Characterizing a World Within the Hot-Neptune Desert: Transit Observations of LTT 9779 b with the Hubble Space Telescope/WFC3

Abstract

We present an atmospheric analysis of LTT 9779 b, a rare planet situated in the hot-Neptune desert, that has been observed with Hubble Space Telescope (HST)/WFC3 with G102 and G141. The combined transmission spectrum, which covers 0.8–1.6 μm, shows a gradual increase in transit depth with wavelength. Our preferred atmospheric model shows evidence for H2O, CO2, and FeH with a significance of 3.1σ, 2.4σ, and 2.1σ, respectively. In an attempt to constrain the rate of atmospheric escape for this planet, we search for the 1.083 μm helium line in the G102 data but find no evidence of excess absorption that would indicate an escaping atmosphere using this tracer. We refine the orbital ephemerides of LTT 9779 b using our HST data and observations from TESS, searching for evidence of orbital decay or apsidal precession, which are not found. The phase-curve observation of LTT 9779 b with JWST NIRISS should provide deeper insights into the atmosphere of this planet and the expected atmospheric escape might be detected with further observations concentrated on other tracers such as Lyα.

Muted Features in the JWST NIRISS Transmission Spectrum of Hot Neptune LTT 9779b

“…Due to its extreme level of irradiation, LTT 9779 b receives >2500×more flux from its host star than the Earth; it might be expected to be in the process of losing its atmosphere through photoevaporation (Owen & Wu 2013;Owen & Lai 2018). However, like Edwards et al (2023), we find no clear evidence for He absorption in our spectrum.…”

“…We do not consider the transit light source effect (TLSE; Rackham et al 2018;Fournier-Tondreau et al 2023;Lim et al 2023) in our retrievals as LTT 9779 is known to be slowly rotating and thus inactive (Jenkins et al 2020). Moreover, Edwards et al (2023) included the TLSE in their HST retrievals but found that it was disfavored over models without the TLSE and resulted in unreasonably high-spot and faculaefilling factors.…”

“…Finally, since Edwards et al (2023) find evidence for FeH in their HST/WFC3 spectrum, we perform another free chemistry retrieval, identical to the initial free chemistry retrieval, except also including FeH opacity (Wende et al 2010). The abundance of FeH remains unconstrained by the data, and we are therefore not able to confirm the results of Edwards et al (2023).…”

“…An analysis of follow-up observations with the UVIS mode of the Hubble Space Telescope (HST) is also underway to confirm this high albedo in the near-ultraviolet. 11 However, transmission observations with HSTʼs Wide Field Camera 3 (WFC3) G102 and G141 grisms presented by Edwards et al (2023) suggest a cloud-free terminator and a very low (subsolar) atmospheric metallicity. They also find no evidence for atmospheric escape via an analysis of the He 1.083 μm metastable triplet.…”

A High-resolution Non-detection of Escaping Helium in the Ultrahot Neptune LTT 9779b: Evidence for Weakened Evaporation

“…Also, the planet has a restricted Jeans parameter Λ = GM p m H /(k B T eq R p ) ≈ 27, a gravitational potential Φ = GM p /R p ≈ 4 × 10 12 erg g −1 , and a Roche lobe filling factor R p /R Roche ≈ 50% (Eggleton 1983). Photoevaporation and/or Roche lobe overflow should be vigorous for a planet so close to its host star with such low ρ p , Λ, Φ, and R Roche (Salz et al 2016;Kubyshkina et al 2018;Caldiroli et al 2022;Edwards et al 2023). In fact, escaping atmospheres have been detected for many Jupiter-and Neptunesized planets with similarly small ρ p , Λ, Φ, and/or R Roche .…”