A giant planet 124 light-years from Earth has provided the strongest evidence yet that alien life could thrive outside our solar system, astronomers say, writes "The Guardian".
Observations from the "James Webb" Space Telescope on a planet called K2-18 b, appear to reveal the chemical fingerprints of two compounds known only to be produced by life on Earth.
The discovery of the chemicals, dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), could bring us closer to answering the question of whether we are alone in the universe.
“This is the strongest evidence yet for biological activity outside the solar system,” said Prof. Niku Madhusudhan, an astrophysicist at the University of Cambridge who led the observations. “We are very cautious. We need to ask ourselves whether the signal is real and what it means.”
“This could be the turning point in the search for an answer to the fundamental question of whether we are alone in the universe”.
Others are more skeptical, raising questions about whether the general conditions on K2-18 b are conducive to life and whether DMS and DMDS, which are largely produced by marine phytoplankton on Earth, can reliably be considered biosignatures.
K2-18 b, which is located in the constellation Leo, is almost nine times more massive than Earth and 2.6 times larger, and orbits its star, a cool red dwarf less than half the size of the sun. When the Hubble Space Telescope spotted water vapor in its atmosphere in 2019, scientists declared it the “most habitable world known” outside the solar system.
The supposed water signal was shown to be methane in follow-up observations by Madhusudhan’s team in 2023. But they argue that K2-18 b’s profile is consistent with a habitable world covered by a vast, deep ocean – a view that remains controversial. More provocatively, the Cambridge team has reported a preliminary hint of DMS.
Planets outside our solar system are too distant to be photographed or reached by robotic spacecraft. But scientists can estimate their size, density and temperature, and study their chemical composition, by tracking the exoplanet as it passes across the face of its host star and measuring the starlight that has been filtered through its atmosphere. In the latest observations, the wavelengths absorbed by DMS and DMDS suddenly decrease as K2-18 b wanders in front of the red dwarf.
“The signal came in loud and clear,” Madhusudhan said. “This is the first time we’ve been able to do this…it’s mind-boggling that it’s possible.”
The findings, published in The Astrophysical Journal Letters, suggest concentrations of DMS, DMDS, or both (their signatures overlap) thousands of times stronger than levels on Earth. The results are reported to “three sigma” level of statistical significance (0.3% chance of occurring by chance), although this falls short of the gold standard for discoveries in physics.
“There could be processes that we don’t know about that produce these molecules,” says Madhusudhan. “But I don’t think there’s a known process that can explain this without biology.”
A challenge in identifying potential other processes is that the conditions on K2-18 b remain controversial. The Cambridge team suggests it has an ocean, while others say the data points to a gas planet, or oceans made not of water but of magma.
The question remains whether DMS could have been delivered to the planet by comets – This would require intense bombardment, which seems unlikely, or it could have been produced in hydrothermal vents, volcanoes or thunderstorms by exotic chemical processes.
“Life is one possibility, but it’s one among many,” said Dr. Nora Haney, a chemist at the Institute of Physics at the University of Bern whose research revealed that DMS was present on an icy, lifeless comet. “We would have to rigorously rule out all other options before we settle on life.”
Dr. Joe Barstow, a planetary scientist at the Open University, also considered the discovery significant, but said: “My skepticism about any claim of evidence for life is always maximum, not because I don’t believe there is other life, but because I think that for such a profound and significant discovery the burden of proof must be very, very high. I don't think this latest discovery crosses that threshold.
At a distance of 120 light-years, there's no prospect of resolving the debate through close-up observations, but Madhusudhan notes that this hasn't been an obstacle to the discovery of black holes or other cosmic phenomena.
“In astronomy, it's never a question of going,” he said. “We're trying to find out if the laws of biology are universal in nature. We don't see it as, ‘We have to go and throw ourselves in the water to catch the fish.’”