Scientists are preparing for our best shot yet to find alien life on exoplanets, which are worlds that orbit other stars. As the James Webb Space Telescope (JWST), a space observatory with unparalleled sensitivity, gears up to take its first observations, researchers are gaming out the potential identification of biosignatures, which are signs of life, on the exoplanets in JWST’s sights.
Now, a team led by Maggie Thompson, a graduate student in astronomy and astrophysics at UC Santa Cruz, has presented an updated guide to interpreting detections of methane gas on exoplanets, which can be produced by living and abiotic processes.
“This is a super exciting time!” said Thomspon in an email. “JWST is going to revolutionize our understanding of exoplanets and will allow us to begin characterizing the atmospheres of rocky, potentially habitable worlds. I'm very excited to see what JWST discovers and what sorts of interesting targets it identifies that we will want to continue observing with future telescopes.”
Thompson and her colleagues note that “methane is the only biosignature that the James Webb Space Telescope could readily detect in terrestrial atmospheres,” making it “imperative to understand methane biosignatures to contextualize these upcoming observations,” according to a study published Monday in Proceedings of the National Academy of Sciences.
Scientists are already trying to find biosignatures in the atmospheres of exoplanets that could hint at the presence of life, such as oxygen, oxone, and carbon dioxide. However, the molecular properties of methane line up with the JWST’s sweet observational spot at near-infrared wavelengths, making it a particularly important compound in the search for extraterrestrial life.
“The way we will observe methane, or any atmospheric gas, in an exoplanet's atmosphere with JWST is going to be through spectral observations in the infrared,” Thompson explained. “Methane's absorption features in the near-infrared, where JWST is most sensitive, are stronger than that of molecular oxygen (O2) and ozone (O3), making methane more easily detectable than oxygen.”
“In addition, other studies have simulated JWST data, including a paper by co-author Joshua Krissansen-Totton in 2018, found that it will likely be possible to detect and constrain methane abundances but detecting oxygen would be much more challenging,” she added.
To anticipate the complexities of a methane detection on an alien world, Thompson and her colleagues assessed the broader contexts that could help distinguish between bonafide biosignatures and emissions from natural geological processes, such as volcanic eruptions.
Methane is short-lived in atmospheres compared to other compounds, such as carbon dioxide, so detecting it would be an indication that there is a huge supply of the gas constantly rising into the skies of another planet. One of the keys to determining whether that supply stems from living creatures or geological processes is to look at the overall composition of the atmosphere, the study suggests.
For instance, volcanic eruptions and tectonic processes belch out methane, but these abiotic events also emit gasses such as carbon dioxide and carbon monoxide. Since carbon monoxide is an easy gas for many lifeforms to consume here on Earth, the team suggests that a planet with carbon dioxide and methane in its atmosphere, but relatively little carbon monoxide, is more likely to host life.
“Although JWST alone will likely not be able to fully assess habitability, it may identify interesting targets, like a rocky exoplanet with abundant methane and carbon dioxide with little to no carbon monoxide, which would motivate observations with future telescopes to uncover the broader planetary context necessary to determine if the methane is being produced by life,” Thompson said.
In addition to using past and present Earth as an analogue for studying atmospheric methane, the team looked to Mars, which appears to produce whiffs of the gas, and Saturn’s moon Titan, a world awash in volatile elements, including liquid and gas versions of methane. Atmospheric methane on Mars and Titan is likely to be abiotic, making these worlds useful analogs of lifeless exoplanets that nonetheless produce a powerful biosignature. These examples can help scientists avoid false positives when looking for aliens in other star systems.
“Mercury is another interesting analog because, although its small size and proximity to the Sun preclude it from having an atmosphere, its crust is enriched in graphite (a crystalline form of carbon) and it serves as an example of worlds with more reducing interiors where you could imagine magmatic outgassing (i.e. volcanic activity) causing a methane-rich atmosphere,” Thompson noted.
“We investigated this possibility in our study and found that it is unlikely for planets with very reduced interiors like Mercury to magmatically outgas significant methane because most of the carbon will actually remain in the solid form as graphite,” she continued. “That being said, Mercury is still an interesting analog for rocky exoplanets that have very reduced interior compositions, and more work is needed to fully understand the atmospheres that could form abiotically via outgassing of such reduced interiors.”
The new study presents a revamped framework for assessing methane biosignatures on exoplanets, but much more research is needed to tease out all the many ambiguous forms that the gas is bound to take in the skies of other worlds. Interestingly, many revelations about distant worlds are likely to be solved by continuing to look at planets closer to home.
“There are a lot of avenues for future research based on this study,” Thompson said. “I'm particularly excited to further explore the possibility of exoplanets that are like Saturn's moon Titan that have large inventories of volatile species. If such planets exist at the outer edge of the habitable zone (so colder than Earth, but still potentially habitable), I'd like to determine if such planets could have atmospheres rich in methane due to abiotic sources.”
“I also think there is a lot of work to be done to understand the ability of chemical reactions between water and rock to generate abiotic methane under different planetary environments, which will require more laboratory experiments and theoretical modeling,” she concluded.