Free Astronomy Magazine September-October 2024

SEPTEMBER-OCTOBER 2024 T his transmission spectrum, captured using NASA’s Hubble and James Webb space telescopes, shows the amounts of different wavelengths (col- ors) of starlight blocked by the atmosphere of the gas-giant exoplanet WASP-107 b. The spectrum includes light collected over five separate ob- servations using a total of three different instruments: Hubble’s WFC3 (0.8–1.6 microns), Webb’s NIRCam (2.4–4.0 microns and 3.9–5.0 microns), and Webb’s MIRI (5–12 microns). Each set of measurements was made by observing the planet-star system for about 10 hours before, during, and after the transit as the planet moved across the face of the star. By comparing the brightness of light filtered through the planet’s atmosphere (transmit- ted light) to unfiltered starlight, it is possible to calculate the amount of each wavelength that is blocked by the atmosphere. Since each molecule ab- sorbs a unique combination of wavelengths, the transmission spectrum can be used to constrain the abundance of various gases. This spectrum shows clear evidence for water (H 2 O), carbon dioxide (CO 2 ), carbon monoxide (CO), methane (CH 4 ), sulfur dioxide (SO 2 ), and ammonia (NH 3 ) in the planet’s atmosphere, allowing researchers to estimate the interior temperature and mass of the core. This wavelength coverage from optical to mid- infrared is the broadest of any exoplanet transmission spectrum to date, and includes the first reported space telescope detection of ammonia in an exoplanet atmosphere. [NASA, ESA, CSA, Ralf Crawford (STScI) – Luis Welbanks (ASU), JWST MANATEE Team] Daniel Thorngren from JHU. It turns out that the core is at least twice as massive as originally estimated, which makes more sense in terms of how planets form. All together, WASP-107 b is not as mysterious as it once appeared. “The Webb data tells us that planets like WASP-107 b didn’t have to form in some odd way with a super small core and a huge gassy envelope,” explained Mike Line from ASU. “In- stead, we can take something more like Neptune, with a lot of rock and not as much gas, just dial up the temperature, and poof it up to look the way it does.” posed that tidal heating could be the cause of WASP-107 b’s puffiness, but until the Webb results were in, there was no evidence. Once they established that the planet has enough internal heat to thoroughly churn up the atmosphere, the teams realized that the spectra could also provide a new way to estimate the size of the core. “If we know how much energy is in the planet, and we know what proportion of the planet is heavier elements like car- bon, nitrogen, oxygen, and sulfur, versus how much is hydrogen and helium, we can calculate how much mass must be in the core,” explained with the cooler layers higher up,” explained Sing. “Methane is unsta- ble at high temperatures. The fact that we detected so little, even though we did detect other carbon- bearing molecules, tells us that the interior of the planet must be signif- icantly hotter than we thought.” A likely source of WASP-107 b’s extra internal energy is tidal heating caused by its slightly elliptical orbit. With the distance between the star and planet changing continuously over the 5.7-day orbit, the gravita- tional pull is also changing, stretch- ing the planet and heating it up. Researchers had previously pro- !