Hot gas giant exoplanet WASP-39 b (artist’s concept)

This artist’s concept shows what the exoplanet WASP-39 b could look like based on indirect transit observations from the NASA/ESA/CSA James Webb Space Telescope as well as other space- and ground-based telescopes.

WASP-39 b is a hot, puffy gas giant that orbits a G-type star that is slightly smaller and less massive than the Sun. WASP-39 b orbits relatively close to this star, just 0.0486 astronomical units (7,250,000 kilometres) away. WASP-39 b has a mass 0.28 times Jupiter (0.94 times Saturn) and a diameter 1.3 times greater than Jupiter.

WASP-39 b is tidally locked, with one side facing the star at all times. This means the planet has a terminator (a boundary that separates the planet’s dayside and nightside) where there is an eternal sunrise and sunset.

By analysing a transmission spectrum of WASP-39 b from Webb’s NIRSpec (Near-Infrared Spectrograph), a technique that studies the exoplanet’s terminator, astronomers confirmed a temperature difference between the morning and evening, with the evening appearing hotter by about 200 Celsius degrees. They also found evidence for different cloud cover, with the morning being likely cloudier than the evening.

Webb has not captured a direct image of WASP-39 b.

[Image description: Illustration of a planet, zoomed in on the planet’s dayside/nightside boundary. The planet encompasses takes up the full image. At the bottom left, the image is dark, depicting the nightside covering the planet in a dark shadow. In the right side of the image, the planet has a fuzzy orange-pink atmosphere with hints of longitudinal wispy cloud bands. The right upper corner is bright, where the star (not illustrated) shines.]

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