It’s Not Conclusive, But Methane is Probably the Best Sign of Life on Exoplanets

When the James Webb Space Telescope aims at exoplanet atmospheres, it’ll use spectroscopy to identify chemical elements. One of the things it’s looking for is methane, a chemical compound that can indicate the presence of life.

Methane is a compelling biosignature. Finding a large amount of methane in an exoplanet’s atmosphere might be our most reliable indication that life’s at work there. There are abiotic sources of methane, but for the most part, methane comes from life.

But to understand methane as a potential biosignature, we need to understand it in a planetary context. A new research letter aims to do that.

Methane is interesting because it doesn’t last long in an atmosphere. Photochemical reactions destroy it, so detecting a lot of it means something is constantly replenishing it. There has to be a large and prominent source. “Terrestrial planets, which are the focus of this study, require significant methane surface fluxes to sustain high atmospheric abundances,” the study says. “On Earth, life sustains large methane surface fluxes, and so methane has long been regarded as a potential biosignature gas for terrestrial exoplanets.”

But not all methane detections will mean life. Scientists need a way to work through any future methane detections in exoplanet atmospheres. The researchers wanted to create a “…dedicated assessment of the planetary conditions needed for methane to be a good biosignature.”

“We wanted to provide a framework for interpreting observations, so if we see a rocky planet with methane, we know what other observations are needed for it to be a persuasive biosignature.”

Maggie Thompson, lead author, UC Santa Cruz.

The new research letter is titled “The case and context for atmospheric methane as an exoplanet biosignature.” It’s available online at the Proceedings of the National Academy of Sciences (PNAS.) The lead author is Maggie Thompson, a graduate student in astronomy and astrophysics at UC Santa Cruz.

Part of methane’s status as a reliable biosignature is its detectability. Oxygen is a good biosignature, and for some of the same reasons methane is. Like methane, it’s also unstable in an atmosphere, so finding a large quantity of it means a significant source is replenishing it.

The James Webb Space Telescope will be operating soon, and one of its jobs is to examine exoplanet atmospheres spectroscopically. It’ll be characterizing exoplanet atmospheres, and detecting biosignatures is part of that work. But while Webb will detect methane relatively easily in a terrestrial atmosphere, oxygen is more difficult to detect. “Given the imminent feasibility of observing methane with JWST, it is imperative to determine the planetary conditions where methane is a compelling biosignature,” the authors write.

This research letter wants to put scientists in a stronger position to interpret methane’s presence in an exoplanet atmosphere. The idea is to identify what other questions researchers need to ask when they detect methane. What other indications will support the conclusion that methane is a biosignature? What will contradict that?

“We wanted to provide a framework for interpreting observations, so if we see a rocky planet with methane, we know what other observations are needed for it to be a persuasive biosignature,” lead author Thompson said in a press release.

The team examined abiotic methane sources to understand how they might account for methane in an exoplanet atmosphere. Volcanoes are one abiotic source of methane. Methane also comes from reactions in places like mid-ocean ridges, hydrothermal vents, and tectonic subduction zones. Comet and asteroid impacts can also produce methane. Researchers can look for evidence of these sources on an exoplanet where they detect methane.

<Click Image to Enlarge> This figure from the study compares methane sources on Earth. The top row shows biogenic methane flux; all other rows show abiogenic methane flux from other sources. No abiotic source can produce the same methane flux as life can. Image Credit: Thompson et al. 2022.

In planetary atmospheres, methane exists in relation to other gases. So identifying abiotic methane sources is only part of the picture. How do other gases like carbon monoxide and carbon dioxide fit into a methane-rich atmosphere? How do they affect one another?

“Methane is one piece of the puzzle, but to determine if there is life on a planet you have to consider its geochemistry, how it’s interacting with its star and the many processes that can affect a planet’s atmosphere on geologic timescales.”

Maggie Thompson, lead author, UC Santa Cruz.

From the research letter:

“While methane can be produced by a variety of abiotic mechanisms such as outgassing, serpentinizing reactions, and impacts, we argue that—in contrast to an Earth-like biosphere—known abiotic processes cannot easily generate atmospheres rich in CH4 and CO2 with limited CO…” the authors explain in their paper.

Earth’s history provides some clues to methane in exoplanet atmospheres. We know that in the past, Earth had an even more methane-rich atmosphere than it does now. And we know what the source was: life.

“If you detect a lot of methane on a rocky planet, you typically need a massive source to explain that,” said co-author Joshua Krissansen-Totton, a Sagan Fellow at UCSC. “We know biological activity creates large amounts of methane on Earth, and probably did on the early Earth as well because making methane is a fairly easy thing to do metabolically.”

Methane-producing microorganisms called methanogens were one of Earth’s earliest lifeforms, originating between 4.11 and 3.78 billion years ago. They were so effective at producing methane that at several times early Earth likely had a hazy, methane-filled atmosphere similar to Saturn’s moon Titan. Maybe we’ll find an exoplanet with a methane-rich atmosphere similar to early Earth’s one day. If that happens, we’ll likely detect it from a great distance, making it challenging to determine if the source is biotic.

But detecting abiotic sources of methane is potentially much more straightforward. Volcanoes, for example, provide other clues that methane is from a non-living source. Volcanoes not only inject methane into the atmosphere but also carbon monoxide. On the other hand, biological activity is likely to consume carbon monoxide. The researchers found that nonbiological processes cannot readily produce habitable planet atmospheres rich in methane and carbon dioxide and with little to no carbon monoxide.

This diagram shows the geological process of subduction, where a heavier tectonic plate sinks under a lighter one. Alteration of ultramafic rocks in subduction zones plays a major role in methane production via abiotic processes on Earth and beyond. Image Credit: By KDS4444 – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=49035989.

“One molecule is not going to give you the answer—you have to take into account the planet’s full context,” Thompson said. “Methane is one piece of the puzzle, but to determine if there is life on a planet, you have to consider its geochemistry, how it’s interacting with its star and the many processes that can affect a planet’s atmosphere on geologic timescales.”

The authors point out that the detection of methane in an exoplanet’s atmosphere is just the beginning. They found that detecting methane is a strong indicator of life for a rocky planet orbiting a Sun-like star if the atmosphere also contains carbon dioxide. If methane is more abundant than carbon dioxide, that’s also a more robust indicator of life, as long as the planet isn’t too water-rich.

This image is a summary of known abiotic sources of methane on Earth. (©2022 Elena Hartley)

The James Webb Space Telescope is extraordinarily powerful. But no observing tool or method is error-free, even the long-awaited Webb. False positives are an issue in scientific endeavours like the search for biosignatures. The researchers looked at the role false positives play in biosignatures and gave some guidelines for handling methane detections.

“The atmospheres of rocky exoplanets are probably going to surprise us, and we will need to be cautious in our interpretations.”

Joshua Krissansen-Totton, co-author, UCSC.

“There are two things that could go wrong—you could misinterpret something as a biosignature and get a false positive, or you could overlook something that’s a real biosignature,” Krissansen-Totton said. “With this paper, we wanted to develop a framework to help avoid both of those potential errors with methane.”

“With the upcoming technological advancements in exoplanet observations enabling the characterization of potentially habitable exoplanets, it is important to consider possible biosignature gases and the sources of false-positive detections,” the research letter says. “This is particularly urgent for methane since biogenic methane is likely detectable for some terrestrial exoplanets with JWST.”

The researchers acknowledge that various abiotic sources could replenish atmospheric methane in diverse planetary environments. But for a planet to produce a methane flux comparable to Earth’s, the same abiotic sources would also generate “observable contextual clues” that would indicate a false positive. “In every case, abiotic processes cannot easily produce atmospheres rich in CH4 and CO2 with negligible CO…” Life would readily consume the CO.

“Clearly, the mere detection of methane in an exoplanet’s atmosphere is not sufficient evidence to indicate the presence of life given the variety of abiotic methane-production mechanisms. Instead, the entire planetary and astrophysical context must be taken into account to interpret atmospheric methane.”

The hunt for biosignatures in exoplanet atmospheres is a relatively new scientific undertaking. Researchers need to do a lot of groundwork before they can have confidence in detecting things like methane. The recent detection, or non-detection, of methane on Mars, shows how incomplete our understanding of other planets is and how the detection of methane may only be a starting point in painting a complete picture of a planet.

This image illustrates possible ways methane might get into Mars’ atmosphere and be removed from it: microbes (left) under the surface that release the gas into the atmosphere, weathering of rock (right), and stored methane ice called a clathrate. Ultraviolet light can work on surface materials to produce methane and break it apart into other molecules (formaldehyde and methanol) to produce carbon dioxide. Credit: NASA/JPL-Caltech/SAM-GSFC/Univ. of Michigan

“This study is focused on the most obvious false positives for methane as a biosignature,” Krissansen-Totton said. “The atmospheres of rocky exoplanets are probably going to surprise us, and we will need to be cautious in our interpretations. Future work should try to anticipate and quantify more unusual mechanisms for nonbiological methane production.”

“With these results, we provide a tentative framework for assessing methane biosignatures,” the authors write. “If life is abundant in the Universe, then with the correct planetary context, atmospheric methane may be the first detectable indication of life beyond Earth.”

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Evan Gough

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