Webb Telescope Spots Carbon Dioxide in Exoplanet Atmosphere for First Time

From capturing sharp infrared images of Jupiter and three of its moons to capturing images of the farthest, oldest galaxies anyone has ever seen, NASA’s James Webb Space Telescope has blown everyone away.

Artist illustration of WASP-39b with a crescent of its atmosphere illuminated by its white star off to the left
An illustration of what exoplanet WASP-39b could look like, based on current understanding of the planet. WASP-39b is a hot, puffy gas giant with a mass 0.28 times that of Jupiter and a diameter 1.3 times greater than Jupiter, orbiting just 0.0486 astronomical units (4,500,000 miles) from its star. WASP-39b is very hot and is likely to be tidally locked, with one side facing the star at all times. This illustration is based on indirect transit observations from Webb as well as other space and ground-based telescopes. (Credit: NASA/European Space Agency/Canadian Space Agency/Joseph Olmsted (Space Telescope Science Institute))

But Webb just keeps on giving. Scientists now report in a new study accepted for publication in Nature that the telescope has for the first time definitively detected carbon dioxide in the atmosphere of an exoplanet — a planet orbiting another star. It’s a major milestone that underscores Webb’s incredible capabilities and promises exciting results to come when researchers use the telescope to peer at smaller, more Earth-like worlds around the galaxy.

“It’s very exciting!” exclaimed Kevin Stevenson, an astrophysicist at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, and a study co-author. “We’ve never had access to the wavelengths needed to see the molecular signature of carbon dioxide. Webb is turning exoplanet science into planetary science.”

As part of NASA’s Early Release Science Program to evaluate Webb’s full scientific capabilities, the team tested the telescope’s Near-Infrared Spectrograph (NIRSpec) instrument by observing a simmering Jupiter-sized exoplanet called WASP-39b. As the planet passed in front of its star on July 10, molecules in the atmosphere absorbed specific wavelengths of starlight beaming through. NIRSpec measured which wavelengths were affected — from red wavelengths at visible light to invisible infrared ones — and the team then determined which molecules were likely responsible.

Researchers already had some hints from the Spitzer space telescope that WASP-39b’s atmosphere has carbon dioxide. But there, for the first time at the infrared wavelength of 4.3 micrometers (4.3 millionths of a meter across), was the clear, indisputable evidence.

“You could literally see it in the raw data, the signal is that strong,” Stevenson said. “Normally, we’re talking about minuscule signals that are very difficult to see; but here, you don’t need to run models, you don’t even need to compute statistics to determine if it’s significant. It’s just, ‘Yep, there it is!’”

Graph of the transmission spectrum from James Webb of WASP-39b's atmosphere, showing prominently the peak for carbon dioxide
A transmission spectrum of WASP-39b captured by Webb’s Near-Infrared Spectrograph (NIRSpec) on July 10, 2022, reveals the first clear evidence for carbon dioxide in a planet outside the solar system. This is also the first detailed exoplanet transmission spectrum ever captured that covers wavelengths between 3 and 5.5 micrometers. The blue line is a best-fit model that takes into account the data, the known properties of WASP-39b and its star, and assumed characteristics of the atmosphere.
Source: NASA/European Space Agency/Canadian Space Agency/Leah Hustak (Space Telescope Science Institute)/Joseph Olmsted (Space Telescope Science Institute)

The spectrum, which the team posted Aug. 25 on the preprint server arXiv, also showed clear signs of water and carbon monoxide in WASP-39b’s atmosphere. There was even a signal not yet linked to a specific molecule present.

Carbon dioxide, however, was particularly important because it has significance at all levels of exoplanet research. It’s useful for determining how much of the atmosphere is neither hydrogen nor helium — a characteristic called its metallicity. Based on our own solar system, scientists believe more massive planets should have lower metallicity and less massive planets should have higher metallicity. But they haven’t been able to confirm that trend yet in exoplanets.

“Webb will soon tell us the answer, through the detection of carbon dioxide in just a few more planets,” Stevenson said.

It’s also a molecule expected on smaller rocky planets like Earth, Venus and Mars, all of which have carbon dioxide as a major constituent of their atmospheres.

“Just seeing the carbon dioxide signal in an exoplanet atmosphere is a big deal,” said David Sing, an exoplanet scientist at Johns Hopkins University in Baltimore and a co-author on the study. “It shows we broadly can detect this key molecule across the exoplanet landscape, not just in very hot gas giants like this one, but also in more terrestrial environments.”

That’s ultimately the team’s objective: determining whether small, rocky planets orbiting the Milky Way’s red dwarf stars — the most abundant stars in the galaxy — have atmospheres to use as a stepping stone to determine those planets’ habitability.

“To date, we haven’t seen a single spectroscopic signal of an atmosphere around a rocky planet because we’ve been looking at the wrong wavelengths,” Sing said. “With Webb, we can really start to find and study the atmospheres around exoplanets for the first time. It’s like getting to open 15 doors all at once. The hard part is knowing which one to go through first.”

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