The JWST is astronomers’ best tool for probing exoplanet atmospheres. Its capable instruments can dissect the light passing through a distant world’s atmosphere and determine its chemical components. Scientists are interested in everything the JWST finds, but when it finds something indicating the possibility of life it seizes everyone’s attention.
That’s what happened in September 2023, when the JWST found dimethyl sulphide (DMS) in the atmosphere of the exoplanet K2-18b.
K2-18b orbits a red dwarf star about 124 light-years away. It’s a sub-Neptune with about 2.5 times Earth’s radius and 8.6 Earth masses. The exoplanet may be a Hycean world, a temperate ocean-covered world with a large hydrogen atmosphere.
In October 2023, researchers announced the tentative detection of dimethyl sulphide in K2-18b’s atmosphere. They found it in JWST observations of the planet’s atmospheric spectrum. “The spectrum also suggests potential signs of dimethyl sulphide (DMS), which has been predicted to be an observable biomarker in Hycean worlds, motivating considerations of possible biological activity on the planet,” the researchers wrote.
The DMS caught people’s attention because it’s produced by living organisms here on Earth, mostly by marine microbes. So, finding it on an ocean world is cause for a deeper look. A team of researchers from the USA, Germany, and the UK examined the detection to see how it fits with atmospheric models.
“The best biosignatures on an exoplanet may differ significantly from those we find most abundant on Earth today.”
They published their results in a paper in the Astrophysical Journal Letters. It’s titled “Biogenic Sulfur Gases as Biosignatures on Temperate Sub-Neptune Waterworlds.” The lead author is Shang-Min Tsai, a University of California Riverside project scientist.
Most of the thousands of exoplanets we’ve discovered are nothing like Earth. Habitability is impossible according to every known metric. But some are more intriguing. Some, like K2-18b, are more difficult to understand regarding habitability.
There’s some disagreement over what type of planet K2-18b is. It was the first exoplanet scientists ever detected water vapour on. It may be the first example of a Hycean world if they exist.
Artist depiction of the mini-Neptune K2-18 b. Credit: NASA, CSA, ESA, J. Olmstead (STScI), N. Madhusudhan (Cambridge University)There are some clear differences between K2-18b and Earth. Our atmosphere is dominated by nitrogen, which makes up about 78%. K2-18b’s atmosphere is dominated by hydrogen. But it’s enough like Earth in some ways that scientists are keen to understand it better.
“This planet gets almost the same amount of solar radiation as Earth. And if atmosphere is removed as a factor, K2-18b has a temperature close to Earth’s, which is also an ideal situation in which to find life,” said lead author Shang-Min Tsai.
The researchers who found DMS in K2-18b’s atmosphere also found carbon dioxide and methane. Finding CO2 and CH4 is noteworthy, but finding DMS with them is even more intriguing.
“What was icing on the cake, in terms of the search for life, is that last year these researchers reported a tentative detection of dimethyl sulfide, or DMS, in the atmosphere of that planet, which is produced by ocean phytoplankton on Earth,” Tsai said. DMS is oxidized in Earth’s oceans and is the planet’s main source of atmospheric sulphur.
K2-18b’s atmospheric composition as measured by the JWST’s near-infrared instruments. The detection of Dimethyl Sulphide is not holding up under increased scrutiny. Image Credit: NASA/CSA/ESA/STScIHowever, the 2023 findings were not conclusive. There were hints of DMS but nothing strong enough to convince scientists and overcome their professional skepticism. “The potential inference of DMS is of high importance, as it is known to be a robust biomarker on Earth and has been extensively advocated to be a promising biomarker for exoplanets,” the authors of the 2023 paper explained.
“The DMS signal from the Webb telescope was not very strong and only showed up in certain ways when analyzing the data,” Tsai said. “We wanted to know if we could be sure of what seemed like a hint about DMS.”
The JWST has no alarm bell and flashing indicator that lights up and says, ‘Biomarker Detected!’ It produces data that must be processed to tease out its secrets. Scientists also rely on battle-tested climate and atmospheric chemistry models to understand what the JWST sees.
“In this study, we explore biogenic sulphur across a wide range of biological fluxes and stellar UV environments,” the researchers write. They performed experiments with a 2D photochemical model and a 3D general circulation model (GCM.) According to Tsai and his co-researchers, the data is unlikely to show the presence of DMS in K2-18b’s atmosphere.
“The signal strongly overlaps with methane, and we think that picking out DMS from methane is beyond this instrument’s capability,” Tsai said.
That doesn’t mean that DMS is ruled out. It’s possible that the chemical could build up to detectable levels if plankton or some other life form were producing it. But, they’d have to produce about 20 times more DMS than there is on Earth.
Professor Madhusudhan from Cambridge University is the lead author of the 2023 paper on K2-18b’s atmosphere. He’s being touted in the media as the man who discovered alien life on another planet. He’s clearly uncomfortable with some of the hyperbole, but the message is becoming bigger than the messenger.
This study will probably put a damper on the media’s enthusiasm. But for people who follow science, this is just another instance of science correcting itself.
The fact is, we’re only groping our way toward understanding exoplanet atmospheres. Scientists have a powerful tool in the JWST, but it has limitations. It measures light in extreme detail and leaves the rest up to us. “We find that it is challenging to identify DMS at 3.4 ?m where it strongly overlaps with CH4,” the authors explain. But, they continue, “it is more plausible to detect DMS … in the mid-infrared between 9 and 13 ?m,” the authors explain.
This figure from the research compares how detectable DMS is in NIR (left) vs MIR (right.) We’re mostly interested in the 20xSorg (20 x organic sulphur.) Its presence at that concentration is muddy in NIR but stands out more clearly in simulated MIR data. Image Credit: Left: Madhusudhan et al. 2023. Right: Batalha et al. 2017.That means there’s hope for K2-18b. These observations were taken with the JWST’s near-infrared instruments, the NIRISS and the NIRSpec. Sometime next year, the JWST will examine the exoplanet’s atmosphere again, this time with its mid-infrared instrument MIRI. This instrument should tell us definitively whether DMS is present.
This figure shows the wavelength ranges of its instruments and the modes available to them. Image Credit: NASA/STScIScientists’ understanding of biosignatures has grown more detailed. Instead of searching for biosignatures like the ones on Earth, scientists are taking a larger, more holistic view of biosignatures and the nature of the atmospheres they might be present in.
“The best biosignatures on an exoplanet may differ significantly from those we find most abundant on Earth today. On a planet with a hydrogen-rich atmosphere, we may be more likely to find DMS made by life instead of oxygen made by plants and bacteria as on Earth,” said UCR astrobiologist Eddie Schwieterman, a senior author of the study.
The team’s work does show that sulphur could be a detectable biomarker for Hycean worlds. “The moderate threshold for biological production suggests that the search for biogenic sulphur gases as one class of potential biosignature is plausible for Hycean worlds,” they conclude.
