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A distant planet’s atmosphere shows signs of molecules that on Earth are associated only with biological activity, a possible signal of life on what is suspected to be a watery world, according to a report that analysed observations by Nasa’s James Webb Space Telescope. 

The peer-reviewed report in the Astrophysical Journal Letters presents more questions than answers, acknowledges numerous uncertainties and does not declare the discovery of life beyond Earth, something never conclusively detected. 

However, the authors do claim to have found the best evidence to date of a possible “biosignature” on a planet far from our solar system. 

The planet, known as K2-18b, is 124 light-years away, orbiting a red dwarf star. 

Earlier observations suggested that its atmosphere is consistent with the presence of a global ocean. The molecule purportedly detected is dimethyl sulfide (DMS). 

On Earth, it is produced by the decay of marine phytoplankton and other microbes, and it has no other known source. The astronomers want to observe the planet further to strengthen the evidence that the molecule is present. 

The lead author of the new paper, Nikku Madhusudhan, an astrophysicist at the University of Cambridge in England, repeatedly urged reporters in a briefing to be cautious in describing the new research. But his warning bracketed more dramatic statements. 

This possible detection of a biosignature could be “potentially one of the biggest landmarks in the history of science”, Madhusudhan said. 

“This is the first time humanity has ever seen biosignature molecules - potential biosignature molecules, which are biosignatures on Earth - in the atmosphere of a habitable-zone planet,” he added. 

The habitable, or “Goldilocks”, zone is the distance from a star that could allow water to remain liquid at the planet’s surface. 

K2-18b, which is within our galaxy, the Milky Way, cannot be seen by any telescope as a discrete object. But it has a fortuitous orbit that crosses its parent star as seen from Earth. 

Such transits dim the starlight ever so slightly, which is how many exoplanets have been discovered. The transits also change the starlight’s spectrum in a pattern that - if observed with instruments on a telescope as advanced as the Webb - can reveal the composition of the planet’s atmosphere. 

In 2023, Madhusudhan and colleagues reported that two instruments on the Webb had detected carbon dioxide and methane in the atmosphere of K2-18b, as well as hints of DMS. 

His team observed the planet for an additional eight hours last April using the Webb’s mid-infrared instrument. The resulting data boosted their confidence that DMS or an almost identical molecule, dimethyl disulfide (DMDS) - or possibly both - are present. 

The paper describes the result as “an important step forward in the search for signatures of life on exoplanets”, but it adds that strengthening the finding will need “a dedicated community effort in multiple directions - observational, theoretical, and experimental”. 

In a statement, Cambridge University said: “While an unknown chemical process may be the source of these molecules in K2-18b’s atmosphere, the results are the strongest evidence yet that life may exist on a planet outside our solar system”. 

Even if further observations strengthen the case that K2-18b has an atmosphere that contains DMS, the scientific community would probably want a great deal more evidence that this is truly a biosignature and not something with an abiotic origin. 

A molecule glimpsed in the air of a planet 729 trillion miles (1173 trillion km) away is a thin reed upon which to rest what would be the historic discovery of alien life. 

The Webb telescope is an international collaboration among Nasa and the space agencies of Europe and Canada, but the agencies have not sought coverage of the new findings, an indication that the community wants to see more data and robust evidence before saying anything about possible alien life. 

Nasa did not comment on the paper but instead provided a statement about Webb’s search for biosignatures. 

Finding “biosignatures, or gases produced only by life, is challenging for Webb - this requires tens to hundreds of hours of observing time for a single planet, the results may not be conclusive due to evolution of the star and planet atmosphere over time, and the planets Webb can search orbit relatively inhospitable stars,” Nasa said. 

“Finding life elsewhere in the universe will also be a process, and detection of a single potential biosignature would not constitute discovery of life,” the statement added. 

The search for extraterrestrial life has had many false positives over decades. 

In the early 20th century, astronomer Percival Lowell assured the world that Mars was laced with canals. 

In 1976, an experiment on Nasa’s Viking lander showed tantalising signs of possible microbial activity, but other tests showed that the surface appears sterile. 

Reports of alien spaceships visiting Earth, the topic of many a credulous media report as well as congressional hearings with more than a whiff of nonsense, invariably crash upon the unforgiving turf of the scientific method. 

Astronomers studying K2-18b will need to answer three basic questions before the broader scientific community may accept they have indeed discovered a biosignature, Sara Seager, an astrophysicist and a leading figure in the study of exoplanets, said in an email. 

“Is the signal real? Is it correctly attributed to the proposed gas, or could other gases produce the same feature? Can the gas be produced without life?” 

Even if the first two questions were answered satisfactorily, she said, the third “may never be fully resolved with the limited data exoplanets offer”. 

Her bottom-line assessment: “Not yet a biosignature, but a timely prompt to sharpen our methods and expectations”. 

Madhusudhan told reporters that DMS is abundant on K2-18b and that there is no known chemical mechanism to produce the molecule at that level other than through biological processes. 

“Either we are looking at a new chemical process that we haven’t seen before … or we’re witnessing the first signs of biological activity outside of Earth,” said Mans Holmberg, a co-author of the report and a postdoctoral researcher at the Space Telescope Science Institute. 

The solar system may harbour life in hidden realms, such as beneath the surface of Mars or on the ice-crusted moons of Jupiter or Saturn. Nasa’s Europa Clipper spacecraft launched in October in what will be a multiyear mission to study the Jovian moon, believed to have a subsurface ocean with as much water as all the oceans of Earth. 

Life exploits elements common in the universe. Planets are now known to be abundant, and stars can burn for billions of years as reliable power sources. The universe may be primed for life. 

The first planets outside our solar system were detected just 30 years ago, and since then astronomers have discovered thousands. 

K2-18b was first discovered by Nasa’s Kepler Space Telescope in 2015. The planet orbits its parent star, a small, relatively cool one known as a red dwarf, in just 33 days. The planet’s diameter is about 2½ times that of Earth. 

K2-18b’s orbit around the star, which puts it about one-seventh of Earth’s distance from the sun, is within the habitable zone of its planetary system. 

Observations by the Webb suggest that it has a nitrogen-rich atmosphere and a liquid ocean vastly deeper than any on our planet, Holmberg said. Such planets have been dubbed hycean worlds. “Hycean” is a portmanteau, derived from “hydrogen” and “ocean”. 

“Everything about this system is quite alien. We don’t have anything like it in the solar system,” Holmberg said. 

His team is hoping for additional observation time with the Webb to strengthen the case that the DMS detection is real. 

Beyond the question of whether there is life on K2-18b is the more speculative question of what form that life might take. On Earth, life remained one-celled for several billion years before the appearance of anything as complex as a clam. K2-18b is about 2.4 billion years old. 

Emily Mitchell, a biologist at the University of Cambridge who was not part of the new paper but has collaborated with its authors, said K2-18b is warmer than Earth and the evolution of life there could be a more rapid process than on our planet. 

That doesn’t mean fish or crabs or sharks exist, but the planet might have relatively complex microbes, she said. 

“It’s very, very speculative,” Mitchell said. “But our models would be consistent with having complex phytoplankton.” 

The search for biosignatures is likely to remain an endeavour burdened by ambiguities. Definitive discoveries may be unicorns. 

“There’s never going to be one biosignature where everyone says, yes, we all agree, definitely life,” Mitchell said. “Any one signature is always going to be debated.” 

-Joel Achenbach, Washington Post