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James Webb sees evidence of an ocean-covered ‘Hycean’ exoplanet

The James Webb Space Telescope has once again peered into the atmosphere of an exoplanet, and this time it has identified indications that the planet could be covered in oceans. The planet K2-18 b is just 120 light-years away from Earth in the constellation of Leo and is a type of planet called a sub-Neptune which is unlike any planet in our solar system.

Researchers used Webb to investigate K2-18 b, which is more than eight times the mass of Earth and orbits a small, cool dwarf star. It is located within the habitable zone of the star, where it is possible for water to exist on the planet’s surface, and the data suggests that this could be an ocean world.

This illustration shows what exoplanet K2-18 b could look like based on science data. K2-18 b, an exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light years from Earth.
This illustration shows what exoplanet K2-18 b could look like based on science data. K2-18 b, an exoplanet 8.6 times as massive as Earth, orbits the cool dwarf star K2-18 in the habitable zone and lies 120 light years from Earth. Illustration: NASA, ESA, CSA, Joseph Olmsted (STScI); Science: Nikku Madhusudhan (IoA)

The researchers used Webb’s NIRSpec instrument, which can break down light into different wavelengths to see which ones have been absorbed by the target. As different compounds absorb different wavelengths of light, by looking at the missing wavelengths scientists can tell what an object is likely composed of. In this case, the planet showed indications of methane and carbon dioxide, and is thought to have a hydrogen-rich atmosphere with water oceans covering its surface.

Though the planet is in the habitable zone and does seem to have water, that doesn’t necessarily mean that life could survive there. Factors like the temperature of the oceans or the amount of radiation from the star can all affect habitability. However, the fact that data indicate a liquid water ocean does suggest the plant could be a hypothetical type of planet called a Hycean planet, which could be a good place to look for life.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” said lead author of the research, Nikku Madhusudhan of the University of Cambridge, in a statement. “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.”

It’s also notable that the exoplanet is a sub-Neptune, as even though we don’t have a planet like this nearby to study, this is thought to be one of the most common planet types in the wider galaxy. The researchers plan to observe this particular planet with another Webb instrument, MIRI, for more information, and other teams will use Webb for further research into potentially habitable exoplanets as well.

“These results are the product of just two observations of K2-18 b, with many more on the way,” said researcher 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 research has been accepted for publication in The Astrophysical Journal Letters.

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Georgina Torbet
Georgina is the Digital Trends space writer, covering human space exploration, planetary science, and cosmology. She…
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