Monday, September 26, 2022

Webb Detects Carbon Dioxide in Atmosphere of Hot-Saturn WASP-39b

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Carbon dioxide is a key chemical species that is found in a wide range of planetary atmospheres.

This transmission spectrum of WASP-39b was captured by Webb’s Near-Infrared Spectrograph (NIRSpec) on July 10, 2022. Image credit: NASA / ESA / CSA / Leah Hustak, STScI / Joseph Olmsted, STScI.

WASP-39b is a hot gas giant that orbits the G7-type star WASP-39 with a period of 4.1 days.

First discovered in 2011, the planet has approximately the same mass as Saturn (0.28 Jupiter masses) but is 50% larger (1.28 Jupiter radii).

The planet’s extreme puffiness is related in part to its high temperature (about 900 degrees Celsius, or 1,652 degrees Fahrenheit).

“Previous observations of this planet with Hubble and Spitzer had given us tantalizing hints that carbon dioxide could be present,” said Professor Natalie Batalha, an astronomer in the Department of Astronomy and Astrophysics at the University of California Santa Cruz.

“The data from the NASA/ESA/CSA James Webb Space Telescope showed an unequivocal carbon dioxide feature that was so prominent it was practically shouting at us.”

“Carbon dioxide is actually a very sensitive measuring stick — the best one we have — for heavy elements in giant planet atmospheres, so the fact that we can see it so clearly is really great,” said Professor Jonathan Fortney, also from the Department of Astronomy and Astrophysics at the University of California Santa Cruz.

“Stars and gas giant planets are made primarily of the lightest elements, hydrogen and helium, but the abundance of heavier elements — what astronomers call metallicity — is a critical factor in planet formation.”

“The ability to determine the amount of heavy elements in a planet is critical to understanding how it formed, and we’ll be able to use this carbon dioxide measuring stick for a whole bunch of exoplanets to build up a comprehensive understanding of giant planet composition.”

The astronomers observed WASP-39b, which is located approximately 700 light-years away from Earth in the constellation Virgo, as part of a JWST Early Release Science program.

Using Webb’s Near Infrared Spectrograph (NIRSpec), they obtained a high-resolution transmission spectrum showing the light transmitted through the planet’s atmosphere separated into its component wavelengths.

A significant signal — an absorption feature — was detected at wavelengths between 4.1 and 4.6 microns in the infrared.

It is the first clear, detailed, indisputable evidence for carbon dioxide ever detected in a planet outside the Solar System.

“The data yielded exquisite light curves and showed that the NIRSpec instrument is exceeding expectations for transmission spectroscopy,” Professor Batalha said.

“This bodes well for observations of small rocky planets, which are expected to have carbon dioxide in their atmospheres (when they have atmospheres) but won’t give as strong a signal as a giant planet like WASP-39b.”

“This detection will serve as a useful benchmark of what we can do to detect carbon dioxide on terrestrial planets going forward,” she added.

“It’s the most likely atmospheric gas we’ll detect with Webb in terrestrial-size exoplanet atmospheres.”

In addition to the large carbon dioxide feature, the researchers also identified a smaller spectral feature near 4.0 microns that is not matched by their models.

“It’s a mystery feature for now,” Professor Batalha said.

“In this paper, we focused on a narrow range of infrared colors — this is only a preview of the features we expect to see in the full spectrum.”

The results will be published in the journal Nature.


Eva-Maria Ahrer et al. (JWST Transiting Exoplanet Community Early Release Science Team). 2022. Identification of carbon dioxide in an exoplanet atmosphere. Nature, in press; arXiv: 2208.11692

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