Helium-poor winds do not require helium-poor planets

Abstract

Observations of the metastable He,{\sc i},10830,\AA\ triplet have revealed escaping exoplanet atmospheres whose inferred helium abundances range from nearly nebular compositions to strongly helium-depleted winds. Such depletion is commonly interpreted as evidence for atmospheric evolution, preferential escape, or departures from primordial composition. We present a closed-form analytic transport-retention theory for the atmospheric transition region between the homopause and the base of the planetary wind. The theory quantifies the competition between upward transport and molecular separation, demonstrating that transport physics alone can substantially deplete the helium abundance supplied to the escaping flow. Our solution yields a retention factor, $\chi_{\rm He}$, that measures the fraction of helium supplied to the base of the hydrodynamic wind relative to the deep atmospheric abundance. Combining the analytic framework with numerical forward models of the He,{\sc i},10830,\AA\ absorption and a sample of twelve helium-observed sub-Neptunes and mini-Neptunes, we find that observed systems span the full range of predicted retention states, from nearly complete retention to strong helium depletion. These forward models use the helium abundance supplied by the analytic framework as the lower-boundary condition to predict the corresponding He,{\sc i},10830,\AA\ absorption. We further show that increasing helium retention systematically strengthens the expected absorption signal by increasing the helium reservoir available to the upper atmosphere. These results suggest that helium depletion does not necessarily imply intrinsically helium-poor atmospheres or evolutionary transitions toward secondary compositions, but may instead be a natural consequence of transport physics in escaping atmospheres.