Water Vapour Absorption in the Clear Atmosphere of an exo-Neptune

TL;DR

This study presents the first clear detection of water vapour in the atmosphere of a Neptune-sized exoplanet, HAT-P-11b, revealing a hydrogen-rich, largely cloud-free atmosphere consistent with core accretion formation theories.

Contribution

It provides the first clear transmission spectrum of a Neptune-sized exoplanet showing water vapour absorption, challenging previous assumptions about obscuring clouds or hazes.

Findings

Detected water vapour at 1.4 micrometres in HAT-P-11b's atmosphere

Atmosphere is predominantly clear down to ~1 mbar pressure

Atmosphere has a low heavy element abundance, consistent with core accretion theory

Abstract

Transmission spectroscopy to date has detected atomic and molecular absorption in Jupiter-sized exoplanets, but intense efforts to measure molecular absorption in the atmospheres of smaller (Neptune-sized) planets during transits have revealed only featureless spectra. From this it was concluded that the majority of small, warm planets evolve to sustain high mean molecular weights, opaque clouds, or scattering hazes in their atmospheres, obscuring our ability to observe the composition of these atmospheres. Here we report observations of the transmission spectrum of HAT-P-11b (~4 Earth radii) from the optical to the infrared. We detected water vapour absorption at 1.4 micrometre wavelength. The amplitude of the water absorption (approximately 250 parts-per- million) indicates that the planetary atmosphere is predominantly clear down to ~1 mbar, and sufficiently hydrogen-rich to exhibit a large scale height. The spectrum is indicative of a planetary atmosphere with an upper limit of ~700 times the abundance of heavy elements relative to solar. This is in good agreement with the core accretion theory of planet formation, in which gas giant planets acquire their atmospheres by directly accreting hydrogen-rich gas from the protoplanetary nebulae onto a large rocky or icy core.