Is Methane the Key to Finding Life on Other Worlds?

How would detecting methane help astronomers identify if exoplanets, or even exomoons, have life as we know it, or even as we don’t know it? This is what a recent study published in The Astronomical Journal hopes to address as a team of researchers led by the NASA Goddard Space Flight Center investigated how a method called BARBIE (Bayesian Analysis for Remote Biosignature Identification on exoEarths) could be used on a future space mission to detect methane (CH4) on Earth-like exoplanets in optical (visible) and near-infrared (NIR) wavelengths. This study builds on past studies using BARBIE, known as BARBIE 1 and BARBIE 2, and has the potential to help scientists and engineers develop new methods for finding life beyond Earth and throughout the cosmos.

Here, Universe Today discusses this incredible study with Natasha Latouf, who is a PhD Candidate in the Department of Physics and Astronomy at George Mason University and lead author of the study, regarding the motivation behind the study, significant results, potential follow-up studies, next steps for BARBIE, the significance of detecting methane on Earth-like exoplanets, and if Natasha thinks we’ll ever find life on Earth-like exoplanets. Therefore, what was the motivation behind the study?

Latouf tells Universe Today, “We developed the BARBIE methodology in order to quickly investigate large amounts of parameter space and make informed decisions about the resultant observational trade-offs. Methane is a key contextual biosignature that we would be very interested in detecting, especially with other biosignatures like O2.”

As its name states, BARBIE used what’s known as a Bayesian inference, which is a statistical method employed to determine data probability outcomes based on a given input of data, meaning the probabilities change based on additional data input. As noted, this work builds off previous studies involving BARBIE, with those investigating parameters including observing exoplanets in optical wavelengths with planetary parameters including surface pressure, surface albedo, gravity, along with water (H20), oxygen (O2), and ozone (O3) abundance. However, those results indicated that only oxygen-rich atmospheres were observable in optical wavelengths, with the authors noting the parameters were too limited. With this work, known as BARBIE 3, the team added NIR wavelengths and CH4 to the parameters to broaden the parameters for more desirable results. Therefore, what were the most significant results from this study?

“The most significant results from this study is the interesting interplay between H2O and CH4 in the near-infrared (NIR),” Latouf tells Universe Today. “While we knew that the spectral features H2O and CH4 overlap heavily in the NIR, and would probably cause some issues with detectability, what we didn’t realize was how much that effect mattered. In fact, we find that at sufficiently high CH4, the signal-to-noise ratio (SNR) required to strongly detect H2O shoots up, and the same vice versa. Essentially, we need to be careful before claiming a planet has no H2O or CH4, because if both are present, we might be missing one! There are follow up studies happening currently, led by my fantastic post-bac Celeste Hagee, studying how the detectability of biosignatures in the NIR changes if we add CO2 into the mix!”

Along with building off previous BARBIE studies, this study focuses on contributing to the planned NASA Habitable Worlds Observatory (HWO) mission, which was recommended by National Academies of Sciences, Engineering, and Medicine (NASEM) Decadal Survey on Astronomy and Astrophysics 2020 and is currently planned to launch sometime in the 2040s. The goal of HWO will be to analyze 25 potentially habitable exoplanets, which contrast past and current exoplanet-hunting missions like NASA’s Kepler and NASA’s TESS (Transiting Exoplanet Survey Satellite) missions, respectively, whose objectives were to locate and identify as many exoplanets as possible.

HWO will use a combination of the direct imaging method to find the exoplanets and its spectroscopy instruments to analyze their respective atmospheres for biosignatures, specifically oxygen and methane. Along with identifying and analyzing potential habitable exoplanets, the other science goals include galaxy growth, element evolution from the Big Bang until now, and our solar system and its place in the universe. Therefore, what next steps need to be taken for BARBIE to become a reality on a future exoplanet imaging mission like HWO?

“The reason why BARBIE is useful is because it provides a huge swath of information about lots of parameter space very quickly – that means we can use that data to build future telescopes!” Latouf tells Universe Today. “For instance, if we’re trying to understand whether we need a 20% or a 40% coronagraph in order to strongly detect biosignatures in the optical regime, we can look at how the 20% and 40% influences detection of biosignatures, and from there make the decision on whether the science benefit of a 40% is worth the increased cost.”

This isn’t the first time scientists have postulated that methane might be a key indicator of life on exoplanets, as a 2022 study published in the Proceedings of the National Academy of Sciences (PNAS) discussed how atmospheric methane should be considered an exoplanet biosignature and be targeted by space telescopes like NASA’s James Webb Space Telescope (JWST). Within our own solar system, methane is a key component of Saturn’s largest moon, Titan, with researchers hypothesizing that its crust could contain methane. Additionally, Mars experiences seasonal changes in methane gases that keep scientists puzzled regarding its origin. Therefore, what is the significance of identifying methane on Earth-like exoplanets?

Latouf tells Universe Today, “CH4 is a contextual biosignature – if we find sufficient amounts of CH4 and O2 in an atmosphere together, it means the atmosphere is in disequilibrium. That means that there must be something PRODUCING those levels of CH4 and O2, and depending on the abundances of each, the signs would point to some form of life behind that production.”

This study comes as the number of confirmed exoplanets currently totals 5,832 with 212 being designated as terrestrial (rocky) exoplanets, or exoplanets that are Earth-sized or smaller. A primary example of terrestrial exoplanets includes the TRAPPIST-1 system that resides just over 40 light-years from Earth and is currently hypothesized to host seven Earth-sized exoplanets with at least three orbiting in its star’s habitable zone, which is the right distance from the star to support surface liquid water like Earth.

The closest known terrestrial exoplanet to Earth is Proxima Centauri b, which is 4.24 light-years from Earth and orbits within its star’s HZ despite its orbit only being 11.2 days. However, this also means Proxima Centauri b is blasted by ultraviolet radiation, meaning its surface might not be suitable for life as we know it. Therefore, does Latouf believe we will ever find life on Earth-like exoplanets and which Earth-like exoplanets are particularly interesting to her?

“In my opinion, I think that we will,” Latouf tells Universe Today. “Will that happen in my lifetime? That I’m not sure of – but I do believe we’re going to find life eventually! Although it’ll sound boring the most Earth-like planet I’m interested in is…Earth. We have this wonderful gift in this planet, with all the exact right conditions. We need to be making sure we’re preserving it and understanding our own planet before we dive into the search for others!”

For now, BARBIE remains on the drawing board, but it demonstrates the tireless commitment of the scientific community to improve upon previous designs with the goal of answering whether life exists beyond Earth and throughout the cosmos. Going forward, the authors note that future work will continue to enhance BARBIE’s capabilities, including detecting all molecules across HWO’s entire wavelength range like ultraviolet in addition to optical and NIR. They also plan to test whether coronagraph detectors, which block light from a star to both reveal and improve exoplanet analysis, are suitable for identifying molecules in an exoplanet’s atmosphere.

Latouf concludes by telling Universe Today, “I want to emphasize that it’s very easy to see a completed paper and think to yourself, especially as an early career, “I could never do that.” BARBIE was a project that was created by a team – sure, I put my special branding on it and did the work, but the project was born of open collaboration and communication. The process of doing the work for BARBIE1, 2, and 3 took about 3.5 years, and many, many setbacks. This work is hard, it’s not easy, and no one finds it easy. All this to say – if you’re working on something, and looking at others thinking you can’t do it like they can, just know: they’re learning and growing too, and science is never as easy as it looks.”

Is methane the correct biosignature to identify life as we know it on exoplanets and how will BARBIE help the continued search for life beyond Earth in the coming years and decades? Only time will tell, and this is why we science!

As always, keep doing science & keep looking up!

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