The systematic discovery of exoplanets began in 1995 with 51 Pegasi b (Nobel Prize 2019 to Mayor and Queloz). NASA’s Kepler and TESS missions have brought the confirmed exoplanet count past 5,500, with thousands of candidates awaiting confirmation.
## Detection Methods
**Transit photometry**: when a planet crosses its host star, it dims starlight by roughly 1% in a periodic signal. Kepler and TESS’s primary method. Enables atmospheric characterization through transmission spectroscopy, but requires favorable orbital alignment — only a small fraction of planetary systems are detectable.
**Radial velocity**: planetary gravity causes periodic stellar velocity variations (meters to hundreds of meters per second) along the line of sight, measurable via Doppler shifts in stellar spectra. Discovered 51 Peg b and remains a critical confirmation method.
**Direct imaging**: photographing planets directly rather than inferring them from starlight variations. Requires extreme contrast ratios (stars are millions of times brighter than planets). Currently limited to massive, young, wide-orbit planets; JWST and next-generation ground telescopes (ELT) are extending this capability.
**Microlensing**: a foreground star and its planets gravitationally magnify a background star’s light as they pass in front of it. Sensitive to planets thousands of light-years away and capable of detecting “rogue planets” (planets without host stars).
**Astrometry**: detecting the stellar wobble from precise positional measurements on the sky. ESA’s Gaia satellite is expected to discover tens of thousands of exoplanets through astrometry.
## JWST and Atmospheric Characterization
JWST has transformed exoplanet atmospheric science. Through transmission spectroscopy during transits, JWST has detected CO₂, H₂O, SO₂ (in WASP-39b), and methane (K2-18b). The simultaneous detection of methane and CO₂ in K2-18b’s atmosphere is consistent with a “Hycean world” (liquid water ocean beneath a hydrogen atmosphere) model — not direct evidence of life, but scientifically significant.
**Habitable zone targets**: The TRAPPIST-1 system (40 light-years away) hosts 7 Earth-sized planets, 3 in the habitable zone, and is the highest-priority target for biosignature searches. JWST is conducting systematic observations of TRAPPIST-1 planet atmospheres.
See [James Webb Telescope Discoveries](https://sunqi.org/james-webb-telescope-discoveries-en/) and the [NASA Exoplanet Archive](https://exoplanetarchive.ipac.caltech.edu/).
