A hot Jupiter’s clear atmosphere

When an exoplanet transits the disk of its parent star, starlight that filters through the exoplanet’s limb can reach an observer. If the exoplanet’s atmosphere is largely free of clouds, haze, and other absorbers, the spectral signatures imprinted by the atmosphere’s constituents can, in principle, be detected and analyzed.

Compared with the disk of the star, the limb of an exoplanet’s atmosphere is tiny. Current telescopes struggle to detect those miniscule spectral signatures. Prospects will improve following the launch of the James Webb Space Telescope (JWST) later this year. They will improve further if astronomers find suitable targets whose atmospheres are clear and whose positions on the sky ensure continuous observation uninterrupted by Earth’s shadow.

Munazza Alam of the Center for Astrophysics | Harvard & Smithsonian and her collaborators have identified a promising target: WASP-62b, a so-called hot Jupiter in orbit around a main-sequence star of 1.11 solar masses. The exoplanet’s hotness comes from its proximity to its stellar parent. The orbit is favorably inclined for observing transits and, at 4.4 days, conveniently short for observing multiple transits. WASP-62b’s location in the sky also favors observers. A member of the constellation Dorado, the exoplanet lies close to the south ecliptic pole and within the JWST‘s continuous viewing zone.

But does WASP-62b have a clear atmosphere? Is it feasible for the JWST‘s spectrometers to identify molecular species? To find out, Alam and her collaborators observed WASP-62b transit its star three times with the Hubble Space Telescope and twice with the Spitzer Space Telescope, which was decommissioned last year.

The spectra and light curves the researchers obtained included such small signals that Gaussian processes, Markov chain Monte Carlo, and other sophisticated numerical techniques were needed to tease out statistically significant atmospheric information. Indeed, only one absorption line, from neutral sodium at 0.59 μm, was unambiguously detected. Nevertheless, the detection, with a significance of 5.1σ, was strong enough that the line’s pressure-broadened wings were discernible.

Given that a cloudy, hazy atmosphere obscures pressure-broadened wings, their detection constitutes evidence that WASP-62b’s atmosphere is clear. With that knowledge in hand, Alam and her collaborators simulated various atmospheric observations made by three of the JWST‘s instruments. They predict that a host of molecular species—including H₂O, FeH, CO, CO₂, and CH₄—will be detected if they are present at expected abundances.

From such measurements, the carbon-to-oxygen ratio can be determined. The ratio is astrophysically important because its value, theorists say, can reveal the conditions that prevailed in the protoplanetary disk out of which WASP-62b first formed. (M. K. Alam et al., Astrophys. J. Lett. 906, L10, 2021 .)