In 2023, debate erupted in the astronomy community about whether life could exist on an exoplanet called K2-18b. It started when a group of scientists published a paper suggesting a specific chemical, dimethyl sulfide, or DMS, may exist in the planet’s atmosphere. A consensus wasn’t reached at the time, and conversation has certainly continued into the present. Many astronomers wonder if the DMS signature from K2-18b can really be trusted, and even question whether DMS is a reliable proxy for the presence of life to begin with.
If not, maybe the discussion about K2-18b’s habitability is, more or less, pointless.
Now, new research raises major questions about the original K2-18b study — and, in fact, the answers could have considerable implications for the quest to find life beyond Earth as a whole. Scientists think they’ve found evidence of DMS on a comet, meaning that life may not be required for its creation, which casts doubt on the use of the chemical as a marker of life.
In an ideal world, researchers could look for life on exoplanets by sending spacecraft or astronauts to these worlds’ surfaces to look for molecules that are only produced by life, explained Edward Schwieterman, an astrobiologist at the University of California at Riverside, who is not associated with the original DMS findings. Researchers could deploy a probe to search for things like RNA, DNA and other biomolecules we know are connected to life (as we know it, of course) and could use their findings as evidence of extraterrestrial activity.
However, a few major challenges stand in the way of this strategy. For one, it is immensely time-consuming and expensive to send spacecraft to other planets in the solar system — let alone beyond.
Without collecting samples from a planet’s surface, we don’t yet have the technology to identify specific biological molecules with ease. “Molecules like DNA simply cannot accumulate in the atmosphere of an exoplanet in a way that could be identifiable by space-based or ground-based telescopes,” Schwieterman told Space.com.
Scientists are therefore left searching for signs of life beyond Earth using what telescopes can indeed see — light spectra. Astronomers can collect information about light wavelengths traveling through and getting blocked by a planet’s atmosphere and, based on various properties of these wavelengths, make educated guesses about composition of the atmosphere itself. This data “can often have multiple different interpretations, so this is really, really tricky,” Joanna Barstow, a planetary scientist at The Open University, told Space.com.
Did researchers really detect DMS?
A chemical found in the atmosphere around an exoplanet that leads researchers to believe the planet may harbor life is called a biosignature. On Earth, DMS is mostly produced by bacteria and phytoplankton in oceans, so many astrobiologists consider it a biosignature.
That’s why it was a surprise to Nikku Madhusudhan, an astrophysicist at the University of Cambridge and lead author on the paper that spurred debate about K2-18b, when his team discovered signs of DMS in the planet’s atmosphere.
K2-18b orbits a star over 700 trillion miles away from Earth and is considered potentially habitable because of the amount of starlight it gets; there is also speculation of water vapor existing in its atmosphere. Furthermore, Madhusudhan and some other astronomers suspect the exoplanet has mild temperatures and a liquid ocean, both crucial factors for life we’re familiar with on Earth.
Madhusudhan’s team’s finding of DMS therefore only seemed to strengthen the case for life on K2-18b. “It’s weak evidence,” he told Space.com, but “if it turns out to be actually right that there is DMS, then that is a big deal.”
But some scientists don’t accept that what Madhusudhan found around K2-18b really was DMS.
“I do not believe we yet have convincing evidence of the presence of DMS in K2-18b’s atmosphere,” Schwieterman said, attributing his suspicion to the new publication’s fairly low statistical confidence.
When other researchers claim to have detected a biosignature, he considers two factors before trusting them. First, he confirms the detected signal is real — that the observers have actually found the molecule they say they have. Then, he ensures people are attributing the creation of that molecule to the correct source — in this case, some sort of life. Schwieterman and many other astronomers believe Madhusudhan’s detection of DMS fails both steps of verification.
Ryan MacDonald, an astrophysicist at the University of Michigan who was not involved in the new research, explained that he’s found molecular signatures at similar confidence levels that “completely vanish” when the same data is run through different models. The outcomes of analysis can change “depending on the fine minutiae of how you treat the data,” he told Space.com, adding that “we’re all still learning” how to analyze this kind of data because its quality is higher than ever before. He would need to see stronger statistics to be convinced that DMS is really present in the atmosphere of K2-18b.
Schwieterman also has his own concerns about attributing this signature to DMS. He explained that the specific way DMS interacts with light can yield signals similar to those of some other gases, like methane, or signals that represent nothing at all. “It would be easy to attribute a signature to DMS that’s actually the result of another gas or noise source,” he said. “What we’d want is a lot more data in order to confirm our attribution of DMS to that measurement.”
Madhusudhan defended his team’s decision to report what they found, even at a relatively low confidence level: “If you’re honestly doing your research and you found the signal at [this confidence level], you ought to be reporting that. Whether you consider it to be potential evidence or not is subjective to some extent… but you should still report it.”
Some online articles, however, went as far as to suggest the group may have found alien life on the distant planet. Even Madhusudhan is hesitant to agree with such conclusions, even preliminarily.
Does the existence of DMS really imply the existence of life?
Beyond the question of whether DMS was actually detected, researchers are investigating whether the molecule is a good marker of life in the first place. The best way to decide is to see if there are other explanations for its presence at a high enough quantity to be observed, Barstow explained.
There are several cases of scientists finding ways to produce other molecules traditionally thought of as biosignatures without the use of life. In 2023, for example, researchers found a way to turn carbon dioxide and helium into oxygen, which is often used as an indicator of life.
Similarly, in 1975 chemists were able to produce DMS in the lab using hydrogen sulfide, methane and electricity, proving that life isn’t necessary for its creation. (Madhusudhan doesn’t think this process would yield enough DMS on K2-18b to be observable.)
In November 2024, a separate research group published evidence of DMS signatures from a comet. Their research supports a new idea of DMS production, one that uses basic elements from across space and again doesn’t rely on life.
Nora Hänni, a chemist at the University of Bern and lead investigator of the new study, explained how DMS could transfer from comets to planets. A comet could potentially land on a planet and deposit chemicals into its atmosphere, she told Space.com, “so it could be like a spaceship, basically.”
“If you want to detect [DMS] in an atmosphere,” Hänni added, “it has to also survive the atmospheric chemistry, the irradiation … Maybe you would have to observe a potential contamination right after the impact, or you would have to have a lot of impacts and some material could accumulate.”
Many unknowns remain.
The properties of the exoplanet K2-18b and its atmosphere are largely a mystery, so it’s unclear how long — or even whether — DMS could truly survive. There’s also little information about how comets function outside of our solar system. Hänni isn’t pushing the hypothesis that comets are responsible for DMS on K2-18b, but she wants to ensure that this scenario is considered as a possibility before any conclusions are drawn about the DMS findings.
Madhusudhan is dismissive of the comet theory, however, because of the amount of DMS he says is needed to be observable in an exoplanet’s atmosphere. “How much do you need to deliver for it to be observable in a planetary atmosphere? Comets don’t happen by the hour,” he said. He thinks that for this method of production to explain the signature he found, the number of comets crashing into K2-18b and bringing DMS with them would need to be unrealistically high.
Clearly, there’s more work to be done before there’s a consensus on if this signature is real and if DMS is a reliable biosignature. MacDonald explained that more telescope observations of K2-18b’s atmosphere from multiple instruments would be the “gold standard” to prove if DMS is actually present.
And that work is in progress.
“We are getting more observations, others are getting more observations,” Madhusudhan said. “So, over the next year, we will see whether or not the molecule is there.”