Water absorption in the transmission spectrum of the water-world candidate GJ 9827 d

Pierre-Alexis Roy ( Université de Montréal )

Recent work on the characterization of sub-Neptunes has allowed us to cumulate growing evidence that the sub-Neptunes with radii greater than ∼ 2.5 R⊕ often host H2/He dominated atmospheres both from measurements of their low bulk densities and direct detections of their low mean-molecular-weight atmospheres. However, the smaller sub-Neptunes in the 1.5-2.2 R⊕ size regime are much less understood, and often have bulk densities that can be explained either by the H2/He-rich scenario, or by a volatile-dominated composition known as the “water world” scenario. Here, we report the detection of water vapor in the transmission spectrum of the 1.955 ± 0.075 R⊕ sub-Neptune GJ 9827 d obtained with the Hubble Space Telescope. We observed 11 HST/WFC3 transits of GJ 9827 d and find an absorption feature at 1.4μm in its transit spectrum, which is best explained (at 3.4σ) by the presence of water in GJ 9827 d’s atmosphere. Our atmosphere retrieval analysis reveals that this water absorption feature can be similarly well explained by a small amount of water vapor in a cloudy H2/He dominated atmosphere, or by a water vapor dominated envelope on this ∼650 K sub-Neptune. Given that recent studies have inferred an important mass-loss rate (> 0.5 M⊕/Gyr) for GJ 9827 d making it unlikely to retain a H-dominated envelope, our water detection highlights GJ 9827 d as a promising water world candidate that could host a water vapor envelope. Our water detection also anchors GJ 9827 d as the smallest exoplanet and the first water world candidate with an atmospheric molecular detection to date.