A sub-Neptune exoplanet with a low-metallicity methane-depleted atmosphere and Mie-scattering clouds

Abstract

18] NASA Hubble FellowWith no analogues in the Solar System, the discovery of thousands of exoplanets with masses and radii intermediate between Earth and Neptune was one of the big surprises of exoplanet science. These super-Earths and sub-Neptunes likely represent the most common outcome of planet formation 1,2 . Mass and radius measurements indicate a diversity in bulk composition much wider than for gas giants 3 ; however, direct spectroscopic detections of molecular absorption and constraints on the gas mixing ratios have largely remained limited to planets more massive than Neptune 4-6 . Here, we analyze a combined Hubble/Spitzer Space Telescope dataset of 12 transits and 20 eclipses of the sub-Neptune GJ 3470 b, whose mass of 12.6 M⊕ places it near the half-way point between previously studied exo-Neptunes (22-23 M⊕) 5-7 and exoplanets known to have rocky densities (7 M⊕) 8 . Obtained over many years, our data set provides a robust detection of water absorption (>5σ) and a thermal emission detection from the lowest irradiated planet to date. We reveal a low-metallicity, hydrogendominated atmosphere similar to a gas giant, but strongly depleted in methane gas. The low, near-solar metallicity (O/H=0.2-18) sets important constraints on the potential planet formation processes at low masses as well as the subsequent accretion of solids. The low methane abundance indicates that methane is destroyed much more efficiently than previously predicted, suggesting that the CH4/CO transition curve has to be revisited for close-in planets. Finally, we also find a sharp drop in the cloud opacity at 2-3 µm characteristic of Mie scattering, which enables narrow constraints on the cloud particle size and makes GJ 3470b a keystone target for mid-IR characterization with JWST.

Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets

“…While the metallicity of the atmosphere may be higher (see §4.2.2), current observations do not provide sufficient constraints on the atmospheric metallicity of exo-Neptunes and sub-Neptunes (e.g. Fraine et al 2014;Wakeford et al 2017;Benneke et al 2019;Chachan et al 2019). The resulting temperature-pressure profiles all have the same general features: A deep adiabat in the convective region, a transition region around the RCB, and 0 200 400 600 800 1000 1200 1400 Temperature (K) 10 9 10 7 10 5 10 3 10 1 10 1 10 3…”

Optical to Near-infrared Transmission Spectrum of the Warm Sub-Saturn HAT-P-12b

“…We (Tennyson et al 2012), as well as the collision-induced absorption of H 2 and He. Following Benneke et al (2019), we use a threeparameter Mie-scattering cloud description for the retrieval analysis defining the mean particle size R part , the pressure level P τ =1 at which the clouds become optically opaque to grazing starlight at 1.5 µm, and the scale height of the cloud profile relative to the gas pressure scale height H part /H gas as free parameters. All free parameters are allowed to vary independently in the retrieval.…”

“…When calculating the cloud opacity, the retrieval is agnostic to the particular composition of the spherical cloud particles, considering only their size and vertical distribution; the former is assumed to be a logarithmic Gaussian distribution with a fixed width of σ R = 1.5. This three-parameter cloud description is motivated by the information content of transmission spectra and captures the wavelength-dependent opacities of a wide range of finite-sized cloud particles near the cloud deck in a highly orthogonal way, ideal for retrieval (Benneke et al 2019). It reduces to Rayleigh hazes in the limit of small particles and a gray cloud deck for large particles while simultaneously allowing for any finite-sized Mie-scattering particles in between.…”

Understanding and mitigating biases when studying inhomogeneous emission spectra with JWST

“…One method of studying the atmospheric constituents of an exoplanet is by employing an atmospheric retrieval (Madhusudhan & Seager 2009;Line et al 2013;Waldmann et al 2015a,b;Blecic et al 2017;Cubillos et al 2017;Mollière et al 2019;Benneke et al 2019). Atmospheric retrieval is the act of obtaining the physical and chemical characteristics of an atmosphere based on observations.…”