Biosignature Gases: A Needle in a Haystack

By Sara Seager, Massachusetts Institute of Technology (MIT)

Somewhere out there, a living, breathing world peacefully orbits its star. Inhabiting the world is a dynamic ecosystem full of thriving bacteria. The life itself has no consciousness or intelligence, but the planet as a whole is an active world, connected through cycles of geophysics, chemistry, and biology in a landscape with liquid water oceans, continents, mountains and volcanoes. We imagine there could be millions or even billions of such planets in our Galaxy. Is Proxima Centauri lucky enough to host such a planet?

To find out whether or not a planet has life, it is not enough to know that the planet is rocky and if the planet orbits in the host star’s habitable zone. We must be able to investigate the planet’s atmosphere. For Proxima Cenaturi, as in most cases, we will need a to use a different telescope than one used to discover the planet (see post by Ignas Snellan). More specifically we want to observe the planet atmosphere to first assess the greenhouse power of the atmosphere to estimate whether or not the surface temperature is suitable for life. Next we want to determine if there are gases that indicate the planet is habitable. Most challenging but most exciting, we want to know if the exoplanet atmosphere contains gases that might indicate if the planet is possibly inhabited.

How to measure the chemical signatures in the atmosphere of a transiting exoplanet. The total light measured off-transit (B in the lower left figure) decreases during the transit, when only the light from the star is measured (A). By subtracting A from B, we get the planet counterpart, and from this the “chemical fingerprints” of the planet atmosphere can be revealed. Credits: NASA/JPL-Caltech.

By habitable I mean a planet with surface liquid water oceans, since all life as we know it needs liquid water. But, since oceans are hard to identify from afar, we will look for water vapor as an indicator of water oceans. By possibly inhabited I mean the identification of signs of life by way of biosignature gases—gases that are produced by life and can accumulate in an exoplanet atmosphere to be detectable from afar. Even if we are lucky enough to detect a biosignature gas on a rocky planet orbiting Proxima Centauri, we will not know if the suspect gases are produced by tiny microbes, enormous animals, or intelligent humanoids. We also will not know if the life producing the gases is carbon based or something more exotic. We focus on what life does—life metabolizes and produces byproduct gases, not on what life is.

Oxygen is our most compelling biosignature gas. Oxygen fills Earth’s atmosphere to 20% by volume. But, without plants or photosynthetic bacteria, Earth’s atmosphere would have virtually no oxygen. Oxygen, and its photochemical byproduct ozone, have strong spectral features at a range of wavelengths, accessible with future ground- and space-based telescopes that might be able to study atmospheres of any planets discovered orbiting Proxima Centauri. If we detect oxygen, astronomers and the public alike will be absolutely ecstatic. But does an oxygen detection mean we have found alien life? No. Unfortunately the attribution of oxygen—or any gas—to life is an in depth, complicated, and somewhat subjective process. The reason is that there are many ways oxygen can be produced, and accumulate, in an exoplanet atmosphere that has nothing to do with life. We must be able to rule out all other possibilities of oxygen generation by non-biological processes. Even then, we will only be able to claim a strong suggestion of life detection not a robust detection.

A flurry of recent activity has detailed a number of different oxygen-producing scenarios that are not related to life. Most of the scenarios have to do with a lack of oxygen “sinks”. If oxygen is not destroyed, then even small quantities of oxygen can accumulate over a large amount of time. One of the more compelling oxygen false positive scenarios is related to the ultraviolet (UV) radiation of exoplanet host stars. The UV radiation splits apart molecules in the planet atmosphere, setting off a chain of chemical reactions that produce byproducts that can destroy oxygen. A major player, OH, is nicknamed the “garbage eater of the atmosphere” because of its power to destroy oxygen and other gases. M dwarf stars typically have a high far-ultraviolet radiation flux (< 200 nm) and a lower near-ultraviolet radiation flux (200-300 nm) compared to our Sun. Any exoplanet orbiting an M star will therefore be subject to different photochemistry than Earth’s atmosphere. Specifically, the chains of reactions that produces OH are weaker, owing to strong far-ultraviolet radiation. With a much smaller amount of OH compared to Earth’s atmosphere, abiotic oxygen can accumulate. To identify this false positive scenario we would need to be able to measure Proxima Cenaturi’s far-UV and near-UV radiation. Other oxygen false-positive scenarios include planets with a carbon dioxide-dominated atmosphere but little volcanic emission, an M star that took a very long time to reach a stable hydrogen burning phase, a planet undergoing a transient ocean evaporation from a runaway greenhouse effect, and more. If we are so lucky to find oxygen on Proxima Centauri, we will have a lot of further observations and atmosphere modeling work to do to understand if the oxygen can be attributed to life or if it might be a false positive.

Beyond oxygen, astronomers also consider a wide range of other biosignature gases, including methane, nitrous oxide, dimethyl sulfide, and others. Despite a growing list and detailed studies, I worry that the list of gases may be too limited, or that the types of planets modeled—usually small deviations from an Earth twin—are not broad enough to anticipate the range of what planet types are out there. If Proxima Cenaturi has a rocky planet in its habitable zone, we should do all we can to make sure we don’t miss a sign of life, just because we were too constrained in our thinking.

Life on Earth produces literally thousands of gases. Most are produced in too small quantities to accumulate to any reasonable level in Earth’s atmosphere. In addition, most are produced for highly organism-specific reasons—such as stress and signaling—that appear to be whims of evolution. Some molecules could be produced in larger quantities on another planet and/or accumulate in an exoEarth atmosphere to high levels, depending on the exoEarth ecology and surface and atmosphere chemistry. In other words, there is a possibility that any gas might be a biosignature gas, if it is present in very high quantities in an exoplanet atmosphere and can’t otherwise be explained away.

Motivated by this reasoning, my team spent a few years constructing and curating a list of all molecules that exist in gas form in a planet atmosphere with a similar temperature and pressure to Earth’s. We both combinatorically constructed lists and also exhaustively searched the literature and found about 14,000 molecules. About 2500 of these are hydrocarbons. We plan to work through this list in classes of molecules to understand their atmospheric and surface chemistry, photochemistry, and spectral properties. From this we can select both promising chemical candidates, and promising ways to search the spectrum that could capture the most diverse range of such candidates.

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Schematic for the concept of considering all small molecules in the search for biosignature gases. The goal is to generate a list of all small molecules and filter them for the set that is stable and volatile in temperature and pressure conditions relevant for exoEarth planetary atmospheres. Further investigation relates to the detectability: the sources and sinks that ultimately control the molecules’ accumulation in a planetary atmosphere of specific conditions as well as its spectral line characteristics. Geophysically or otherwise generated false positives must also be considered. In the ideal situation, this overall conceptual process would lead to a finite but comprehensive list of molecules that could be considered in the search for exoplanet biosignature gases. Credit: S. Seager and D. Beckner.

Does this sound like a lot of work for a library of gases even though the study of atmospheres of any planets found to orbit Proxima Cenaturi and others lie a decade or more in the future? It is. But it will take a long time to fully prepare so we don’t miss out on a biosignature gas detection.

Despite an exhuberant realization that the search for and detection of biosignature gases is within reach, there is a long road ahead. Nonetheless the coming decades are opportune for extensive progress in finding and characterizing other Earths, and full of hope for biosignature gas detection. I remain as hopeful as ever as I plan to devote the rest of my career to the search for life on exoplanets.

Professor Seager

About the Author

Sara Seager is an astrophysicist and planetary scientist at MIT. Her science research focuses on theory, computation, and data analysis of exoplanets. Her research has introduced many new ideas to the field of exoplanet characterization, including work that led to the first detection of an exoplanet atmosphere. Professor Seager also works in space instrumentation and space missions for exoplanets, including CubeSats, as a co-I on the MIT-led TESS, a NASA Explorer Mission to be launched in 2017, and chaired the NASA Science and Technology Definition Team for a “Probe-class” Starshade and telescope system for direct imaging discovery and characterization of Earth analogs. Professor Seager was elected to the National Academy of Sciences in 2015, is a 2013 MacArthur Fellow, and in 2012 was named in Time Magazine’s 25 Most Influential in Space.