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SYFY WIRE The Ark

JWST Explores Possible Sign of Alien Life on Exoplanet K2-18 b

A new study casts doubt on JWST's possible detection dimethyl sulfide on exoplanet K2-18b, but there's still hope.

By Cassidy Ward

In SYFY’s The Ark, a brave crew of explorers embarks on a one-way journey to another world. They’re headed for Proxima centauri b, a tidally locked planet orbiting the star next door, a small cosmic hop of just 4.2 light-years from here. Proxima centauri b has the benefit of being close by, an important factor when traveling interstellar distances, but it isn’t the friendliest place for life.

How to Watch

Catch up on The Ark on Peacock.

If we want to find someplace where life might be a little more comfortable, we’re going to have to look farther afield, but possibly not much farther. In September of 2023, NASA announced a possible biosignature detection on the exoplanet K2-18 b, just 120 light-years from here. Now, a new study casts down on that detection, but leaves room for finding signs of alien life there in the future.

For More on Exoplanets:
JWST Provides a Weather Report from the Exoplanet Wasp-43 b
Astronomers Find a Planet Half-Covered in Lava
Planets in Rare Star System Dan in Perfect Mathematical Harmony

JWST Detected Possible Sign of Alien Life on K2-18b

Spectra of K2-18 b, obtained with Webb’s NIRISS (Near-Infrared Imager and Slitless Spectrograph) and NIRSpec (Near-Infrared Spectrograph)

The exoplanet K2-18 b is about 2.5 times the diameter of Earth, has 8 times the mass, and orbits inside the habitable zone of a red dwarf star. Its size and composition makes it a sub-Neptune, a class of planets which are common in the Milky Way but which aren’t represented in our own solar system. That means we don’t have any nearby allegories to compare sub-Neptunes to, making them an interesting and mysterious kind of world.

Earlier observations by Hubble suggested that K2-18 b might be a Hycean world, one which has a hydrogen-rich atmosphere and liquid water oceans. When the James Webb Space Telescope (JWST) aimed its Near Infrared Spectrograph (NIRSpec) at the exoplanet, it confirmed a hydrogen-rich atmosphere along with methane, carbon dioxide, and a possible detection of dimethyl sulfide (DMS). On Earth, DMS is produced only by living things, mostly oceanic phytoplankton.

The NIRSpec data is consistent with the previous hypothesis of a hydrogen atmosphere covering a global ocean of liquid water, and the DMS detection suggested there might be something living in those waters. However, the possible DMS detection was weak and astronomers were cautious to warn that more data was needed. “Upcoming Webb observations should be able to confirm if DMS is indeed present in the atmosphere of K2-18 b at significant levels,” said Cambridge University astronomer Nikku Madhusudhan, in the NASA announcement.

There are plans for JWST to make those observations later in 2024 using stronger onboard instruments, in the hope of getting more detailed infrared readings from the atmosphere of K2-18 b. In the meantime, scientists have reanalyzed the initial readings and published the results in The Astrophysical Journal Letters.

New Study Casts Doubt on Biosignature Detection on Exoplanet K2-18b

Illustration of K2-18b

On Earth, DMS is continuously produced by plankton as a secondary metabolite. Despite that renewable source of dimethyl sulfide, the compound vanishes rapidly from the atmosphere making it difficult to detect. With that in mind, researchers at the University of California, Riverside wondered if it were even possible for DMS to accumulate to detectable levels in a hydrogen-rich atmosphere.

“The DMS signal from the Webb telescope was not very strong and only showed up in certain ways when analyzing the data. We wanted to know if we could be sure of what seemed like a hint about DMS,” said study author Shang-Min Tsai. “The signal strongly overlaps with methane, and we think that picking out DMS from methane is beyond this instrument’s capability.”

Tsai and colleagues modeled the physics and chemistry of DMS in a hydrogen-based environment and found that it was unlikely to accumulate enough for NIRSpec to have detected it. According to their models, a DMS detection is possible, but only if the amount in the atmosphere were 20 times higher than on Earth. The new findings douse some of the previous excitement surrounding K2-18 b, but researchers note that life on the exoplanet is still possible. The new study doesn’t negate the possibility of biosignatures on K2-18 b, but it does conclude that JWST hasn’t detected any yet.

The Ark will continue its journey to an exoplanet in Season 2, this summer on Peacock!