A correlation between giant-planet mass and atmospheric heavy elemental abundance was first noted in the past century from observations of planets in our own Solar System, and has served as a cornerstone of planet formation theory. Using data from the Hubble and Spitzer Space Telescopes from 0.5 to 5 microns, we conducted a detailed atmospheric study of the transiting Neptune-mass exoplanet HAT-P-26b. We detected prominent H2O absorption bands with a maximum base-to-peak amplitude of 525ppm in the transmission spectrum. Using the water abundance as a proxy for metallicity, we measured HAT-P-26b's atmospheric heavy element content [4.8 (-4.0 +21.5) times solar]. This likely indicates that HAT-P-26b's atmosphere is primordial and obtained its gaseous envelope late in its disk lifetime, with little contamination from metal-rich planetesimals.
@article{arxiv.1705.04354,
title = {HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance},
author = {Hannah R. Wakeford and David K. Sing and Tiffany Kataria and Drake Deming and Nikolay Nikolov and Eric D. Lopez and Pascal Tremblin and David S. Amundsen and Nikole K. Lewis and Avi M. Mandell and Jonathan J. Fortney and Heather Knutson and Björn Benneke and Thomas M. Evans},
journal= {arXiv preprint arXiv:1705.04354},
year = {2017}
}