HAT-P-26b: A Neptune-Mass Exoplanet with a Well Constrained Heavy Element Abundance

TL;DR

This study analyzes the atmosphere of the Neptune-mass exoplanet HAT-P-26b using space telescope data, revealing its primordial composition and providing insights into planet formation and atmospheric metallicity.

Contribution

It presents the first detailed atmospheric metallicity measurement of HAT-P-26b, linking water abundance to heavy element content in a Neptune-mass exoplanet.

Findings

Detected prominent H2O absorption bands in the atmosphere.

Measured atmospheric heavy element content as approximately 4.8 times solar.

Indicates the atmosphere is likely primordial with late gas accretion.

Abstract

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.