Is There More to Life than Oxygen?

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Title: There’s More to Life than O2: Simulating the Detectability of a Range of Molecules for Ground-based, High-resolution Spectroscopy of Transiting Terrestrial Exoplanets
Authors: Miles H. Currie, Victoria S. Meadows, and Kaitlin C. Rasmussen
First Author’s Institution: University of Washington
Status: Published in PSJ

Searching for Life Elsewhere

The need for characterizing planetary atmospheres grows with the increasing number of discovered exoplanets. In the quest to find life on other planets, astronomers turn to one of the main techniques we can use to study the atmosphere of an exoplanet: transmission spectroscopy. This technique can be used if an exoplanet transits in front of its host star, meaning it passes in front of the star and blocks part of the light that we receive from the star. If the transiting exoplanet has an atmosphere, astronomers can use this technique to try to infer the composition of the atmosphere. This technique is promising in the search for life because we can use it to search for biosignatures, which are molecules that could help us determine whether life is present on these exoplanets!

On Earth, oxygen is present in our atmosphere due to photosynthesis, which occurs in plants, algae, and some bacteria. This process takes in carbon dioxide and produces oxygen and energy stored as a sugar. We, as human beings, breathe oxygen to live. Therefore, in our knowledge of what life is on Earth, we look for oxygen on other planets as a sign of life. However, today’s authors argue that there’s more to life than oxygen and explore what other molecules could help indicate whether life could be present on exoplanets. They also explore which of those molecules can be detected by extremely large ground-based telescopes including the European Southern Observatory’s Extremely Large Telescope, the Thirty Meter Telescope, and the Giant Magellan Telescope.

A Novel Detectability Pipeline

The authors of today’s research article aim to estimate how detectable different molecules can be in the atmospheres of terrestrial exoplanet atmospheres by using a technique called cross-correlation analysis (see Figure 1). This technique works by comparing the simulated transmission spectrum put in at the beginning of the pipeline to a model template of the molecular absorption bands from other studies. To assess how they compare to each other, the authors calculate a correlation coefficient. A higher value of the correlation coefficient means a better match between the two spectra. For this article, the authors chose seven gases — oxygen, methane, carbon dioxide, water vapor, ozone, carbon monoxide, and ethane — to be studied in four main types of atmospheres. The four types of atmospheres they focused on were 1) pre-industrial Earth, 2) Archean Earth, 3) an atmosphere with an accumulation of oxygen due to the breakdown of carbon dioxide (also known as photolysis) with no lightning, and 4) an oxygen-containing atmosphere with lightning occurring. Lastly, the authors chose to study the seven selected molecules in these four different types of atmospheres around five different M-dwarf stars at two different distances from the observer. This was all done through a new detectability pipeline made by the authors and can be seen in Figure 1.

flowchart of the new pipeline

Figure 1: This figure illustrates the new pipeline set forth by the authors for studying the detectability of different molecules. The red rounded boxes show the beginning and end points of the pipeline, the green boxes show the different models or software that they used, the white parallelograms show the input or output data, and the white rhombuses represent the parameters used for this study. [Currie et al. 2023]

What Can We See?

By using this new pipeline, the authors were able to determine which of the seven molecules is more likely to be detected with the extremely large ground-based telescopes. The best two potential biosignatures that could be discovered on terrestrial exoplanets are carbon dioxide and methane followed by oxygen, water vapor, and carbon monoxide. If carbon dioxide is detected with methane in the atmosphere of an exoplanet, this constitutes a “disequilibrium biosignature pair” that could be driven by life and only requires between 5 and 20 transits to be detected for their most optimistic scenario of Earth-like planets at two different distances from observer (see Figure 2). Similarly, if oxygen and methane are detected together in an exoplanet’s atmosphere, that is also a disequilibrium biosignature pair that could be driven by life. Both of these pairs could easily be acquired in as few as 39 transits for their “most optimistic scenario of an Earth-like planet transiting a star 5 parsecs (16 light-years) away.” The other two molecules, ozone and ethane, are not accessible with ground-based transmission spectroscopy.

plot of the number of transits required to get a three-sigma detection of certain molecules under certain conditions

Figure 2: This figure shows the total number of observed transits required for a three-sigma detection of the five molecules detectable with extremely large telescopes: oxygen, methane, carbon dioxide, carbon monoxide, and water vapor. All four types of atmospheres are depicted in this figure orbiting a variety of different host stars at a distance of 12 parsecs (39 light-years) from the observer. If there is no marker it means that the atmosphere doesn’t have that molecule or it requires more than 300 transits to be detected. [Currie et al. 2023]

This study is an exciting step in our understanding and search for life on terrestrial exoplanets using transmission spectroscopy through extremely large ground-based telescopes. This article demonstrates the need for these ground-based observatories and highlights how they can complement our current observations with space telescopes such as JWST. By using and studying these observations together, we are one step closer to being able to characterize these exoplanets and potentially find life.

Original astrobite edited by Pranav Satheesh.

About the author, Junellie Gonzalez Quiles:

Junellie Gonzalez Quiles is a PhD Student and NSF Graduate Research Fellow in the Department of Earth and Planetary Sciences at Johns Hopkins University. Her current research focuses on modeling geochemical cycles and outgassing on exoplanets to help us understand the evolution of the atmospheric composition and its effect on planetary climate. She is deeply passionate about outreach, science communication, and diversity, equity, and inclusion (DEI) initiatives. Outside of work, she loves to knit, embroider, and do other arts and crafts. She also plays the trombone and enjoys practicing yoga.