WASP-39 b resides in the Codex as the puffy Saturn-mass world that opened JWST’s exoplanet chemistry era. It transits a Sun-like G-type star about 698 light-years away, with a radius of 14.3 × Earth and a mass near 89 Earths, Saturn-like heft under a swollen envelope. Early Release Science spectra delivered the first unambiguous carbon-dioxide detection in an exoplanet atmosphere, a towering 4.3-micrometer feature, along with water, sodium, potassium, carbon monoxide, and photochemical sulfur dioxide. Too hot for habitability at roughly 1,170 K, it still rewrote how we read alien air. This entry stands eternally in the Galactic Codex as the spectrum that made JWST’s first chemical fingerprints real.
