HST PanCET program: A Cloudy Atmosphere for the promising JWST target WASP-101b

TL;DR

This study uses HST observations to analyze the atmosphere of hot Jupiter WASP-101b, revealing a cloudy atmosphere with no significant water absorption, and compares it to similar exoplanets to inform future observations.

Contribution

First HST PanCET observation of WASP-101b showing a cloudy atmosphere and establishing criteria for future exoplanet spectral analysis.

Findings

WASP-101b has no significant H2O features in near-infrared spectrum.

WASP-101b's atmosphere is not clear, ruling it out as a JWST ERS target.

Similarities between WASP-101b and WASP-31b suggest other planets in the same parameter space are also cloudy.

Abstract

We present results from the first observations of the Hubble Space Telescope (HST) Panchromatic Comparative Exoplanet Treasury (PanCET) program for WASP-101b, a highly inflated hot Jupiter and one of the community targets proposed for the James Webb Space Telescope (JWST) Early Release Science (ERS) program. From a single HST Wide Field Camera 3 (WFC3) observation, we find that the near-infrared transmission spectrum of WASP-101b contains no significant H$_2$O absorption features and we rule out a clear atmosphere at 13{\sigma}. Therefore, WASP-101b is not an optimum target for a JWST ERS program aimed at observing strong molecular transmission features. We compare WASP-101b to the well studied and nearly identical hot Jupiter WASP-31b. These twin planets show similar temperature-pressure profiles and atmospheric features in the near-infrared. We suggest exoplanets in the same parameter space as WASP-101b and WASP-31b will also exhibit cloudy transmission spectral features. For future HST exoplanet studies, our analysis also suggests that a lower count limit needs to be exceeded per pixel on the detector in order to avoid unwanted instrumental systematics.