University of Warwick: Webb Space Telescope Detects Carbon Dioxide in Exoplanet Atmosphere

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After years of preparation and anticipation, exoplanet researchers are ecstatic. The James Webb Space Telescope (JWST) has captured an astonishingly detailed rainbow of near-infrared starlight filtered through the atmosphere of a hot gas-giant planet 700 light-years away.

The transmission spectrum of exoplanet WASP-39 b, based on a single set of measurements made using Webb’s Near-Infrared Spectrograph and analysed by dozens of scientists, represents a hat trick of firsts: Webb’s first official scientific observation of an exoplanet; the first detailed exoplanet spectrum covering this range of near-infrared colours; and the first indisputable evidence for carbon dioxide in the atmosphere of a planet orbiting a distant star.

The results accepted for publication in Nature, are indicative of Webb’s ability to spot key molecules like carbon dioxide in a wide variety of exoplanets – including smaller, cooler, rocky planets – providing insights into the composition, formation, and evolution of planets across the galaxy.

PhD student Eva-Maria Ahrer, from the University of Warwick, Centre for Exoplanets and Habitability, who contributed to the development of the data analysis methods used in the detection, said: “This result is incredibly exciting; detecting this carbon dioxide feature so clearly in the atmosphere of WASP-39b is astonishing. It was an emotional and exhilarating moment for me when we had our first meeting after the data came down, seeing the transmission spectrum for the first time and all the hard work of our scientific collaboration coming together.”

Her colleague at the University of Warwick, Professor Peter Wheatley, Centre for Exoplanets and Habitability, added “It is particularly exciting for us that this first detection was made in the atmosphere of WASP-39b, since we discovered this planet in 2011 using the UK-led SuperWASP telescopes. WASP-39b is a hot planet located very close to its parent star, with a temperature of 900 degrees Celsius, leading to a puffy atmosphere that makes it an excellent target for JWST. The planet has a volume more than twice that of Jupiter, but only a quarter of its mass, making it nine times less dense.”

Transiting planets like WASP-39b, whose orbits we observe edge-on rather than from above, can provide researchers with ideal opportunities to probe planetary atmospheres. During a transit of the planet across the star, some of the starlight is eclipsed by the planet completely (causing an overall dimming) and some is transmitted 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.

The research team used Webb’s Near-Infrared Spectrograph (NIRSpec) for its observations of WASP-39b. In the resulting spectrum of the exoplanet’s atmosphere, a small hill between 4.1 and 4.6 microns presents the first clear, detailed evidence for carbon dioxide ever detected in a planet outside the solar system.

Eva-Maria Ahrer explains “Molecular features such as carbon dioxide tell us about the composition and beginnings of the planet from when it formed around the star. Seeing such clear features made possible by high-quality observations makes me incredibly excited to see what we will be able to detect with the JWST in the next coming years, in particular when pushing to smaller, Earth-sized planets.”

Professor Peter Wheatley added “Different molecules were present at different distances around the young star when the planet was forming, so the detection of carbon dioxide (and other molecules in the future) allow us to look back in time to understand how and where different types of planets assembled.”

The JWST NIRSpec observation of WASP-39b announced today is just one part of a larger investigation that includes observations of WASP-39b using multiple Webb instruments, one of which is led by Warwick researcher Eva-Maria Ahrer.

Professor Peter Wheatley explains, “This Early Release Science program is just the beginning of the exploration of planetary atmospheres with the Webb telescope, and we can look forward to a series of startling discoveries in the coming years, including the detection of molecules in atmospheres of smaller, rocky planets”.

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