Two new studies have sparked fresh debate about a faraway planet with a weird atmosphere.
One of the studies claims additional evidence for the presence of life on the planet K2-18 b, based on chemical clues.
The other study argues that such clues can be produced on a lifeless world covered with hot magma.
The hubbub illustrates how tricky it can be to determine whether life exists beyond Earth by looking for “biosignatures” with powerful telescopes — in this case, NASA’s James Webb Space Telescope.
It also illustrates the potential pitfalls of reporting provocative results.
A study published today in the Astrophysical Journal Letters attracted widespread attention when it reported that the chemical signatures of dimethyl sulfide and dimethyl disulfide had been found in K2-18 b’s atmosphere.
The findings were a follow-up to an earlier study published by the same researchers, which detected carbon-bearing molecules including methane and carbon dioxide in the atmosphere.
What’s so special about dimethyl sulfide and dimethyl disulfide? The study noted that on Earth, those two chemicals are produced only by life, primarily microbial life such as marine plankton. The researchers claimed that the findings were consistent with their view that K2-18 b was what’s known as a Hycean planet — a habitable, ocean-covered world with a hydrogen-rich atmosphere.
“Given everything we know about this planet, a Hycean world with an ocean that is teeming with life is the scenario that best fits the data we have,” Cambridge astronomer Nikku Madhusudhan, the study’s lead researcher, said in a news release.
The teeming-with-life scenario drew skeptical comments from an assortment of other scientists who have been studying the potential for exoplanetary life. The planetary science community has long debated the case for Hycean planets and detecting life on such planets.
“This paper adds (weak) evidence to one model amongst many of a complicated planet atmosphere,” Oxford astronomer Chris Lintott wrote in a series of posts to Bluesky. “Currently, it’s not much more of a hint of life as the lottery ticket in my back pocket is a hint of my having a million quid by Monday.”
Arguing against the teeming-with-life scenario, researchers have noted that the chemical signature of dimethyl sulfide has been detected in the lifeless environments of a comet and the interstellar medium. One study, published last year, suggested that the sort of atmospheric chemistry documented previously by JWST could be due to interactions with a boiling-hot ocean of magma.
The magma hypothesis is front and center in a study that’s just been accepted for publication in the Astrophysical Journal. That study was based on JWST observations of a different sub-Neptune planet, a cousin of K2-18 b known as TOI-270 d. A research team led by Christopher Glein of the Southwest Research Institute found that TOI-270 d’s weird atmospheric chemistry could be explained by cooking methane and carbon dioxide in the presence of hot hydrogen and magma.
Glein said the findings could apply to K2-18b as well. “There appears to be a lack of diagnostic evidence for a Hycean state on these types of planets, at least on the ones we’ve observed so far,” he said in an email.
John Stott, an astrophysicist at Lancaster University, phrased a similar assessment more colorfully in a Bluesky quip: “It will probably turn out to be vulcanism rather than Vulcans.”
The questions surrounding dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) are still up in the air. “I have not yet looked into whether this model can produce DMS and DMDS to the reported abundances (I just learned about this new work a day ago),” Glein wrote. “That’s an important question, but first we need to confirm that these species are confidently detected.”
The confidence level for the JWST observations reported in Madhusudhan’s paper is at the 3-sigma level. That’s statistically significant, but it doesn’t quite hit the 5-sigma level that’s associated with a confirmed discovery. Madhusudhan and his colleagues say that 16 to 24 hours of follow-up observation time with JWST may help them reach the 5-sigma level.
In a news release, Glein said the study of alien atmospheric chemistry was worthwhile even if it doesn’t produce conclusive evidence of extraterrestrial life.
“While it is a bit disappointing to find that TOI-270 d is unlikely to be habitable, this planet still offers a fantastic opportunity to explore alternative paths of planetary origins and evolution,” Glein said. “We are learning much more about the crazy configurations of planets that nature comes up with.”
This week’s debate raises a larger issue about the search for chemical biosignatures. How strong would the evidence have to be to convince scientists that the chemical compounds associated with life on Earth could only be produced by biological activity?
Stott was skeptical any such evidence would be strong enough.
“If we ever did find an exoplanet with robust biomarkers, it would be interesting philosophically,” he wrote. “Personally, I’d only believe it if either: 1. We had a physical sample of the life (impossible anytime soon unless in solar system); or 2. A clear broadcast signal from intelligent life.”
Written by Alan Boyle/Universe Today.
