Astronomy
The Spectral Signatures of Ozone on Exoplanets as a Biosignature
Quick fact
Ozone has a very strong absorption feature in the ultraviolet (UV) region of the spectrum, which happens to be a region where the Earth's own atmosphere absorbs almost all incoming sunlight. This makes ozone one of the most detectable potential biosignature gases on transiting exoplanets, despite being present in only trace amounts.
Why this is interesting
Imagine reading the chemical ingredients of a distant world's air from millions of light-years away—and one of those ingredients turns out to be a byproduct of life. How can we do that?
Read the full explanation
Understanding The Spectral Signatures of Ozone on Exoplanets as a Biosignature
Every molecule absorbs light at specific wavelengths, like a unique barcode. When a planet passes in front of its star (a transit), some starlight filters through the planet's atmosphere. During that filter, molecules like ozone (O3) remove particular wavelengths of the star's light. By measuring the dimming of the star at each wavelength, astronomers can build a transmission spectrum. If the spectrum shows a dip in the UV where ozone absorbs, that is a strong hint that ozone exists in that atmosphere. Ozone is made from oxygen, and on Earth, abundant oxygen is produced by photosynthetic life. So finding ozone is like seeing a distant 'life signature'.
A deeper explanation
On Earth, the ozone layer forms high in the stratosphere when ultraviolet (UV) radiation from the Sun splits oxygen molecules (O2) into individual oxygen atoms. These atoms then combine with other O2 molecules to form ozone (O3). This photochemical reaction is a direct result of the presence of O2, which itself is overwhelmingly produced by biological photosynthesis. The ozone molecule exhibits a strong absorption band in the ultraviolet (around 200-310 nm), which is one of the sharpest and most prominent spectral features of any atmospheric gas. When an exoplanet transits its host star, the gas layers absorb a small fraction of the starlight, and the ozone UV band creates a clear and measurable absorption signal. This makes ozone an excellent indirect biosignature because its existence, in large quantities, implies the prior presence of abundant molecular oxygen—a gas that is highly reactive and would not persist without continual production. However, caution is needed: some non-biological processes, like the photolysis of CO2, can also produce oxygen (and thus ozone), so ozone must be analyzed alongside other spectral features and the planet's context (such as water vapor and temperature) to strengthen the case for life.