Signs of CO2 in a planet beyond our solar system

In a remarkable display of its precision and accuracy, the James Webb Space Telescope (JWST), a collaboration between NASA, the European Space Agency, and the Canadian Space Agency, has captured definitive evidence for carbon dioxide in the atmosphere of a gas giant planet orbiting a Sun-like star 700 light-years away.

The result, which is accepted for publication in Nature, provides important insights into the composition and formation of the exoplanet and is indicative of Webb’s ability to detect and measure carbon dioxide in the thinner atmospheres of smaller rocky planets. And beyond that, better understanding such exoplanets could lead to finding worlds that could harbour extraterrestrial life.

The team that made the discovery obtained time on the telescope through an Early Release Science program, which was chosen to collect some of Webb’s first data after the start of its scientific operations in late June.

Led by Natalie Batalha of the University of California Santa Cruz, the team includes astronomers from all over the world, including Université de Montréal's Björn Benneke, who is also a member of the Institute for Research on Exoplanets (iREx).

The observing program’s target, WASP-39 b, is a hot gas giant planet 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 puffiness 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 orbit 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.

During a transit, some of the starlight is blocked by the planet completely (causing the overall dimming) and some passes through the planet’s atmosphere. Because different gases absorb different combinations of colours, researchers can analyse small differences in brightness of the transmitted light across a spectrum of wavelengths to determine exactly what an atmosphere is made of.

With its combination of an inflated atmosphere and frequent transits, WASP-39 b is an ideal target for transmission spectroscopy. The team used Webb’s Near-Infrared Spectrograph (NIRSpec) to make this detection.