The NASA/ESA/CSA James Webb Space Telescope has captured the distinct signature of water, along with evidence for clouds and haze, in the atmosphere surrounding WASP-96b, a hot, puffy gas giant located roughly 1,150 light-years away in the southern constellation of Phoenix.
WASP-96b was discovered in 2013 by astronomers with the Wide Angle Search for Planets (WASP) survey.
Located 1,150 light-years away in the constellation of Phoenix, it represents a type of gas giant that has no direct analog in our Solar System.
WASP-96b orbits its 8-billion-year-old Sun-like star, WASP-96, every 3.4 days and is very hot (1,881 degrees Fahrenheit, or 1,027 degrees Celsius).
With a mass less than half that of Jupiter and a diameter 1.2 times greater, it is much puffier than any planet orbiting our Sun.
The combination of large size, short orbital period, puffy atmosphere, and lack of contaminating light from objects nearby in the sky makes WASP-96b an ideal target for atmospheric observations.
On June 21, 2022, the Near-Infrared Imager and Slitless Spectrograph (NIRISS) onboard Webb measured light from the WASP-96 system for 6.4 hours as the planet moved across the star.
The result is a light curve showing the overall dimming of starlight during the transit, and a transmission spectrum revealing the brightness change of individual wavelengths of infrared light between 0.6 and 2.8 microns.
The transmission spectrum reveals previously hidden details of the atmosphere: the unambiguous signature of water, indications of haze, and evidence of clouds that were thought not to exist based on prior observations.
“A transmission spectrum is made by comparing starlight filtered through a planet’s atmosphere as it moves across the star to the unfiltered starlight detected when the planet is beside the star,” the Webb astronomers explained.
“We are able to detect and measure the abundances of key gases in a planet’s atmosphere based on the absorption pattern — the locations and heights of peaks on the graph.”
The spectrum of WASP-96b is not only the most detailed near-infrared transmission spectrum of an exoplanet atmosphere captured to date, but it also covers a remarkably wide range of wavelengths, including visible red light and a portion of the spectrum that has not previously been accessible from other telescopes (wavelengths longer than 1.6 microns).
This part of the spectrum is particularly sensitive to water as well as other key molecules like oxygen, methane, and carbon dioxide, which are not immediately obvious in the WASP-96b spectrum but which should be detectable in other exoplanets planned for observation by Webb.
The blue line on the graph is a best-fit model that takes into account the data, the known properties of WASP-96b and its star (e.g., size, mass, temperature), and assumed characteristics of the atmosphere.
“The extraordinarily detailed spectrum — made by simultaneously analyzing 280 individual spectra captured over the observation — provides just a hint of what Webb has in store for exoplanet research,” the astronomers said.
“Over the coming year, we will use spectroscopy to analyze the surfaces and atmospheres of several dozen exoplanets, from small rocky planets to gas- and ice-rich giants.”
“Nearly one-quarter of Webb’s Cycle 1 observation time is allocated to studying exoplanets and the materials that form them.”
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