The Webb Telescope searches for the atmospheres of exoplanets

Physics 15, 151

The last detection


About a distant Jupiter-like planet spurs optimism by the discovery of atmospheric gases around Earth-like planets orbiting distant stars.

ESO/m. Cornmeiser

An artist’s impression of the TRAPPIST-1 planetary system shows an extremely cold dwarf star and the rocky planets orbiting it.

In 2017, astronomers observed seven Earth-sized planets orbiting a cool little star called TRAPPIST-1 in the constellation Aquarius. Three of these exoplanets lie within the star’s habitable zone, the distance from the star at which a planet with Earth’s atmosphere could host liquid water. But do these planets and other exoplanets orbiting distant stars have the atmospheric conditions necessary for liquid water? Do they have any atmosphere at all? Using the recently launched James Webb Space Telescope (JWST), researchers now hope to answer these questions.

Jacob Lustig-Jeiger, an astronomer at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, is participating in a program on JWST to search for signs of the atmosphere on several exoplanets, including one in the TRAPPIST-1 system. His observations include sifting through the spectrum of light absorbed by the planet’s atmosphere as it passes between us and its host star. It looks for the spectral signature of common molecules – such as water and carbon dioxide (CO .).2) and methane β€” found in Earth’s atmosphere and some of its rocky neighbors. “If we find any of these particles in the planet’s spectra that we got from JWST, we’ll have discovered the atmosphere,” Lustig-Yaeger says. Under the right conditions, some of these molecules may also represent so-called biosignature gases, which are defined as those that can be produced by life and can accumulate in the atmosphere to detectable levels.

The JWST has already shown that it is up to the task of discovering molecules around distant worlds. In August, JWST detected a clear signal of carbon dioxide2 In the absorption spectrum of WASP-39b, a Jupiter-like planet 700 light-years from Earth [1]. This observation was the first direct detection of this important gas in the atmosphere on an exoplanet. But the main goal of JWST’s exoplanet watchers is to detect carbon dioxide2 Signatures in the spectra of rocky Earth-like planets. The presence of this greenhouse gas would not only reveal information about these exoplanets, but may also help piece together the history of Venus and Mars, both of which are rich in carbon dioxide.2. If you have CO2Dominate the atmosphere [around exoplanets]”So you can extrapolate the pathways that led to what we see in our solar system,” says Nestor Espinosa, an astrophysicist at the Space Telescope Science Institute in Maryland.

NASA. ESA; CSA; L. Hustak/STScI; JWST Science Team Early Release Society of Transiting Exoplanets

Transmission spectrum of hot gas giant exoplanet WASP-39b captured by JWST’s near-infrared spectrometer on July 10, 2022. The bulge reveals first clear evidence of carbon dioxide2 On a planet outside the solar system.

JWST is not the first space telescope to look for infrared signals from the atmospheres of exoplanets. Its predecessor, the Hubble Space Telescope, searched for water vapor in a limited range of wavelength measurements between 1.1 to 1.7.


. “Hubble struggled for many years to find a small bump of water in the spectra of exoplanets,” says Everett Schlowin of the University of Arizona and a member of the JWST Near Infrared Camera team. It was not until 2013 that Hubble discovered a signature of water vapor in the atmosphere of an exoplanet. In contrast, JWST can search for many molecules, including carbon dioxide2 and other carbon-bearing species, using spectrophotometers covering the wavelength range from 0.6 to 12


. The telescope also benefits from the high spectral resolution that allows for the detection of relatively faint molecular features. “We’re now looking for spurs of other particles, perhaps cloud particles or haze that could be in the atmosphere,” Schlawen says.

The telescope can study exoplanets in other ways. Using JWST’s 6.5-meter-wide primary mirror, the researchers will be able to collect infrared light coming directly from the surface of an exoplanet. These measurements can be made at different points in the planet’s orbit, providing clues about temperature and pressure conditions in different regions on the planet’s surface. The result will be maps of the exoplanets that show seasonal variations, atmosphere composition, presence or absence of clouds, details of the planet’s heat distribution, and more. JWST’s detailed observations will allow to refine theories about planetary evolution and habitability. β€œBefore JWST, it was very difficult to test these high-dimensional models,” Espinosa says.

However, JWST does not need to disclose atmospheric signatures to prove its value. For small M-dwarf stars, such as TRAPPIST-1, astronomers aren’t sure if conditions are right for planetary atmospheres to evolve. Therefore, finding a planet without an atmosphere is just as important as finding a planet with a rich atmosphere, says Megan Mansfield, an exoplanet spectrologist at the University of Arizona. It will use the James Webb Space Telescope to observe the exoplanet Gl 486b orbiting the M dwarf star in the constellation Virgo. “If a planet didn’t have an atmosphere, we would still be able to get a spectrum of light from the planet, but it would come directly from the planet’s surface,” she says, providing an opportunity to learn about surface features and how rocks formed.

With a surface temperature of 700 K, planets like GI 486b are too hot to live in. High-energy flares from host stars may strip young, rocky planets of their atmospheres early in their lives. Studying these hot exoplanets could provide a better understanding of the conditions needed for a planet to retain its atmosphere. “Mercury, for example, has its atmosphere blown away because it’s so close to the sun,” Espinosa says. “The lack of an atmosphere tells you a lot about the history of planet formation.”

JWST researchers have no doubt that upcoming observations will reveal surprises that lead to new questions and ultimately important new insights about the planets’ atmospheres. The first JWST spectra of WASP-39b and other hot Jupiters are consistent with both Hubble observations and theoretical predictions. “But [those spectra] It also contains interesting hints of the unexpected. That’s pretty much the best-case scenario for science,” says Lustig-Jeger.

– Rachel Berkowitz

Rachel Berkowitz is a correspondent at Physics magazine Based in Vancouver, Canada.


  1. JWST Transit Exoplanet Society Early Launch Science Team, “Identification of Carbon Dioxide in the Atmosphere of an Exoplanet,” temper nature (2022).

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