WASP-39 b is a gas giant exoplanet that orbits a G-type star. Its mass is 0.28 Jupiters, it takes 4.1 days to complete one orbit of its star, and is 0.0486 AU from its star. Its discovery was announced in 2011. In July 2022 it became the first exoplanet to be studied by NASA's James Webb Space Telescope.

The Webb Space Telescope captured the first clear evidence for carbon dioxide in the atmosphere of a planet outside the solar system with its study of this world. This observation of a gas giant planet orbiting a Sun-like star 700 light-years away provides important insights into the composition and formation of the planet.

In November 2022, five science papers reported on WASP-39 b discoveries made using JWST. Those included the first observation of sulfer dioxide and photochemistry (chemical changes influenced by starlight) in an exoplanet atmosphere.

WASP-39 b is a hot giant about the same size as Saturn. Its extreme puffiness is related in part to its high temperature (about 1,600 degrees Fahrenheit or 900 degrees Celsius). Unlike the cooler, more compact gas giants in our solar system, WASP-39 b orbits very close to its star – only about one-eighth the distance between the Sun and Mercury – completing one circuit in just over four Earth-days. The planet’s discovery was made based on ground-based detections of the subtle, periodic dimming of light from its host star as the planet transits, or passes in front of the star.

Previous observations from other telescopes, including NASA’s Hubble and Spitzer space telescopes, revealed the presence of water vapor, sodium, and potassium in the planet’s atmosphere. Webb’s unmatched infrared sensitivity has now confirmed the presence of carbon dioxide on this planet as well.