Examples of potential Earth-like exoplanets. Credit: NASA/Ames/JPL-Caltech
What role can the relationship between oxygen (O2) and ozone (O3) in exoplanet atmospheres have on detecting biosignatures? This is what a study submitted to Astronomy & Astrophysics hopes to address as an international team of researchers investigating novel methods for identifying and analyzing Earth-like atmospheres.
The research is available on the pre-print server arXiv.
This study has the potential to help scientists develop new methods for identifying exoplanet biosignatures, and potentially life as we know it.
For the study, the researchers used a series of climate models to examine how O3 could be used to identify O2 due to their nonlinear relationship, meaning their data follows a curved shape instead of a straight line. This means low levels of O2 equal low levels of O3, and vice versa.
The climate models included all types of stars, including sizes (largest to smallest: O, B, A, F, G, K, M), temperature classification (0 to 9), and current state of evolution (Roman numerals 0 to VI), including if it’s a main-sequence star (identified by a V). For context, our sun is a G2V star.
This study is the third paper in a series of studies by this same team of researchers with the overall goal of using O3 to identify O2 in Earth-like atmospheres. The first paper examined the overall O2-O3 relationship, the second paper examined how nitrous oxide (N2O) influenced the O2-O3 relationship, and this most recent paper examines how methane (CH4) influences the O2-O3 relationship.
In the end, the researchers found that while fluctuating levels of CH4 alters the O2-O3 relationship, there is heavy reliance on the amount of O2 and the host star, specifically its temperature.
Additionally, the team found that model scenarios that had high levels of CH4 and O2 orbiting stars with higher temperatures resulted in CH4 being converted to water (H2O), thus altering the atmospheric temperatures and influencing the amount of O3.
The study notes, “These results further complicate the usage of O3 as a proxy for O2, but also provide additional guidance for future observations. We have now shown in this study that varying CH4 impacts the O2-O3 relationship just as much as N2O, but in different ways.
“There are many scenarios where high CH4 could be increasing the O3 of an atmosphere, while high N2O would be working at the same time to deplete that O3. This shows that we would be required to think about variations of both species in order to use an O3 measurement to learn about the O2 content of the atmosphere.”
Of the nearly 6,000 confirmed exoplanets, there are currently dozens of examples of potential Earth-like exoplanets, including Kepler-186f, Kepler-1649c, and TRAPPIST-1e, which are located approximately, 580, and 301, and 40 light-years from Earth, respectively. While Kepler-186f and Kepler-1649c are both estimated to have masses and radii slightly larger than Earth, TRAPPIST-1e is estimated to have a mass and radius at 0.69 and 0.92 of Earth, respectively.
Additionally, all these exoplanets orbit M-type stars, which are smaller and cooler than our sun. This similar pattern is observed with other potential Earth-like exoplanets, as more than half of them orbit M-type stars. This has altered the understanding of where we can identify Earth-like worlds since our sun is a G-type star, thus scientists originally anticipated finding Earth-like exoplanets around similar stars.
However, while M-type stars are smaller and cooler, they also have longer lifespans than G-type stars. While G-type stars have lifespans of approximately 10 billion years, it is estimated that M-type stars can have lifespans of potentially hundreds of billions to trillions of years, which enhances the possibility of life potentially existing on exoplanets that orbit M-type stars.
More information:
Thea Kozakis et al, Is ozone a reliable proxy for molecular oxygen?. III. The impact of CH_4 on the relationship for Earth-like atmospheres, arXiv (2025). DOI: 10.1051/0004-6361/202556015
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Photochemistry and climate modeling of Earth-like exoplanets (2025, September 1)
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