Exoplanets and Stellar Physics

Abstract:

Atmospheric metal enrichment (that is, elements heavier than helium, also called 'metallicity') is a key diagnostic of the formation of giant planets^1-3. The giant planets of the Solar System show an inverse relationship between mass and both their bulk metallicities and atmospheric metallicities. Extrasolar giant planets also display an inverse relationship between mass and bulk metallicity⁴. However, there is significant scatter in the relationship and it is not known how atmospheric metallicity correlates with either planet mass or bulk metallicity. Here we show that the Saturn-mass exoplanet HD 149026b (refs. ^5-9) has an atmospheric metallicity 59-276 times solar (at 1σ), which is greater than Saturn's atmospheric metallicity of roughly 7.5 times solar¹⁰ at more than 4σ confidence. This result is based on modelling CO₂ and H₂O absorption features in the thermal emission spectrum of the planet measured by the James Webb Space Telescope. HD 149026b is the most metal-rich giant planet known, with an estimated bulk heavy element abundance of 66 ± 2% by mass^11,12. We find that the atmospheric metallicities of both HD 149026b and the Solar System giant planets are more correlated with bulk metallicity than planet mass.

Abstract:

There are no planets intermediate in size between Earth and Neptune in our Solar System, yet these objects are found around a substantial fraction of other stars [1]. Population statistics show that close-in planets in this size range bifurcate into two classes based on their radii [2, 3]. It is hypothesized that the group with larger radii (referred to as "sub-Neptunes") is distinguished by having hydrogen-dominated atmospheres that are a few percent of the total mass of the planets [4]. GJ 1214b is an archetype sub-Neptune that has been observed extensively using transmission spectroscopy to test this hypothesis [5-14]. However, themeasured spectra are featureless, and thus inconclusive, due to the presence of high-altitude aerosols in the planet's atmosphere. Here we report a spectroscopic thermal phase curve of GJ 1214b obtained with JWST in the mid-infrared. The dayside and nightside spectra (average brightness temperatures of 553 ± 9 and 437 ± 19 K, respectively) each show >3σ evidence of absorption features, with H2O as the most likely cause in both. The measured global thermal emission implies that GJ 1214b's Bond albedo is 0.51 ± 0.06. Comparison between the spectroscopic phase curve data and three-dimensional models of GJ 1214b reveal a planet with a high metallicity atmosphere blanketed by a thick and highly reflective layer of clouds or haze.