‘These are the first hints we are seeing of an alien world that is possibly inhabited.”
These words from a Cambridge astrophysicist signalled an “astounding” leap forward in the quest to find extraterrestrial life.
Thanks to data from the James Webb Space Telescope (JWST), there is now a 99.7 per cent certainty that the atmosphere of a planet called K2-18b contains at least one gas that, on Earth, is uniquely produced by living organisms in nature.
The James Webb Space Telescope
NASA/GETTY IMAGES
The best explanation is that K2-18b — which orbits a star 124 light years away, a distance it would take 187,000 years to reach in our fastest spacecraft — is a water-covered planet “teeming” with algae-like microbes in its oceans, the lead scientist has suggested.
Dr Nikku Madhusudhan of the Institute of Astronomy at the University of Cambridge, who led the study, said of the findings: “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.”
The word “hycean” is a combination of the terms “hydrogen” and “ocean”, coined to describe a planet such as K2-18b, which is thought to have a hydrogen-rich atmosphere and is entirely covered in liquid water on its surface. It orbits once every 33 days within the habitable zone of its star, K2-18, which can be found in the constellation of Leo, and the planet is about 2.6 times the radius of Earth. Scientists have sounded a note of caution and have stopped short of declaring that they have certainly discovered alien life, but Madhusudan added: “It was astounding … it was an incredible realisation seeing the results emerge.” He described it as a “shock to the system” to detect such a strong biomarker, the term for a tell-tale clue on an alien world that could potentially be produced by living things. “This is a very fundamental advancement in our search for life.” • The Times view: Planet could be among most momentous discoveries in history The JWST detected strong signs that one or both of the gases dimethyl sulphide (DMS) and dimethyl disulphide (DMDS), compounds made up of carbon, hydrogen and sulphur, are present in concentrations of at least ten parts per million in the planet’s atmosphere. Scientists have struggled to conceive of any way that the gases, which decay within hours once produced, could be present and replenished in the planet’s atmosphere at such high levels without the involvement of living creatures. On Earth, the gases are overwhelmingly produced by photosynthetic plankton, but also by some bacteria, seaweeds and plants. DMS is generally produced and released when the organisms become stressed. A study, published in The Astrophysical Journal Letters, said that DMS and DMDS are “both molecules uniquely produced by life on Earth and predicted as promising biosignatures in habitable exoplanets”. The discovery could cause K2-18b to leap above much closer sites in our own solar system, such as Mars and Europa and Enceladus, the icy moons of Jupiter and Saturn with subterranean oceans, to find itself “at the very top of the league” as a candidate for humanity’s first detection of alien life, according to Madhusudhan. The scientist was, however, keen to sound a loud note of caution. Another 16 to 24 hours of observations with the JWST would be needed to boost the confidence levels for the detection of the DMS gases up from a 3-sigma level, a certainty of 99.7 per cent, to the gold standard 5-sigma level, a certainty of 99.99994 per cent, required to make a definitive declaration. Further theoretical work, to conceive of any non-living process that could explain the presence of the gas, is also underway. Some recent studies have suggested that DMS can be present in deep space in interstellar gases and even comets. “We cannot at this stage make the claim that, even if we detect DMS and DMDS, it is [certainly] due to life, let me be very clear about that,” Madhusudhan said. “But if you take published studies so far, then there is no mechanism in the literature that can explain what we are seeing without life.” More than 5,500 “exoplanets” — planets outside our solar system — have been detected. They are much too far away for any spacecraft to reach and it is at present impossible to take a direct photographic image of a planet such as K2-18b, because it is too distant and too close to the glare of its star. However, when one of these planets passes across the face of its star as seen from Earth, known as a transit, some of the star’s light skims through the planet’s atmosphere en route to our telescopes. The JWST can then analyse the chemical make-up of the atmosphere because different elements and compounds in the planet’s air absorb different parts of the spectrum, leaving a tell-tale fingerprint in the starlight. The Times met Madhusudhan last year just before the JWST made its latest groundbreaking observations, which took almost 12 months to analyse. The telescope had made a discovery in 2023 that showed not only carbon dioxide and methane, but also surprising “hints” of DMS in the planet’s atmosphere, but the confidence level of these observations ranged from 95 per cent down to 0 per cent. New observations were made using a different instrument that observes light in the mid-infrared range. All DMS detection reached a confidence level of 99.7 per cent, according to the study. Madhusudhan’s work has faced challenges from other exoplanetary scientists, with some claiming the oceans on K2-18b could be too hot to support life and others saying that the planet may have no water on its surface at all or may be uninhabitable because it is “tidally locked”, with one hemisphere constantly facing towards its star. However, Madhusudhan remains convinced that the data points to K2-18b being a hycean world and that meteorites could deliver enough nutrients to sustain life on a planet with no rocky land. Dr Michael Steinke, an expert in DMS from the University of Essex, who was not involved in the study, said the presence of water, methane and DMS seems to “tick quite a few boxes” to make K2-18b a leading site for finding alien life. He said it was important to consider recent studies suggesting that the gas can potentially be produced in space by “abiotic” means without life, but said: “I’m excited to read the paper and see whether there might be life similar to Earth on another planet.”