@article{477d57b2880f4c4a81fa5cd348b9166a,
title = "Signatures of strong magnetization and a metal-poor atmosphere for a Neptune-sized exoplanet",
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-11 b, a low-mass (0.08 MJ) 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.",
author = "Lotfi Ben-Jaffel and Ballester, {Gilda E.} and Mu{\~n}oz, {Antonio Garc{\'i}a} and Panayotis Lavvas and Sing, {David K.} and Jorge Sanz-Forcada and Ofer Cohen and Tiffany Kataria and Henry, {Gregory W.} and Lars Buchhave and Thomas Mikal-Evans and Wakeford, {Hannah R.} and Mercedes L{\'o}pez-Morales",
note = "Funding Information: We are very grateful to the staff at the Space Telescope Science Institute, in particular to P. Royle, S. Penton, W. Januszewski and J. Debes, for their careful work in the scheduling of our observations and for various instrument insights. We also thank C. Carvalho at the Institut d{\textquoteright}Astrophysique de Paris and T. Forrester at the Lunar & Planetary Laboratory for the setup and managing of various computing resources for this project. We thank V. Bourrier, D. Ehenreich and A. Lecavelier for helpful discussions about the analysis of the Lyα transit data. L.B.-J. and P.L. acknowledge support from the Centre National des Etudes Spatiales CNES (France) and the Centre National de la Recherche Scientifique (CNRS) under project PACES. All US co-authors acknowledge support from the Space Telescope Science Institute under the PanCET GO 14767 programme, and G.E.B., G.W.H. and T.K. were also funded under GO 14625. This work is based on observations with the National Aeronautics and Space Administration/European Space Agency HST, obtained at the Space Telescope Science Institute operated by AURA, Inc. This work is also based on observations with XMM-Newton. Publisher Copyright: {\textcopyright} 2021, The Author(s), under exclusive licence to Springer Nature Limited.",
year = "2022",
month = jan,
doi = "10.1038/s41550-021-01505-x",
language = "English (US)",
volume = "6",
pages = "141--153",
journal = "Nature Astronomy",
issn = "2397-3366",
publisher = "Nature Publishing Group",
number = "1",
}