When we look up at the night sky, every star reminds us of a possibility: the chance that another world, circling that distant sun, might harbor life. But finding life light-years away is not as simple as pointing a telescope and waiting for something to wave back. Instead, scientists search for biosignatures—indirect clues in a planet’s atmosphere that might suggest living processes. Among these clues, oxygen (O₂) and ozone (O₃) play starring roles.
An international team of researchers has been working to understand how the delicate balance between oxygen and ozone could help us detect alien life. Their latest study, submitted to Astronomy & Astrophysics and available on the pre-print server arXiv, dives into this puzzle by asking how oxygen and ozone interact in atmospheres that may—or may not—resemble our own.
The results are both exciting and humbling: ozone might be a useful tool for tracking oxygen, but the relationship is far from simple. And as it turns out, other gases like methane (CH₄) and nitrous oxide (N₂O) complicate the story even further.
Why Oxygen and Ozone Matter
On Earth, oxygen is a hallmark of life. It fills our atmosphere thanks to billions of years of photosynthesis from tiny organisms, plants, and algae. Ozone, meanwhile, is formed when oxygen molecules are broken apart by sunlight and then recombine in new ways. Our ozone layer is not just a byproduct of oxygen—it is also a planetary shield, absorbing harmful ultraviolet radiation and making Earth more hospitable to life.
This link between oxygen and ozone makes ozone an attractive proxy for detecting oxygen on distant planets. If telescopes detect ozone in an atmosphere, it might point to oxygen, and in turn, to biology. But the new study shows that the O₂–O₃ relationship is nonlinear, meaning it doesn’t follow a simple straight line. Low oxygen generally means low ozone, and high oxygen means high ozone—but other factors can twist this balance in surprising ways.
Modeling Alien Atmospheres
To explore this complexity, the research team turned to climate models that simulate how different gases interact under different stellar conditions. They accounted for a wide range of star types—from blazing O-class stars to tiny, cool M-class stars. For context, our own Sun is a G2V star, sitting in the middle of the stellar family.
By running thousands of simulations, the researchers examined how methane influences the oxygen–ozone balance. Methane is a particularly important gas to consider because, like oxygen, it is linked to life on Earth. On our planet, microbes produce much of the methane in the atmosphere. But methane is chemically active—it reacts with oxygen, ozone, and other molecules, reshaping the chemistry of the atmosphere in ways that complicate detection.
When Methane Meets Oxygen
The models revealed that high levels of methane can alter how much ozone forms in an atmosphere, depending on the host star’s temperature and radiation. For instance, in hotter stellar environments, methane breaks down more quickly and can transform into water vapor (H₂O). This not only changes the thermal structure of the atmosphere but also reshapes the delicate ozone balance.
The researchers found that in some scenarios, methane could increase ozone, while in others, nitrous oxide might work to deplete it. This tug-of-war means that scientists cannot rely on ozone alone as a straightforward indicator of oxygen—or life. Instead, ozone must be considered alongside other gases, in the broader context of planetary and stellar conditions.
The Bigger Picture of Biosignature Detection
This paper is part of a larger effort by the same research team to refine how we interpret possible biosignatures. Their first study examined the overall oxygen–ozone link, the second looked at nitrous oxide’s role, and this third focuses on methane. Together, these studies build a more realistic and nuanced picture of how planetary atmospheres behave.
For future telescopes like the James Webb Space Telescope (JWST) and the planned Habitable Worlds Observatory, these insights will be crucial. Detecting ozone in an exoplanet atmosphere might one day be heralded as evidence of life—but only if we also understand how other gases and stellar environments could mimic or distort that signal.
Exoplanets as Laboratories of Possibility
As of today, astronomers have confirmed nearly 6,000 exoplanets, and dozens of them are considered potentially Earth-like. Worlds such as Kepler-186f, Kepler-1649c, and TRAPPIST-1e have captured imaginations because they orbit within the so-called habitable zone—the region around a star where liquid water could exist.
Interestingly, many of these worlds orbit M-type stars, which are smaller and cooler than our Sun. This discovery has shifted the search for life dramatically. While astronomers once expected Earth-like planets to circle Sun-like G stars, the data now suggest that the majority of promising candidates may orbit cooler, longer-lived M stars.
And this matters for biosignatures. M stars emit different types of radiation, which changes how gases like oxygen, ozone, methane, and nitrous oxide interact in planetary atmospheres. In other words, the type of star is just as important as the composition of the atmosphere itself.
The Promise and Challenge Ahead
The new study underscores an essential truth about astrobiology: detecting life is going to be complicated. There is no single “smoking gun” biosignature. Instead, scientists must learn to read the symphony of gases in an atmosphere and how they dance under the influence of starlight.
But there is also beauty in this complexity. The fact that oxygen, ozone, methane, and nitrous oxide interact in different ways means that life may leave multiple fingerprints on a planet. And the more we understand these patterns, the better we can recognize life—even if it looks different from what we know on Earth.
A Cosmic Invitation
Every exoplanet discovered is a reminder that Earth is not unique in having the conditions for life. By studying the interplay of oxygen and ozone, and by carefully modeling the atmospheric chemistry of other worlds, scientists are slowly building the tools to answer one of humanity’s oldest questions: Are we alone?
The latest research shows that ozone may not be a simple shortcut to finding oxygen, but it remains a vital piece of the puzzle. And in that challenge lies excitement. Each new model, each new telescope, each new discovery brings us closer to glimpsing life’s reflection in the skies of another world.
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
