The Upper Edge of the Neptune Desert is Stable Against Photoevaporation

Transit surveys indicate that there is a deficit of Neptune-sized planets on close-in orbits. If the entirety of this “Neptune desert” is 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. We strongly detect helium absorption signals for three planets, tentatively detect signals for two planets, and do not detect signals for the remaining two planets. We interpret these measured excess absorption signals using grids of one-dimensional Parker wind models to derive mass-loss rates, and using these rates we empirically benchmark 1D hydrodynamical outflow models for the first time. The resulting outflow efficiencies are too small 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.