Free Astronomy Magazine January-February 2023

34 JANUARY-FEBRUARY 2023 ASTRO PUBLISHING T he atmospheric com- position of the hot gas giant exoplanet WASP-39 b has been re- vealed by NASA’s James Webb Space Telescope. This graphic shows four transmission spectra from three of Webb’s instruments operated in four instrument modes. All are plotted on a common scale extend- ing from 0.5 to 5.5 mi- crons. A transmission spectrum is made by comparing starlight fil- tered through a planet’s atmosphere as it moves in front of the star, to the unfiltered starlight detected when the planet is beside the star. Each of the data points (white circles) on these graphs represents the amount of a specific wavelength of light that is blocked by the planet and absorbed by its at- mosphere. Wavelengths that are preferentially absorbed by the atmos- phere appear as peaks in the transmission spectrum. The blue line is a best-fit model that takes into account the data, the known proper- ties of WASP-39 b and its star (e.g., size, mass, temperature), and as- sumed characteristics of the atmosphere. At upper left, data from NIRISS shows finger- prints of potassium (K), water (H 2 O), and carbon monoxide (CO). At upper right, data from NIRCam shows a prominent water signature. At lower left, data from NIRSpec indicates water, sulfur dioxide (SO 2 ), carbon dioxide (CO 2 ), and carbon monoxide (CO). At lower right, additional NIRSpec data reveals all of these molecules as well as sodium (Na). [NASA, ESA, CSA, Joseph Olmsted (STScI)] Meanwhile, carbon monoxide (CO) was detected, but obvious signa- tures of both methane (CH 4 ) and hy- drogen sulfide (H 2 S) were absent from the Webb data. If present, these molecules occur at very low levels. To capture this broad spec- trum of WASP-39 b’s atmosphere, an international team numbering in the hundreds independently analyzed data from four of the Webb tele- scope’s finely calibrated instrument modes. “We had predicted what [the telescope] would show us, but it was more precise, more diverse, and more beautiful than I actually be- lieved it would be,” said Hannah Wakeford, an astrophysicist at the