The James Webb Space Telescope observed the exoplanet WASP-80 b as it passed in front of and behind its host star, revealing spectra indicative of an atmosphere containing methane gas and water vapor.
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While water vapor has been detected in over a dozen planets to date, until recently, methane—a molecule found in abundance in the atmospheres of Jupiter, Saturn, Uranus, and Neptune within our solar system—has remained elusive in the atmospheres of transiting exoplanets when studied with space-based spectroscopy.
Taylor Bell from the Bay Area Environmental Research Institute (BAERI), working at NASA's Ames Research Center in California's Silicon Valley, and Luis Welbanks from Arizona State University tell us more about the significance of discovering methane in exoplanet atmospheres and discuss how Webb observations facilitated the identification of this long-sought-after molecule.
These findings were recently published in Nature.
"With a temperature of about 825 Kelvin (about 1,025 degrees Fahrenheit), WASP-80 b is what scientists call a 'warm Jupiter,' which are planets that are similar in size and mass to the planet Jupiter in our solar system but have a temperature that's in-between that of hot Jupiters, like the 1,450-K (2,150-F) HD 209458 b (the first transiting exoplanet discovered), and cold Jupiters, like our own which is about 125 K (235 F)."
"WASP-80 b goes around its red dwarf star once every three days and is situated 163 light-years away from us in the constellation Aquila. Because the planet is so close to its star and both are so far away from us, we can't see the planet directly with even the most advanced telescopes like Webb.
"Instead, researchers study the combined light from the star and planet using the transit method (which has been used to discover most known exoplanets) and the eclipse method."
"Using the transit method, we observed the system when the planet moved in front of its star from our perspective, causing the starlight we saw to dim a bit. It's kind of like when someone passes in front of a lamp, and the light dims."
"During this time, a thin ring of the planet's atmosphere around the planet's day/night boundary is lit up by the star, and at certain colors of light where the molecules in the planet's atmosphere absorb light, the atmosphere looks thicker and blocks more starlight, causing a deeper dimming compared to other wavelengths where the atmosphere appears transparent.
"This method helps scientists like us understand what the planet's atmosphere is made of by seeing which colors of light are being blocked."
"Meanwhile, using the eclipse method, we observed the system as the planet passed behind its star from our perspective, causing another small dip in the total light we received. All objects emit some light, called thermal radiation, with the intensity and color of the emitted light depending on how hot the object is."
"Just before and after the eclipse, the planet's hot dayside is pointed toward us, and by measuring the dip in light during the eclipse, we were able to measure the infrared light emitted by the planet.
"For eclipse spectra, absorption by molecules in the planet's atmosphere typically appears as a reduction in the planet's emitted light at specific wavelengths. Also, since the planet is much smaller and colder than its host star, the depth of an eclipse is much smaller than the depth of a transit."
"The initial observations we made needed to be transformed into something we call a spectrum; this is essentially a measurement showing how much light is either blocked or emitted by the planet's atmosphere at different colors (or wavelengths) of light.
"Many different tools exist to transform raw observations into useful spectra, so we used two different approaches to make sure our findings were robust to different assumptions." ■