Free Astronomy Magazine September-October 2024

53 SEPTEMBER-OCTOBER 2024 ASTRO PUBLISHING the Sun), it doesn’t receive enough energy from its star to be so in- flated. “WASP-107 b is such an interesting target for Webb because it’s sig- nificantly cooler and more Nep- tune-like in mass than many of the other low-density planets, the hot Jupiters, we’ve been studying,” said David Sing from the Johns Hopkins University (JHU), lead author on a parallel study also published in Nature . “As a result, we should be able to detect methane and other molecules that can give us informa- tion about its chemistry and internal dynamics that we can’t get from a hotter planet.” WASP-107 b’s giant radius, ex- tended atmosphere, and edge-on orbit make it ideal for transmission spectroscopy, a method used to identify the various gases in an exo- planet atmosphere based on how they affect starlight. Combining observations from Webb’s NIRCam (Near-Infrared Cam- era), Webb’s MIRI (Mid-Infrared In- strument), and Hubble’s WFC3 (Wide Field Camera 3), Welbanks’ team was able to build a broad spec- trum of 0.8- to 12.2-micron light ab- sorbed by WASP-107 b’s atmos- phere. Using Webb’s NIRSpec (Near- Infrared Spectrograph), Sing’s team built an independent spectrum cov- ering 2.7 to 5.2 microns. The precision of the data makes it possible to not just detect, but ac- tually measure the abundances of a wealth of molecules, including water vapor (H 2 O), methane (CH 4 ), carbon dioxide (CO 2 ), carbon monoxide (CO), sulfur dioxide (SO 2 ), and ammonia (NH 3 ). Both spectra show a surprisingly lack of methane in WASP-107 b’s atmosphere: one-thousandth the amount expected based on its as- sumed temperature. “This is evi- dence that hot gas from deep in the planet must be mixing vigorously T his artist’s concept shows what the exoplanet WASP-107 b could look like based on recent data gath- ered by NASA’s James Webb Space Telescope, along with previous ob- servations from Hubble and other space- and ground-based telescopes. [NASA, ESA, CSA, Ralf Crawford (STScI)] explain how WASP-107 b can be so inflated without resorting to ex- treme theories of how it formed. The results, which were made pos- sible by Webb’s extraordinary sensi- tivity and accompanying ability to measure light passing through exo- planet atmospheres, may explain the puffiness of dozens of low-den- sity exoplanets, helping solve a long-standing mystery in exoplanet science. At more than three-quar- ters the volume of Jupiter but less than one-tenth the mass, the “warm Neptune” exoplanet WASP- 107 b is one of the least dense plan- ets known. While puffy planets are not uncommon, most are hotter and more massive, and therefore easier to explain. “Based on its radius, mass, age, and assumed internal temperature, we thought WASP-107 b had a very small, rocky core surrounded by a huge mass of hydrogen and he- lium,” explained Luis Welbanks from Arizona State University (ASU), lead author on a paper published in Nature . “But it was hard to under- stand how such a small core could sweep up so much gas, and then stop short of growing fully into a Jupiter-mass planet.” If WASP-107 b instead has more of its mass in the core, the atmosphere should have contracted as the planet cooled over time since it formed. Without a source of heat to re-expand the gas, the planet should be much smaller. Although WASP-107 b has an orbital distance of just 5 million miles (one-seventh the distance between Mercury and