Signatures of strong magnetization and a metal-poor atmosphere for a Neptune-sized exoplanet

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-11 b, a low-mass (0.08 M_J) exoplanet showing strong, phase-extended transit absorption of neutral hydrogen (maximum and tail transit depths of 32 ± 4% and 27 ± 4%) and singly ionized carbon (15 ± 4% and 12.5 ± 4%). We show that the atmosphere should have less than six times the solar metallicity (at 200 bar), and the exoplanet must also have an extended magnetotail (1.8–3.1 au). The HAT-P-11 b equatorial magnetic field strength should be about 1–5 G. 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.