Update (April 27, 2024):
NASA’s James Webb Space Telescope just completed another observation of K2-18b on April 26, which lasted for a reported eight hours. One of the objectives was to try to detect the dimethyl sulfide again, after the tentative and highly debated result from September last year. The new results will now be analyzed and it will likely be at least a few months before we hear more about whether Webb found the dimethyl sulfide signal again or more details about the planet itself.
The Times (UK) published a new article on April 25 and there is also a new video interview (April 26) with Nikku Madhusudhan, a University of Cambridge astrophysicist involved in last year’s research.
Did the James Webb Space Telescope spot signs of life on a distant planet? On September 11, 2023, NASA announced that Webb made some exciting discoveries while observing an exoplanet called K2-18 b. They said the planet has methane and carbon dioxide in its atmosphere, which indicates it might be a Hycean world, one with a deep hydrogen atmosphere and global water ocean. But the extraordinary news is that Webb found hints of a molecule called dimethyl sulfide (DMS). On Earth, only life produces dimethyl sulfide, such as bacteria and phytoplankton in oceans.
Does that mean Webb has found signs of life on exoplanet K2-18 b? Well, first, the finding of dimethyl sulfide needs to be confirmed. And then, scientists have to learn more about the exoplanet. While it is in the habitable zone of its star, its environment could still be unfavorable to life. As NASA said, it may have a hostile environment due to its active star, or its ocean may also be too hot to be habitable. Still, the new discovery is tantalizing.
While studying the exoplanet K2-18 b, @NASAWebb hinted at a detection of dimethyl sulphide (DMS). On Earth, this molecule is only produced by microbial life. Because the detection needs to be confirmed, the team will follow up with additional observations. https://t.co/lYRNSVgI0M pic.twitter.com/Ui3c1fDbSf
— NASA Exoplanets (@NASAExoplanets) September 11, 2023
More about K2-18 b
K2-18 b is what scientists call a sub-Neptune or mini-Neptune. The exoplanet is 2.6 times larger and 8.6 times more massive than Earth. The new data from Webb show that the exoplanet is also likely a Hycean world. And unlike many sub-Neptunes, scientists say that Hyceans may be habitable. In this instance, habitable means the conditions could be right for life to form. It does not mean that we are planning to send earthly life to the exoplanet.
The Webb discovery is also the first time that astronomers have discovered carbon molecules in the atmosphere of a planet in its star’s habitable zone.
The new research paper has been accepted for publication in The Astrophysical Journal Letters. In the meantime, a draft version of the paper is available from ESA.
A Hycean world with an ocean?
K2-18 b orbits in the habitable zone of a red dwarf star about 124 light-years from Earth, in the constellation Leo the Lion. The habitable zone is the region around a star where temperatures could allow liquid water to exist. To be clear, this doesn’t prove that K2-18 b is actually habitable, although the new findings from Webb support that it is possible.
Also, Hyceans are a newer identified class of exoplanet, and there is still a lot that we don’t know about them. Unlike most sub-Neptunes, however, they may be able to have solid surfaces beneath their deep hydrogen atmospheres. And below the atmosphere, a global water ocean. They would still be hotter than Earth, but perhaps not too hot for life of some kind.
The discovery of methane (CH4) and carbon dioxide (CO2) – and a lack of ammonia – support the hypothesis that K2-18 b is a Hycean planet, the researchers said. Lead author Nikku Madhusudhan, an astronomer at the University of Cambridge, said that:
Our findings underscore the importance of considering diverse habitable environments in the search for life elsewhere. 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.
The researchers said that K2-18 b likely has a mantle of high-pressure ice, similar to Neptune. The Webb results support the possibility of a planet-wide ocean on the surface, although it is still possible that the planet is too hot. Or at least too hot for the ocean to be habitable. Only additional observations will help astronomers determine the actual conditions.
Webb detects possible biosignature
The presence of methane, carbon dioxide and a possible ocean are certainly interesting on their own of course. But Webb also potentially detected something even more tantalizing in the atmosphere of K2-18 b: dimethyl sulfide (DMS). Dimethyl sulfide is an organic sulfur compound. On Earth, it is only produced naturally by biological processes from organisms such as bacteria and phytoplankton in marine environments.
This is still tentative, however. While there is a possible detection, it is less robust than the detections of methane and carbon dioxide. The paper stated:
The potential inference of DMS is of high importance as it is known to be a robust biomarker on the Earth and has been extensively advocated to be a promising biomarker for exoplanets.
Overall, we find CH4 and CO2 to be our most confident detections, followed by DMS, with the abundance estimates reported above. While our results provide important first insights into the chemical composition of K2-18 b, upcoming observations will be able to verify our present findings.
These findings support the Hycean nature of K2-18 b and the potential for biological activity on the planet.
Madhusudhan said:
The possibility of DMS in the atmosphere is highly promising, but we are planning to take another look to robustly establish its presence.
Most detailed spectrum of a habitable zone sub-Neptune
Webb’s observations are the most detailed yet of a sub-Neptune in the habitable zone. Co-author Subhajit Sarkar of Cardiff University said:
Although this kind of planet does not exist in our solar system, sub-Neptunes are the most common type of planet known so far in the galaxy. 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.
Savvas Constantinou of the University of Cambridge added:
These results are the product of just two observations of K2-18 b, with many more on the way. This means our work here is but an early demonstration of what Webb can observe in habitable-zone exoplanets.
Future observations with Webb
Of course, now we want to know if Webb will be doing any follow-up observations. The answer is yes. The team will use the telescope’s MIRI (Mid-Infrared Instrument) spectrograph to further analyze the planet’s atmosphere. As Madhusudhan said:
Our ultimate goal is the identification of life on a habitable exoplanet, which would transform our understanding of our place in the universe. Our findings are a promising step toward a deeper understanding of Hycean worlds in this quest.
Bottom line: NASA’s Webb telescope looked at the sub-Neptune exoplanet K2-18 b. It found methane, carbon dioxide, a possible ocean, and even … maybe … a biosignature.
Source: Carbon-bearing Molecules in a Possible Hycean Atmosphere
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