A Hubble PanCET Study of HAT-P-11b: A Cloudy Neptune with a Low Atmospheric Metallicity

TL;DR

This study provides a detailed transmission spectrum of HAT-P-11b, revealing a cloudy atmosphere with low metallicity, and demonstrates the importance of broad wavelength coverage for atmospheric characterization.

Contribution

First comprehensive transmission spectrum of HAT-P-11b from 0.35 to 5 μm, combining Hubble and Spitzer data, with new insights into its cloud properties and low atmospheric metallicity.

Findings

Detection of water/methane features at 1.15 μm and 0.95 μm

Clouds dominate the optical and near-infrared spectrum

Atmospheric metallicity constrained to be below 4.6 Z⊙

Abstract

We present the first comprehensive look at the $0.35-5$ $\mu$m transmission spectrum of the warm ($\sim 800$ K) Neptune HAT-P-11b derived from thirteen individual transits observed using the Hubble and Spitzer Space Telescopes. Along with the previously published molecular absorption feature in the $1.1-1.7$ $\mu$m bandpass, we detect a distinct absorption feature at 1.15 $\mu$m and a weak feature at 0.95 $\mu$m, indicating the presence of water and/or methane with a combined significance of 4.4 $\sigma$. We find that this planet's nearly flat optical transmission spectrum and attenuated near-infrared molecular absorption features are best-matched by models incorporating a high-altitude cloud layer. Atmospheric retrievals using the combined $0.35-1.7$ $\mu$m HST transmission spectrum yield strong constraints on atmospheric cloud-top pressure and metallicity, but we are unable to match the relatively shallow Spitzer transit depths without under-predicting the strength of the near-infrared molecular absorption bands. HAT-P-11b's HST transmission spectrum is well-matched by predictions from our microphysical cloud models. Both forward models and retrievals indicate that HAT-P-11b most likely has a relatively low atmospheric metallicity ($<4.6 \; Z_{\odot}$ and $<86 \; Z_{\odot}$ at the $2 \sigma$ and $3 \sigma$ levels respectively), in contrast to the expected trend based on the solar system planets. Our work also demonstrates that the wide wavelength coverage provided by the addition of the HST STIS data is critical for making these inferences.