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 periodically transits WASP-96 (also known as 2MASS J00041112-4721382), a Sun-like star 980 light years away in the southern constellation Phoenix. Image credit: © Sci-News.com.
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 light curve from Webb’s NIRISS instrument shows the change in brightness of light from the WASP-96 star system over time as the planet transits the star. A transit occurs when an orbiting planet moves between the star and the telescope, blocking some of the light from the star. This observation was made using NIRISS’s Single-Object Slitless Spectroscopy (SOSS) mode, which involves capturing the spectrum of a single bright object, like the star WASP-96, in a field of view. To capture these data, Webb stared at the WASP-96 star system for 6 hours 23 minutes, beginning about 2.5 hours before the transit and ending about 1.5 hours after the transit was complete. The transit itself lasted for just under 2.5 hours. The curve includes a total of 280 individual brightness measurements — one every 1.4 minutes. Because the observation was made using a spectrograph, which spreads the light out into hundreds of individual wavelengths, each of the 280 points on the graph represents the combined brightness of thousands of wavelengths of infrared light. The actual dimming caused by the planet is extremely small: the difference between the brightest and dimmest points is less than 1.5%. NIRISS is ideally suited for this observation because it has the ability to observe relatively bright targets over time, along with the sensitivity needed to measure such small differences in brightness: in this observation, the instrument was able to measure differences in brightness as small as 0.02%. Although the presence, size, mass, and orbit of the planet had already been determined based on previous transit observations, this transit light curve can be used to confirm and refine existing measurements, such as the planet’s diameter, the timing of the transit, and the planet’s orbital properties. The background illustration of WASP-96b and its Sun-like star is based on current understanding of the planet from both NIRISS spectroscopy and previous ground- and space-based observations. Webb has not captured a direct image of the planet or its atmosphere. Image credit: NASA / ESA / CSA / STScI.
“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.
A transmission spectrum made from a single observation using Webb’s Near-Infrared Imager and Slitless Spectrograph (NIRISS) reveals atmospheric characteristics of WASP-96b. Image credit: NASA / ESA / CSA / STScI.
“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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