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

Abstract

We present the first comprehensive look at the 0.35−5 µm transmission spectrum of the warm (∼ 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 µm bandpass, we detect a distinct absorption feature at 1.15 µm and a weak feature at 0.95 µm, indicating the presence of water and/or methane with a combined significance of 4.4 σ. 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 µm HST transmission spectrum yield strong constraints on atmospheric cloudtop 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 and < 86 Z at the 2σ and 3σ 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.

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…”

The exoplanet perspective on future ice giant exploration

“…Almost all of these atmospheres show evidence of aerosols (clouds/hazes) in the atmosphere muting or Each spectrum has been fit with a model from a grid of forward models scaled to each exoplanets scale height [10]. Data: HAT-P-11b [11], GJ 436 [12], HAT-P-26b [13], GJ 3470b [14], K2-18b [15], HD 97658b [16], GJ 1214b [17,18]. (Online version in colour.…”

“…The solar system measurements [42][43][44][45] (black squares) are based on the CH 4 abundance and display a trend increasing the atmospheric metallicity with decreasing planetary mass. The measured exoplanets [11,[13][14][15]21,[46][47][48][49][50][51][52][53][54] (circles coloured by temperature) do not currently follow this same trend. However, it is important to note that for the exoplanets many are in single planet systems, most if not all migrated to their current position close to their star, and the measurements are predominantly based on H 2 O absorption or emission features.…”

PLATON II: New Capabilities and a Comprehensive Retrieval on HD 189733b Transit and Eclipse Data

“…A similar lack of retrieval codes for transit spectra motivated us to write PLATON (Zhang et al 2019), a fast, open source, easy to use, and easy to understand forward modeling and retrieval code that traces its lineage back to Exo-Transmit (Kempton et al 2017). PLATON has since been used in several papers: a few exploring the atmospheric properties of observed planets (Chachan et al 2019;Kirk et al 2019;Guo et al 2020), and one demonstrating the possibility of using K-means clustering to speed up retrievals by 40% (Hayes et al 2020). In the latter study, the speed of PLATON was especially useful due to the necessity of running many retrievals.…”