Biosignatures: How Scientists Search for Signs of Life on Exoplanets

We cannot visit exoplanets, but we can analyze their atmospheres from light-years away. Biosignatures — chemical and spectral fingerprints of life — are how scientists hope to answer the biggest question in science: are we alone?

Finding a planet in the habitable zone is just the first step. The real prize is determining whether any of these worlds actually harbor life. Since we cannot send probes to stars hundreds of light-years away, scientists must rely on remote detection — looking for telltale chemical and spectral signatures in a planet's atmosphere or surface that would be difficult or impossible to produce without biology. These are called biosignatures.

What Are Biosignatures?

A biosignature is any observable feature of a planet that provides scientific evidence for past or present life. Biosignatures fall into several categories:

• Atmospheric biosignatures — Gases in a planet's atmosphere that are produced or maintained by living organisms (e.g., oxygen, methane, ozone).
• Surface biosignatures — Spectral features caused by biological materials on the planet's surface (e.g., the vegetation red edge from chlorophyll).
• Temporal biosignatures — Seasonal variations in atmospheric composition that correlate with biological cycles.

No single detection would be conclusive on its own. The strength of a biosignature claim depends on context: the combination of gases present, the type of star, the planet's size and temperature, and whether non-biological explanations can account for the observations. Scientists speak of a "web of evidence" rather than a single smoking gun.

O2/CH4 Disequilibrium: The Strongest Atmospheric Signal

The single most compelling atmospheric biosignature is the simultaneous detection of oxygen (O2) and methane (CH4) in the same atmosphere. Here is why:

Oxygen and methane are chemically reactive with each other. In the presence of UV starlight, they react and destroy each other on geologically short timescales (millions of years). If both gases are detected together in significant quantities, something must be continuously replenishing them.

Detecting O2/CH4 disequilibrium on an exoplanet would not prove life exists — there could be exotic geological processes at work — but it would be extraordinarily difficult to explain without biology, making it the highest-priority target for atmospheric characterization missions.

The Chlorophyll Red Edge

On Earth, photosynthetic organisms exhibit a striking spectral feature called the vegetation red edge (VRE). Chlorophyll strongly absorbs visible light in the blue (~450 nm) and red (~680 nm) wavelengths for photosynthesis, but sharply reflects near-infrared light (~700–750 nm). This creates a dramatic increase in reflectance at the boundary between red and near-infrared — the "red edge."

If an exoplanet hosted widespread photosynthetic life, its disk-averaged spectrum could show a similar feature. The exact wavelength might differ — organisms around M-dwarf stars might evolve pigments optimized for the star's redder spectrum — but the principle of a sharp spectral edge from biological light harvesting would remain.

Transmission Spectroscopy: Reading Atmospheres from Afar

The primary technique for studying exoplanet atmospheres is transmission spectroscopy. During a transit, starlight passes through the thin ring of atmosphere at the planet's limb (edge). Different molecules in the atmosphere absorb light at specific wavelengths, imprinting absorption features onto the stellar spectrum.

By comparing the star's spectrum during transit to its spectrum outside of transit, astronomers can identify which molecules are present in the planet's atmosphere. Key absorption features include:

• Water vapor (H2O) — Multiple bands in the near- and mid-infrared, particularly at 1.4 and 2.7 microns.
• Carbon dioxide (CO2) — Strong absorption at 4.3 microns, readily detectable by JWST.
• Methane (CH4) — Absorption at 3.3 microns and in the near-infrared.
• Ozone (O3) — A proxy for oxygen, with a strong feature at 9.6 microns in the mid-infrared.
• Oxygen (O2) — Narrow absorption at 0.76 microns (the "A-band"), challenging but potentially detectable.

James Webb Space Telescope Capabilities

The James Webb Space Telescope (JWST), launched in December 2021, is the most powerful tool currently available for exoplanet atmospheric characterization. Its 6.5-meter primary mirror and suite of infrared instruments (NIRSpec, MIRI, NIRCam, NIRISS) cover wavelengths from 0.6 to 28 microns — spanning the absorption bands of all major biosignature gases.

JWST has already detected CO2, H2O, and SO2 in the atmospheres of gas giant exoplanets. The greater challenge is rocky, Earth-sized planets, which have thinner atmospheres and much weaker signals. Detecting biosignatures on a rocky world around an M-dwarf will likely require dozens of stacked transits to build sufficient signal-to-noise — a process that may take years of dedicated observations.

Avoiding False Positives

One of the greatest challenges in biosignature science is the risk of false positives — non-biological processes that mimic biological signals. Several abiotic mechanisms can produce oxygen or methane:

• Photodissociation of water — Intense UV radiation can split water molecules in the upper atmosphere, releasing oxygen. If hydrogen escapes to space, oxygen accumulates without any biology.
• Volcanic outgassing — Volcanoes can release methane and other reduced gases, creating a methane-rich atmosphere without life.
• Serpentinization — Geological water-rock reactions can produce methane abiotically.

This is why scientists emphasize context. A single gas detection is not enough. The combination of multiple biosignature gases, the type of host star, the planet's size and location in the habitable zone, and the absence of plausible abiotic explanations all factor into the assessment. The goal is to build a case so strong that biology becomes the most parsimonious explanation.

How S.O.L.A.R.I.S. Contributes to the Search

S.O.L.A.R.I.S. plays a critical upstream role in the biosignature search: finding the right planets to characterize. The project's pipeline includes a biosignature assessment module that flags exoplanet candidates showing preliminary indicators of:

• Plankton-analogue spectral signatures — Photometric variations consistent with surface biological features.
• O2/CH4 atmospheric disequilibrium — Indirect indicators from transit depth variations across different wavelength bands.

These are statistical assessments based on photometric data, not direct spectroscopic confirmations. But by identifying the most promising candidates from tens of thousands of stars, S.O.L.A.R.I.S. helps focus the limited observing time of facilities like JWST on the targets most likely to yield results.

The Road Ahead

The search for biosignatures is entering its most exciting phase. JWST is actively observing rocky exoplanet atmospheres. Future missions like the Habitable Worlds Observatory (HWO), planned for the 2040s, will use a coronagraph to directly image Earth-like planets around Sun-like stars and study their atmospheres in reflected light — opening an entirely new window on biosignature detection.

In the meantime, the most important task is finding the right targets. Every star that S.O.L.A.R.I.S. volunteers process brings us closer to identifying the world where we might first detect signs of extraterrestrial life. Created by Cassius Mehlhopt and launched March 5, 2026, the volunteer software is free, under 1 MB, and runs on macOS, Windows, and Linux.

Join the Search for Life

S.O.L.A.R.I.S. discoveries are exoplanet candidates based on statistical analysis of TESS photometry. Biosignature assessments are preliminary and require spectroscopic confirmation. The project is not affiliated with NASA.