Signatures of Strong Magnetization and Metal-Poor Atmosphere for a Neptune-Size Exoplanet

TL;DR

This study detects signs of a strong magnetic field and a metal-poor atmosphere in the Neptune-sized exoplanet HAT-P-11b through ultraviolet transit observations of neutral hydrogen and ionized carbon.

Contribution

It introduces a novel method using ionized species in UV transit data to simultaneously measure exoplanetary magnetic fields and atmospheric metallicity.

Findings

Detected extended hydrogen and ionized carbon absorption during transit.

Estimated the exoplanet's magnetic field strength to be 1-5 Gauss.

Constrained the atmospheric metallicity to less than six times solar.

Abstract

The magnetosphere of an exoplanet has yet to be unambiguously detected. Investigations of star-planet interaction and neutral atomic hydrogen absorption during transit to detect magnetic fields in hot Jupiters have been inconclusive, and interpretations of the transit absorption non-unique. In contrast, ionized species escaping a magnetized exoplanet, particularly from the polar caps, should populate the magnetosphere, allowing detection of different regions from the plasmasphere to the extended magnetotail, and characterization of the magnetic field producing them. Here, we report ultraviolet observations of HAT-P-11b, a low-mass (0.08 MJ) exoplanet showing strong, phase-extended transit absorption of neutral hydrogen (maximum and tail transit depths of 32 \pm 4%, 27 \pm 4%) and singly ionized carbon (15 \pm 4%, 12.5 \pm 4%). We show that the atmosphere should have less than six times the solar metallicity (at 200 bars), and the exoplanet must also have an extended magnetotail (1.8-3.1 AU). The HAT-P-11b equatorial magnetic field strength should be about 1-5 Gauss. Our panchromatic approach using ionized species to simultaneously derive metallicity and magnetic field strength can now constrain interior and dynamo models of exoplanets, with implications for formation and evolution scenarios.