The discovery was made during an observation of a gas giant planet orbiting a Sun-like star 700 light years away.

The finding suggests that Webb may be able to detect and measure carbon dioxide in the thinner atmospheres of smaller rocky planets too.

WASP-39 b is a hot gas giant with a mass roughly one-quarter that of Jupiter (about the same as Saturn) and a diameter 1.3 times greater than Jupiter. Its extreme size is related in part to its high temperature (about 900°C).

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, reported in 2011, 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...