The Upper Edge of the Neptune Desert Is Stable Against Photoevaporation

Abstract

Transit surveys indicate that there is a deficit of Neptune-sized planets on close-in orbits. If this "Neptune desert" is entirely cleared out by atmospheric mass loss, then planets at its upper edge should only be marginally stable against photoevaporation, exhibiting strong outflow signatures in tracers like the metastable helium triplet. We test this hypothesis by carrying out a 12-night photometric survey of the metastable helium feature with Palomar/WIRC, targeting seven gas-giant planets orbiting K-type host stars. Eight nights of data are analyzed here for the first time along with reanalyses of four previously-published datasets. We strongly detect helium absorption signals for WASP-69b, HAT-P-18b, and HAT-P-26b; tentatively detect signals for WASP-52b and NGTS-5b; and do not detect signals for WASP-177b and WASP-80b. We interpret these measured excess absorption signals using grids of isothermal Parker wind models to derive mass-loss rates, which are typically around 10 10 − 10 11 g s −1 . Our empirically constrained mass-loss rates are in good agreement with predictions from the 1D hydrodynamical outflow code ATES for all planets except WASP-52b and WASP-80b, where our data suggest that the outflows are much smaller than predicted. Excluding these two planets, the outflows for the rest of the sample are consistent with a mean energy-limited outflow efficiency of ε = 0.41 +0.16 −0.13 . Even when we make the relatively conservative assumption that gas-giant planets experience energy-limited outflows at this efficiency for their entire lives, photoevaporation would still be too inefficient to carve the upper boundary of the Neptune desert. We conclude that this feature of the exoplanet population is a pristine tracer of giant planet formation and migration mechanisms.

Non-detection of He I in the Atmosphere of GJ 1214b with Keck/NIRSPEC, at a Time of Minimal Telluric Contamination

“…Alternatively, Salz et al (2016) note many important factors that are missing from their models: metals cooling the outflow and reducing the mass-loss rate, stellar winds forcing the escaping material radially outwards away from the star (e.g., Wang & Dai 2021;MacLeod & Oklopčić 2022), and suppression of the planetary outflow by magnetic fields. This last effect was first studied by Adams (2011), Trammell et al (2011), and Owen & Adams (2014, and was suggested to be occuring on WASP-52b, WASP-80b (Vissapragada et al 2022), and TOI-560b (Zhang et al 2022). Any combination of the above effects, and a heluim abundance of less than 10% in the upper atmosphere, may be the reason for our non-detection.…”

Constraining planetary mass-loss rates by simulating Parker wind profiles with Cloudy

“…However, we do find somewhat lower efficiencies than the hydrodynamic simulations of Salz et al (2016) and Caldiroli et al (2022), who (excluding stable atmospheres) find efficiencies in the range 30%-90% for planets with F XUV /ρ p < 10 4 erg cm s −1 g −1 . We also find lower efficiencies than Vissapragada et al (2022b), who fit the efficiency parameter for a sample of five planets whose mass-loss rates they constrained using Parker wind models. They report an efficiency of 0.41 +0.16 −0.13 , but we note that they divided their mass-loss rates by a factor of four to account for flux averaging over the planet surface, so that their efficiency definition implicitly differs from ours.…”

“…Adding the uncertainties in quadrature, we obtain EW=9.26 ± 1.82 mÅ. The HAT-P-18 b observations were performed by Paragas et al (2021) and reanalyzed by Vissapragada et al (2022b), who report 0.70 ± 0.16% excess absorption in the narrowband filter. The WASP-107 b observations were performed by Spake et al (2018), who report EW=48.02 ± 10.78 mÅ.…”

“…We chose our sample based on the availability of published metastable helium observations. The WASP-69 b observations were performed by Vissapragada et al (2020) and reanalyzed by Vissapragada et al (2022b), who report an excess absorption of 0.512 ± 0.049% in their narrowband filter. We convolved our helium model spectra with the transmission profile of this filter (S. Vissapragada, private communication) to obtain the excess absorption and compared these against the observed value.…”

“…We expect these differences to be mostly due to the use of Cloudy. For HAT-P-18 b, the offset is a bit larger, but also partly stems from the fact that we used the updated transit depth from Vissapragada et al (2022b) that is higher than the value used in Paragas et al (2021). In the case of HD 209458 b, the difference with the constraints from Lampón et al (2020) is on the order or ∼0.5 dex, which might be due to fitting the EW instead of the resolved line shape.…”