For the first time, astronomers have found unambiguous evidence of carbon
dioxide in the atmosphere of an exoplanet (a planet outside our solar
system).
The discovery, accepted for publication in Nature and posted online August
25, demonstrates the power of the James Webb Space Telescope (JWST) to
deliver unprecedented observations of exoplanet atmospheres.
Natalie Batalha, professor of astronomy and astrophysics at UC Santa Cruz,
leads the team of astronomers that made the detection, using JWST to observe
a Saturn-mass planet called WASP-39b which orbits very close to a sun-like
star about 700 light-years from Earth.
“Previous observations of this planet with Hubble and Spitzer had given us
tantalizing hints that carbon dioxide could be present,” Batalha said. “The
data from JWST showed an unequivocal carbon dioxide feature that was so
prominent it was practically shouting at us.”
Carbon dioxide is an important component of the atmospheres of planets in
our solar system, found on rocky planets like Mars and Venus as well as gas
giants like Jupiter and Saturn. For exoplanet researchers, it is important
both as a gas they are likely to be able to detect on small rocky planets
and as an indicator of the overall abundance of heavy elements in the
atmospheres of giant planets.
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| A transmission spectrum of the hot gas giant exoplanet WASP-39 b captured by JWST's Near-Infrared Spectrograph (NIRSpec) on July 10, 2022, reveals the first clear evidence for carbon dioxide in a planet outside the solar system. Credit: NASA, ESA, CSA, Leah Hustak (STScI), Joseph Olmsted (STScI) |
“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 coauthor Jonathan Fortney,
professor of astronomy and astrophysics at UCSC and director of the Other
Worlds Laboratory.
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, Fortney
explained.
“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,” he said.
Batalha’s team observed WASP-39b as part of a JWST Early Release Science
program to study transiting exoplanets. A transiting planet passes in front
of its star as viewed from Earth, enabling astronomers to analyze the
starlight that passes through the planet’s atmosphere, where gases like
carbon dioxide absorb certain wavelengths of light.
Using the Near Infrared Spectrograph (NIRSpec) on JWST, the team obtained a
high-resolution “transmission spectrum” showing the light transmitted
through WASP-39b’s atmosphere separated into its component wavelengths.
Batalha said the data yielded “exquisite light curves” and showed that the
NIRSpec instrument is exceeding expectations for transmission spectroscopy.
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,” Batalha said. “It’s
the most likely atmospheric gas we’ll detect with JWST in terrestrial-size
exoplanet atmospheres.”
In addition to carbon dioxide, the researchers detected another interesting
feature in the spectrum of WASP-39b that they have not yet identified. “It’s
a mystery feature for now,” 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.”
Fortney noted that WASP-39b appears to have a similar composition to Saturn.
Saturn’s metallicity is 10 times that of the sun, and WASP-39b also seems to
be enriched in heavy elements by about 10 times relative to the sun.
“That’s super interesting, and we would love to know if all Saturn-mass
planets have the same metallicity,” he said. “It was exciting to see this in
another system, because we didn’t know what to expect when we went from the
planets in our solar system to the atmospheres of exoplanets.”
Located in the constellation Virgo, WASP-39b is more than 20 times closer to
its star than Earth is to the sun. Although it is about the same mass as
Saturn, it is less dense and about 50 percent larger, probably due to
heating from being so close to its host star. Previous observations showed
it to have relatively clear skies, making it a good target for transmission
spectroscopy.
When the first data from JWST were released in July, the UCSC exoplanet
researchers were hosting 45 visiting astronomers for the Other Worlds
Laboratory’s annual Exoplanet Summer Program. “We were all huddled around
the laptop getting our first look at the spectrum and marveling at it,”
Batalha said. “It’s a tremendous, almost euphoric feeling, seeing something
for the first time that no other human has seen before—that’s what science
is all about.”
Reference:
Identification of carbon dioxide in an exoplanet atmosphere, Nature (2022).
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