Nondetection of Helium in the Upper Atmospheres of Three Sub-Neptune Exoplanets

Abstract

We present a search for helium in the upper atmospheres of three sub-Neptune-sized planets to investigate the origins of these ubiquitous objects. The detection of helium for a low-density planet would be strong evidence for the presence of a primary atmosphere accreted from the protoplanetary nebula because large amounts of helium are not expected in the secondary atmospheres of rocky planets. We used Keck+NIRSPEC to obtain high-resolution transit spectroscopy of the planets GJ 1214b, GJ 9827d, and HD 97658b around the 10833 Å He triplet feature. We did not detect helium absorption for any of the planets despite achieving a high level of sensitivity. We used the nondetections to set limits on the planets' thermosphere temperatures and atmospheric loss rates by comparing grids of 1D models to the data. We also performed coupled interior structure and atmospheric loss calculations, which suggest that the bulk atmospheres (winds) of the planets would be at most modestly enhanced (depleted) in helium relative to their primordial composition. Our lack of detections of the helium triplet for GJ 1214b and GJ 9827d is highly inconsistent with the predictions of models for the present-day mass loss on these planets. Higher signal-to-noise data would be needed to detect the helium feature predicted for HD 97658b. We identify uncertainties in the extreme-ultraviolet fluxes of the host stars and the lack of detailed mass-loss models specifically for cool and metal-enhanced atmospheres as the main limitations to the interpretation of our results. Ultimately, our results suggest that the upper atmospheres of sub-Neptune planets are fundamentally different from those of gas giant planets.

The Nature and Origins of Sub‐Neptune Size Planets

“…Beyond the attempts to directly observe the bulk atmospheres of sub-Neptune size planets, there have also been clever attempts to deduce the composition of these planets' atmospheres through observations of their thermospheres, exospheres, and winds. These observations have been obtained for neutral hydrogen at Lyman α (Bourrier et al, 2017;dos Santos et al, 2020;Ehrenreich et al, 2012;García Muñoz et al, 2020;Waalkes et al, 2019) and in the helium IR triplet (Kasper et al, 2020). The detection of neutral hydrogen in an escaping wind would constrain the mass-loss rate of the atmosphere but would have an ambiguous interpretation with regards to the composition because neutral hydrogen could be produced from the photodissociation of H 2 and H 2 O.…”

Modelling the He I triplet absorption at 10 830 Å in the atmospheres of HD 189733 b and GJ 3470 b

“…In addition to the spectral shape and intensity of the XUV stellar irradiation (see, e.g. Oklopčić 2019), high H/He ratios could help to explain the non-detection of the He(2 3 S) in some highly irradiated exoplanets, such as GJ 1214 b and GJ 9827 d (as suggested by Kasper et al 2020) as well as K2-100b (Gaidos et al 2020).…”

Metastable Helium Reveals an Extended Atmosphere for the Gas Giant HAT-P-18b

“…Since then, this line has been used to detect extended atmospheres in six other planets (five gas giants and one sub-Neptune, with masses ranging from 0.044M J to 1.116M J ) using both space-and ground-based facilities (Allart et al 2018;Mansfield et al 2018;Nortmann et al 2018;Salz et al 2018;Alonso-Floriano et al 2019;Ninan et al 2019;Palle et al 2020). Understanding how the metastable helium signal scales with stellar EUV flux, planetary gravity, and stellocentric distance on a sample of large, well-characterized planets is a prerequisite for He I observations of smaller planets, where measurements and interpretation are inherently more difficult (Kasper et al 2020). Additionally, detections of outflowing gas giant atmospheres in the He I line provide useful constraints on crucial radiative and collisional processes in theoretical atmospheric mass loss models (e.g., Salz et al 2016;Oklopčić & Hirata 2018).…”