For most of human history, planets orbiting other stars — exoplanets — were philosophical speculation. The first confirmed exoplanet detections around a Sun-like star came in 1995. Since then, the catalogue has exploded: as of 2025, over 5,700 exoplanets have been confirmed, with thousands more candidates awaiting confirmation.
The most prolific detection method is the transit method: if a planet passes in front of its host star from our perspective, it blocks a tiny fraction of the star's light, causing a measurable dip. NASA's Kepler space telescope, operational from 2009 to 2018, found over 2,600 confirmed exoplanets using this method. Its successor, TESS, continues the work.
The radial velocity (Doppler) method detects the tiny wobble a planet induces in its star's velocity via the Doppler shift in the star's spectral lines. Direct imaging — actually photographing exoplanets — is extraordinarily challenging but has been achieved for a handful of large, young planets orbiting far from their stars.
The 'habitable zone' (HZ) — sometimes called the Goldilocks zone — is the range of orbital distances at which a planet with an Earth-like atmosphere could maintain liquid water on its surface. This is considered crucial for life as we know it. The boundaries depend strongly on the star's luminosity.
Thousands of roughly Earth-sized planets have been found in or near their stars' habitable zones. Among the most intriguing are the TRAPPIST-1 system: seven Earth-sized planets around an ultracool dwarf star 40 light-years away, three firmly in the habitable zone. The James Webb Space Telescope has been studying their atmospheres.
However, 'habitable zone' doesn't mean 'habitable.' A planet needs the right atmosphere, geology, magnetic field, and chemistry. Venus is in the Sun's habitable zone yet is hellishly hostile. Conversely, moons like Europa (Jupiter) and Enceladus (Saturn) may host liquid water oceans under their ice shells, far outside the conventional habitable zone.
A biosignature is a measurable property that provides evidence of past or present life. In exoplanet science, the most promising approach is transmission spectroscopy: as a planet transits its star, starlight filters through its atmosphere. Different molecules absorb different wavelengths — their spectral fingerprints can reveal the atmosphere's composition.
Atmospheric oxygen (O₂) and ozone (O₃) are considered strong biosignatures because known abiotic processes don't produce them in significant quantities. Methane (CH₄) alongside oxygen is even stronger — as they rapidly react, steady simultaneous presence implies continuous production (e.g., by biology).
The James Webb Space Telescope (JWST), launched December 2021, has begun probing exoplanet atmospheres with unprecedented sensitivity. Detecting unambiguous biosignatures on a rocky exoplanet remains one of the most sought-after goals in all of science — and may happen within the next decade.
Test what you've just learned.
1.What is the transit method of exoplanet detection?
2.What is the 'habitable zone'?
3.Why is the TRAPPIST-1 system particularly interesting to astrobiologists?
4.Why would detecting oxygen AND methane together in an exoplanet's atmosphere be significant?