What Is an Exoplanet?
An exoplanet is any planet that orbits a star outside our solar system. The first confirmed exoplanet around a Sun-like star was discovered in 1995; since then, astronomers have confirmed more than 5,000 exoplanets, with thousands more candidates awaiting verification. They range from scorching gas giants skimming their stars to rocky worlds in temperate orbits where liquid water could exist.
How Do Astronomers Find Exoplanets?
Exoplanets are too small and faint to photograph directly in most cases, so astronomers rely on indirect methods that detect a planet's effect on its host star.
The transit method watches for the tiny dip in a star's brightness when a planet crosses in front of it. NASA's Kepler and TESS telescopes have used this technique to discover thousands of worlds. The radial velocity method measures the wobble of a star induced by an orbiting planet's gravity, which also reveals the planet's minimum mass.
Other techniques include direct imaging of young, massive planets far from their stars, and gravitational microlensing, where a planet's gravity briefly magnifies the light of a background star.
What Is the Habitable Zone?
The habitable zone, sometimes called the Goldilocks zone, is the orbital region around a star where temperatures could allow liquid water to exist on a rocky planet's surface. It is not a guarantee of life — a planet also needs an atmosphere, the right chemistry, and protection from stellar radiation — but it is the first filter astronomers apply when ranking potentially habitable worlds.
The habitable zone moves outward for hotter, larger stars and inward for cooler, smaller red dwarfs, which are the most common stars in the galaxy.
How Does JWST Study Exoplanet Atmospheres?
The James Webb Space Telescope uses transmission spectroscopy to read an exoplanet's atmosphere. As a planet transits its star, starlight filters through the planet's atmosphere, and different molecules absorb different wavelengths. Webb's infrared vision can detect water vapor, carbon dioxide, methane, and other gases — the chemical fingerprints that reveal what an alien sky is made of.
In late August 2026, Webb detected water vapor in the atmosphere of a distant exoplanet, a milestone in the search for habitable worlds and a preview of the atmospheric catalog Webb is building.
The Search for Biosignatures
A biosignature is a chemical signature that suggests biological activity — for example, oxygen and methane existing together in an atmosphere, since they would normally react and disappear without being continually replenished. Finding a convincing biosignature on an exoplanet would be one of the greatest discoveries in human history, and it is the long-term goal driving exoplanet atmospheric studies.
Key Takeaways
- An exoplanet is any planet orbiting a star beyond our solar system
- The transit and radial velocity methods are the most successful discovery tools
- The habitable zone is where liquid water could exist on a rocky surface
- JWST reads exoplanet atmospheres through transmission spectroscopy
- Biosignatures — combined gases like oxygen and methane — could hint at life