NASA detects methane, carbon dioxide on ‘Hycean’ exoplanet

New data from NASA’s James Webb Space Telescope has revealed that K2-18 b, an exoplanet nearly nine times the size of Earth, could be an example of a theorized class of planets with hydrogen-rich atmospheres, vast liquid oceans and the potential to support life.

Observations taken by the telescope detected the presence of carbon-bearing molecules such as methane and carbon-dioxide, according to a NASA press release published on Monday.

Astronomers also picked up signs of a molecule called dimethyl sulfide (DMS) – which, on Earth, is only produced by living things.

K2-18 b is an exoplanet orbiting a dwarf star roughly 120 light years away from Earth in the constellation of Leo. It is categorized as a sub-Neptune, a class of planets that are between the size of Earth and Neptune and common in the universe, although not in our solar system.

NASA says any planet that lies outside of our own solar system is considered an exoplanet.

Scientists have known about K2-18 b for a while, with initial observations taken by NASA’s Hubble Space Telescope, but these new observations taken with the James Webb Telescope have provided details that astronomers had previously only wondered about.

“Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere,” Nikku Madhusudhan, an astronomer at the University of Cambridge and lead author of a paper announcing these results, said in the release. “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.”

A hydrogen-dominated atmosphere and rolling oceans of liquid water – these are the key features of a Hycean world, first theorized by a team of researchers including Madhusudhan in 2021. Liquid water has long been held up as the holy grail for life to develop on another planet, making its detection high on the priority list for astronomers.

The abundance of methane and carbon-dioxide detected in K2-18 b’s atmosphere, along with the lack of ammonia, point to the right chemical conditions for a Hycean world, researchers say.

The results have been accepted for publication in the Astrophysical Journal Letters.

In order to identify the chemicals present in the atmosphere of a planet light years away from the Earth, researchers are using a technique based on wavelengths of light, which is where James Webb’s superior vision comes into play.

“We have obtained the most detailed spectrum of a habitable-zone sub-Neptune to date, and this allowed us to work out the molecules that exist in its atmosphere,” Subhajit Sarkar, a member of the research team with Cardiff University, said in the release.

Researchers analyzed the light from the exoplanet’s parent star where it passed through the atmosphere of the planet itself, and were able to pick up information about what chemicals the light had interacted with.

“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,” Madhusudhan said. “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.”

The possible detection of DMS brings up more questions, according to researchers. On Earth, DMS plays a key role in the oceanic sulfur cycle, released most often by phytoplankton, algae and other marine bacteria. However, the detection of DMS is less robust than the evidence of methane and carbon dioxide.

“Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” Madhusudhan said.


Hycean worlds are a fairly new idea, but they’ve generated excitement among the science community because of their expanded concept of what a planet that can support life can look like.

Data from NASA’s Kepler mission, a gigantic exoplanet survey which operated between 2009 and 2018, has been used to identify several candidate exoplanets for further study that could be Hycean worlds based on their size and density, one of which was K2-18 b.

However, a study published in the Astrophysical Journal in August suggests current Hycean worlds may not be capable of supporting life as we know it due to a runaway greenhouse effect keeping them from sustaining a liquid ocean.

The study, which mentioned K2-18 b being the focus of extensive research attempting to confirm or disqualify it as a Hycean world, modelled how a Hycean world would actually function and suggested that the atmospheric pressure predicted for these worlds could cause huge issues.

Earth’s atmosphere is thick, but the elements it contains allows for specific wavelengths of light to enter while preventing others from doing so, a delicate balance that helps the planet’s surface to stay within a liveable range of warmth.

A hydrogen-dominated atmosphere would block and allow different wavelengths of light, creating a different solar interaction at the planet’s surface. If a Hycean world were placed in Earth’s orbit around our Sun, its oceans would boil away to nothing, modelling from the August study suggested.

This means that the habitable zone for a Hycean world – the distance it has to be from its star and still sustain a liquid ocean – would likely require it to be much farther away than Earth is from our Sun. Also according to the study, all currently known Hycean candidates are too close to their stars to actually have liquid oceans.

K2-18 b lies at the edge of what is currently understood as the habitable zone based on data that assumes it is a rockier, smaller planet like Earth, which puts it too close according to the August study.

Researchers acknowledged in Monday’s press release that there is a possibility that K2-18 b could be too hot to be habitable or to have liquid water.

So, does this mean there’s no way K2-18 b could contain or sustain life?

Not necessarily – research into Hycean worlds is brand new, and this week’s announcement of carbon-bearing molecules present on K2-18 b indicates there is definitely more to be discovered as scientists continue to study these unique exoplanets.

“Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe,” Madhusudhan said. “Our findings are a promising step towards a deeper understanding of Hycean worlds in this quest.” 

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