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In 2023, astronomers reported a tentative detection of dimethyl sulfide — which is predominately produced by marine microbes on Earth and regarded as a biosignature gas — in the atmosphere of the super-Earth exoplanet K2-18b. In a paper published in the Astrophysical Journal Letters, University of California, Riverside astronomer Shang-Min Tsai and colleagues challenge this finding, but also outline how the NASA/ESA/CSA James Webb Space Telescope might verify the presence of dimethyl sulfide.
K2-18 is a red dwarf located approximately 111 light-years away in the constellation of Leo. Also known as EPIC 201912552, the star hosts two massive exoplanets: K2-18b and K2-18c. first discovered in 2015, K2-18b has a radius of 2.2 times that of Earth and is about 8 times as massive. The planet orbits its star every 33 days at a distance of approximately 0.15 AU and has an Earth Similarity Index of 0.73; It receives 1.28 times the light intensity of Earth, and its equilibrium temperature is 28 degrees Fahrenheit (minus 2 degrees Celsius).
K2-18c, discovered in 2017, has a mass about 7.5 times that of Earth, orbits the host star one every 9 days, and is probably too hot to be in the habitable zone.
In 2023, astronomers reported a tentative detection of dimethyl sulfide (DMS) in the atmosphere of K2-18b.
“K2-18b gets almost the same amount of solar radiation as Earth, and if atmosphere is removed as a factor, K2-18b has a temperature close to Earth’s - which is also an ideal situation in which to find life,”
Dr. Tsai said.
“K2-18b’s atmosphere is mainly hydrogen, unlike our nitrogen-based atmosphere, but there was speculation that K2-18b has water oceans, like Earth; that makes K2-18b a potentially Hycean world, which means a combination of a hydrogen atmosphere and water oceans."
What was icing on the cake, in terms of the search for life, is that last year researchers reported a tentative detection of DMS in the atmosphere of that planet, which is produced by ocean phytoplankton on Earth. 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, Dr. Tsai and co-authors wanted to understand whether enough DMS could accumulate to detectable levels on K2-18b.
“The DMS signal from Webb was not very strong and only showed up in certain ways when analyzing the data. We wanted to know if we could be sure of what seemed like a hint about DMS.”
Based on computer models that account for the physics and chemistry of DMS, as well as the hydrogen-based atmosphere, the researchers found that it is unlikely the data show the presence of DMS.
“The signal strongly overlaps with methane, and we think that picking out DMS from methane is beyond this instrument’s capability,”
Dr. Tsai said.
However, the scientists believe it is possible for DMS to accumulate to detectable levels. For that to happen, plankton or some other life form would have to produce 20 times more DMS than is present on Earth. Detecting life on exoplanets is a daunting task, given their distance from Earth, so to find DMS, Webb would need to use an instrument better able to detect infrared wavelengths in the atmosphere than the one used last year.
Fortunately, the telescope will use such an instrument later this year, revealing definitively whether DMS exists on K2-18b.
“The best biosignatures on an exoplanet may differ significantly from those we find most abundant on Earth today,”
said Dr. Eddie Schwieterman, an astrobiologist at the University of California, Riverside.
“On a planet with a hydrogen-rich atmosphere, we may be more likely to find DMS made by life instead of oxygen made by plants and bacteria as on Earth.”