Is it or isn't it? It's not the first time the scientific community has been roiled by a debate over a tantalizing piece of evidence that may or may not prove the existence of life outside of planet Earth.
In 1996, President Bill Clinton made the shocking announcement that a meteor found in the Allan Hills of Antarctica in 1984 contained evidence that life had once existed on Mars. Known as "AH 84001," the meteor contained what was thought to be "microbial fissiles" suggestive of past life on the planet.
Later studies have shown that the "magnetite processes" that scientists believed were indicative of life may not have had anything to do with organics at all. Others cling to the theory that the particles can only occur as a result of a biological process. This is a debate that's been raging for nearly thirty years and shows no sign of flagging.
Similarly, the recent discovery of the molecule dimethyl sulfide (DMS) in the atmosphere of K2-18b, a planet smaller than Neptune that lies about 120 light years from Earth, has become instantly controversial, with skeptics and believers embroiled in a good-old-fashioned scientific brawl. Many scientists who make a living looking for extraterrestrial life believe that DMS is a positive sign of biological processes at work.
Others aren't so sure.
“It is not strong evidence,” says Stephen Schmidt, an astronomer at Johns Hopkins University in Baltimore.
“It’s almost certainly not life,” says Tessa Fisher, an astrobiologist at the University of Arizona in Tucson.
Nikku Madhusudhan, an astronomer at the University of Cambridge and one of the authors of a new study published Wednesday in the Astrophysical Journal Letters, claims that the large amounts of DMS that were discovered in the atmosphere of K2-18b are the result of warm oceans of water, wrapped in atmospheres containing hydrogen, methane, and other carbon compounds. DMS, which is found in Earth's oceans. The kind of planet that K2-18b is classified as is a "mini-Neptune" and is one of the most common of the 5,300 exoplanets discovered.
The scientists detected the molecule by analysing starlight as it filtered through the planet’s atmosphere; different chemical compounds leave identifying imprints in the light’s spectrum. The data might indicate the presence of the related molecule dimethyl disulfide (DMDS), either in addition to DMS or in its stead. These chemicals are intriguing because on Earth they are produced by living organisms such as marine phytoplankton.
In 2023, the researchers reported similar findings. This follow-up work looks at the planet in a different set of wavelengths and is a stronger and cleaner signal that the molecules are present, the team says.
The scientific process that can identify molecules at such a huge distance is not new. It dates to the 1970s and the creative use of spectrography in space that could measure differing wavelengths of light to identify common molecules.
The difference today is that compared to the crude, space-based instruments of the '70s, the James Webb Space Telescope is the gold-plated Cadillac of optical space telescopes.
“What we are seeing is a major paradigm shift in the field of exoplanet science,” said Madhusudhan.
But just because it can be seen or measured more accurately doesn't mean it is what Madhusudhan and his team say it is.
Indeed, many scientists looking at the same data are unconvinced that the Cambridge team has discovered a "signature of life" on K2-18b.
For starters, there are questions about whether K2-18 b even has water — or a surface where anything could live. Modelling studies of it and similar planets suggest that they are probably barren. “A lifeless mini-Neptune scenario remains the most parsimonious explanation,” says Joshua Krissansen-Totton, a planetary scientist at the University of Washington in Seattle.
Then there’s the issue of whether DMS/DMDS is actually present, or whether it is a spurious signal. The measurement reported by the Cambridge team is “really pushing the limit of what JWST can do,” says Laura Kreidberg, an astronomer at the Max Planck Institute for Astronomy in Heidelberg, Germany.
Schmidt and his colleagues recently re-analysed the 2023 claim from the Cambridge team and found no evidence of biosignature molecules in that data. Schmidt says the new observations are “pretty noisy, and any reported features could still just be statistical fluctuations”. For their part, the Cambridge researchers say there is just a 0.3% probability that the signal could be due to chance.
Back and forth they will go until, sometime into the far future, we actually travel to K2-18b and shake hands with whoever or whatever is there or find it a barren wasteland.
Until then, there will be more observations and probably better and more accurate instruments that will allow us to make better guesses about what exactly is on that alien world.
