A new study questions the discovery of signs of life on a distant planet by the James Webb Space Telescope, although it describes how it could verify the presence of gas of biotic origin.
The research, carried out by the University of California Riverside and published in Astrophysical Journal Letters, does not rule out the possibility of a discovery in the near future.
In September 2023 there were tantalizing reports of a biosignature gas in the atmosphere of planet K2-18b, a potential water world 120 light years away, which appeared to have several conditions that would make life possible.
Many exoplanets, that is, planets orbiting other stars, are not easily comparable to Earth. Their temperatures, atmospheres and climates make it difficult to imagine Earth-type life on them.
However, K2-18b is a little different. “This planet receives almost the same amount of solar radiation as Earth. And if you eliminate the atmosphere as a factor, K2-18b has a temperature close to that of Earth, which is also an ideal situation to find life,” he said in a statement from the UCR project scientist and author of the article, Shang-Min Tsai.
K2-18b’s atmosphere is composed primarily of hydrogen, unlike our nitrogen-based atmosphere. But it was speculated that K2-18b has oceans of water, like Earth. That makes K2-18b a potentially “Hycean” world, meaning a combination of a hydrogen atmosphere and water oceans.
Last year, a Cambridge team revealed methane and carbon dioxide in K2-18b’s atmosphere using JWST, other elements that could indicate signs of life.
“What was the icing on the cake, in terms of searching for life, is that last year these researchers reported a tentative detection of dimethyl sulfide, or DMS, in that planet’s atmosphere, which is produced by oceanic phytoplankton from the earth”. Tsai said. DMS is the main source of airborne sulfur on our planet and may play a role in cloud formation.
Because the telescope data were inconclusive, UCR researchers wanted to understand whether enough DMS could accumulate to detectable levels in K2-18b. As with any planet this far away, obtaining physical samples of atmospheric chemicals is impossible.
“The DMS signal from the Webb telescope was not very strong and only appeared in a certain way when analyzing the data,” Tsai said. “We wanted to know if we could be sure of what looked like a clue about DMS.”
Based on computer models that take into account the physics and chemistry of DMS, as well as the hydrogen-based atmosphere, the researchers found that the data are unlikely to show the presence of DMS. “The signal overlaps strongly with methane, and we believe that detecting DMS from methane is beyond the capability of this instrument,” Tsai said.
However, researchers believe it is possible for DMS to accumulate to detectable levels. For that to happen, plankton or some other form of life would have to produce 20 times more DMS than is present on Earth.
ANALYSIS WITH A NEW INSTRUMENT
Detecting life on exoplanets is a daunting task, given their distance from Earth. To find DMS, the Webb telescope would need to use an instrument more capable of detecting infrared wavelengths in the atmosphere than the one used last year. Fortunately, the telescope will use such an instrument later this year, which will definitively reveal whether DMS exists on K2-18b.
“The best biosignatures on an exoplanet may differ significantly from those we find most abundant on Earth today. On a planet with a hydrogen-rich atmosphere, we are more likely to find DMS produced by life rather than oxygen produced by plants and bacteria like on Earth,” said UCR astrobiologist Eddie Schwieterman, lead author of the study.
Given the complexities of searching for signs of life on remote planets, some wonder about the researchers’ continued motivations.
“Why do we keep scanning the cosmos for signs of life? Imagine you’re camping in Joshua Tree at night and you hear something. Your instinct is to turn on a light to see what’s out there. That’s what we’re doing too, in a way,” Tsai said.
