Detecting Life in the Universe: Detectable Signatures

The Light of Life: A Biosignature Revealed by the Moon

The light of life has a particularity if observed indirectly, a biosignature is reflected on the neighboring star. There is life on Earth and it shows on the Moon.

The existence of life on Earth is not limited to local evidence, detected in our atmosphere or biosphere. It can also be expressed indirectly, through the analysis of light reflected by other celestial bodies. This remarkable phenomenon finds a concrete and accessible illustration in the Earth-Moon system: the terrestrial biosignature is detectable on the Moon itself.

The Moon does not produce its own light, but reflects sunlight. However, when it is partially illuminated, a faint glow — called earthshine — appears on its night side. This light comes from the Earth: it results from the reflection of sunlight on the Earth's surface, then backscattered towards the Moon, and then reflected back to Earth. It is in this secondary light, doubly reflected, that a precious clue lies: the spectral signature of terrestrial life.

When analyzing the spectrum of this earthshine, we find optical characteristics of the Earth's biosphere.

• The molecular oxygen band (O₂) around 760 nm, a gas rare in planetary atmospheres without life, as it is highly reactive and unstable in the long term without continuous photosynthesis.
• The water vapor signature, an indicator of an active hydrological cycle.
• The "vegetation red edge", a reflectance jump in the near infrared due to chlorophyll, typical of terrestrial plant photosynthesis.
• Modulations in the Earth's albedo, linked to the dynamic presence of clouds and oceans.

This indirect observation technique is at the heart of strategies for detecting life on exoplanets. In the terrestrial case, the Moon acts as a cosmic mirror. It allows testing, on the scale of the Earth-Moon system, the analysis protocols that will be applied to the search for biosignatures in the light reflected by exomoons or planets orbiting other stars.

Thus, terrestrial light, when it faintly illuminates the Moon, carries within it the clues of life. This experimental discovery, confirmed by ground-based spectrography (such as those of the Earthshine project), demonstrates that an attentive extraterrestrial observer could, by scrutinizing similar reflected light, deduce the presence of life on our planet, without ever observing it directly.

This phenomenon gives a striking meaning to the poetic expression: terrestrial life is reflected on the Moon. It is an optical manifestation of our planetary biology, made observable in the silence of the night sky.

Biosignatures: In Search of Traces of Life in the Universe

The quest for life beyond the Solar System largely relies on the detection of atmospheric biosignatures, i.e., chemical elements or gas combinations whose biological origin is plausible, or even probable. Among these markers, molecular oxygen (O₂), ozone (O₃), methane (CH₄), carbon dioxide (CO₂), or water vapor (H₂O) are at the heart of planetary spectroscopy programs. Advances in instrumentation (space telescopes such as JWST, Ariel, or upcoming missions like LUVOIR) allow examining the atmospheres of exoplanets in transit in front of their star or via direct imaging.

Spectroscopic Detection of Biosignature Gases

When an exoplanet passes in front of its star (transit method), some of the starlight passes through its atmosphere. This light is filtered by the gases present, with each species absorbing specific wavelengths. By comparing the star's spectrum with and without transit, we can obtain a transmission spectrum of the planetary atmosphere. This method allows detecting the signatures of several gases:

• Molecular Oxygen (O₂): has a characteristic absorption line at 0.76 µm (Fraunhofer A band). Its presence in high concentration is difficult to explain without photosynthetic activity.
• Ozone (O₃): detectable via its absorption band in the ultraviolet (0.25 µm) and mid-infrared (around 9.6 µm). It acts as an indirect tracer of oxygen.
• Methane (CH₄): has strong bands in the infrared at 3.3 µm and 7.6 µm. A durable co-presence of CH₄ and O₂/O₃ in an atmosphere is a strong chemical imbalance, typical of biological activity.
• Carbon Dioxide (CO₂): although abiotic, it is important for characterizing the greenhouse effect and the thermodynamic state of the atmosphere. Its bands are visible at 4.3 µm and 15 µm.
• Water Vapor (H₂O): has several bands in the infrared (notably at 1.4 µm, 1.9 µm, and 6.3 µm). It signals the potential presence of a hydrological cycle, hence a habitable zone.

Combinations and Chemical Imbalances: A Criterion for Life

The key to detection does not rely solely on the presence of an isolated gas, but on the analysis of the overall chemical balance of the atmosphere. A planet whose atmosphere presents both oxygen (highly oxidizing) and methane (easily oxidizable) in a stable manner over long timescales is a case difficult to explain without an active biological source maintaining this imbalance.

Atmospheric models coupled with surface and biosphere models are therefore essential to distinguish true biosignatures from false positives (such as the photodissociation of water on planets without an atmosphere or volcanism emitting CH₄ and SO₂).

Instrumental Perspectives and Future Challenges

Direct spectroscopy (via coronagraphy or interferometry) will soon allow observing non-transiting planets around nearby stars. These methods will offer better spectral and spatial resolution. The detection of biosignatures will, however, require very weak signals and long observations, as the contrasts are on the order of 10⁻⁷ to 10⁻¹⁰ between the star and its planet.

In parallel, the search for non-classical biosignatures (isoprenoids, reduced nitrogen, phosphines, etc.) is expanding to extreme environments potentially analogous to the most primitive terrestrial ecological niches.

How NASA Deciphers Exoplanets: Size and Atmosphere Under the Space Microscope

NASA explains how researchers study the characteristics of exoplanets, particularly size and atmospheric composition. Exoplanets are too distant to be seen directly, but thanks to the light absorbed as they transit in front of their star, scientists can deduce many hidden characteristics indirectly, such as mass, density, composition (rocky or gaseous), and the depth of its atmosphere.

All this information is encoded during the transit in the color of the absorbed light. Each absorbed wavelength in the light spectrum reveals a distinct molecular chemical fingerprint. What interests researchers the most are traces of life such as water vapor (H2O), oxygen (O2), and methane (CH4).