with JWST could confirm whether DMS is indeed present in large quantities on K2-18 b.

At about 2.6 times the width of Earth, K2-18’s size means it has a Neptune-like interior of high-pressure ice, a thinner atmosphere, and a surface ocean. This means the planet could boil away liquid water, making the oceans too hot to support life. Assessing the atmospheric composition of a world as distant as K2-18 b is no easy task because the light reflected from the planet’s atmosphere is very faint compared to that of its host star. Madhusudhan and his colleagues waited for K2-18 b to pass in front of its star from the JWST’s viewing angle. This would allow the star’s light to shine directly through the planet’s atmosphere.

Chemical elements and compounds absorb and emit light at characteristic wavelengths, meaning that when they are in a planet's atmosphere, they leave a unique "fingerprint" in the star's spectrum. The team's findings reflect data collected by JWST during two flybys of K2-18 b in front of its host star. The researchers will continue to observe K2-18 b to learn more about the exoplanet's environmental conditions. Their study will be published in the Astrophysical Journal Letters.

An Khang (According to Space )

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