A team of scientists is doubling down on its claims that a world 124 light-years away in space is likely covered in oceans and full of aquatic life, with new data to support the findings.
The research, led by astronomers at the University of Cambridge in the United Kingdom, kicked off a fervent debate in 2023 when it suggested K2-18b, an exoplanet in the constellation Leo, gave a chemical signal for dimethyl sulfide gas. On Earth, that molecule is only known to be made by living things — primarily phytoplankton, a type of microscopic algae.
The initial report was based on observations from the powerful James Webb Space Telescope, a collaboration of NASA and its European and Canadian counterparts. But the results created a lot of hubbub among habitable world experts. Skeptics criticized the weakness of the signal and other aspects of the study, such as the belief that the planet, about nine times heavier than Earth and 2.5 times wider, is indeed a water world.
Other scientists continue to feel frustrated with the way the work is being described to the public, with news headlines that suggest the group is closer to discovering life beyond Earth than it really is.
Now the team has put forward a follow-up study, using a different instrument on Webb, that offers a fresh view of the planet and more evidence for either dimethyl sulfide or a similar life-related compound, dimethyl disulfide, in its atmosphere.
“The signal came through, strong and clear,” said Nikku Madhusudhan, lead author of the new paper, published this week in The Astrophysical Journal Letters.
Scientists haven’t found a rocky exoplanet with air. But now they have a plan.
Further studies of exoplanet K2-18b, which orbits a cool dwarf star in its so-called “habitable zone,” continue to incite controversy over whether it hosts life.
Credit: NASA / ESA / CSA / Joseph Olmsted (STScI) illustration
K2-18b orbits a red dwarf star, cooler than the sun, in its so-called “habitable zone,” the region around a host star where it’s not too hot or cold for liquid water to exist on the surface of a planet. In our solar system, that sweet spot encompasses Venus, Earth, and Mars.
Based on the new analysis, the scientists seem more confident that K2-18b is a Hycean world, a type of planet predicted to exist in the galaxy, combining the words “hydrogen” and “ocean.” These theorized exoplanets, a subset of worlds that can’t be found in our own solar system, are called mini Neptunes: smaller than Neptune but larger than Earth.
Such planets could be covered in water and surrounded by thick atmospheres full of hydrogen gas, unlike Earth’s nitrogen-based atmosphere. If they exist, Hycean worlds are expected to be easier to see and study with telescopes than Earth-like rocky worlds, because they’re bigger and have puffier atmospheres.
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“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,” Madhusudhan said in a statement. He did not respond to questions from Mashable earlier this week.
Meanwhile, other scientists are offering counter arguments to explain the planet, such as the possibility that K2-18b is a big rock wrapped in a magma ocean, with little likelihood of being habitable. Some also have attempted to apply different computer models to the 2023 data and could not pick out dimethyl sulfide, or DMS for short, from other signals.
The latest Webb data focused on mid-infrared light, a different part of the light spectrum, where DMS and dimethyl disulfide, or DMDS, leave stronger chemical fingerprints.
The way they analyzed the planet’s atmosphere is called transmission spectroscopy. When planets cross in front of their host star, starlight shines through their atmospheres. Molecules within the atmosphere absorb certain light wavelengths, or colors, so by splitting the star’s light into its basic parts — a rainbow — astronomers can look for which light segments are missing to figure out the makeup of an atmosphere.
While Earth has relatively tiny amounts of DMS and DMDS, K2-18b appears to have much more — perhaps thousands of times more, according to the paper, fitting with theories for Hycean planets. The observations reached “three-sigma” significance, the team said, meaning there’s only a 0.3 percent chance the results happened by accident. Their findings could qualify as a formal scientific discovery with just 16 to 24 more hours of Webb telescope observations, they said.
Regardless of the way the Cambridge team has talked about its next steps, there doesn’t appear to be consensus among scientists on the right time to claim a detection of extraterrestrial life. That may be one reason why their work is raising hackles, said Michaela Leung, a planetary scientist at the University of California in Riverside.
Despite the James Webb Space Telescope’s power, scientists question whether it’s capable of definitively identifying specific life-produced gases in exoplanet atmospheres.
Credit: NASA GSFC / CIL / Adriana Manrique Gutierrez illustration
She recently wrote a paper on other molecules researchers could look for with Webb that are linked to biology on Earth.
“Look for another gas,” Leung told Mashable. “A strong claim of life detection here is going to report more than one potential biosignature. Even if what is in that atmosphere is DMS, which I think is not clear at this time, I think we would also have to robustly eliminate all abiotic possibilities.”
The Cambridge team considered how the two gases might form without organisms, such as through starlight or from comets, giant snowballs hurtling through space. Still, they believe a biological explanation is the more likely scenario.
No matter how much more time they spend measuring the chemical composition of K2-18b’s atmosphere, that data alone won’t be enough to convince some scientists, said Sarah Hörst, a professor at Johns Hopkins University in Baltimore, who specializes in planetary atmospheres.
“We have many examples in our own solar system of molecules that could be considered to be signs of life but have eventually been shown to have other explanations,” Hörst told Mashable. “The search for life is quite challenging and will require extraordinary evidence.”