The angle between the spin of a star and the orbital planes of its planets traces the history of the planetary system. Exoplanets orbiting close to cool stars are expected to be on circular, aligned orbits because of strong tidal interactions with the stellar convective envelope1. Spin–orbit alignment can be measured when the planet transits its star, but such ground-based spectroscopic measurements are challenging for cool, slowly rotating stars2. Here we report the three-dimensional characterization of the trajectory of an exoplanet around an M dwarf star, derived by mapping the spectrum of the stellar photosphere along the chord transited by the planet3. We find that the eccentric orbit of the Neptune-mass exoplanet GJ 436b is nearly perpendicular to the stellar equator. Both eccentricity and misalignment, surprising around a cool star, can result from dynamical interactions (via Kozai migration4) with a yet-undetected outer companion. This inward migration of GJ 436b could have triggered the atmospheric escape that now sustains its giant exosphere5.
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This work is based on observations made with the HARPS spectrograph on the 3.6 m ESO telescope at the ESO La Silla Observatory, Chile, under GTO program ID 072.C-0488, and with the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias under OPTICON program 16A/049, ‘Sensing Planetary Atmospheres with Differential Echelle Spectroscopy’ (SPADES). OPTICON has received funding from the European Community’s Seventh Framework Programme (FP7/2013-2016) under grant agreement number 312430. This project has also received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 Research and Innovation Programme under grant agreement number 724427 (FOUR ACES). This work was carried out in the framework of the National Centre for Competence in Research ‘PlanetS’ supported by the Swiss National Science Foundation (SNSF). R.A., N.A.-D., V.B., D.E., C.L. and A.W. acknowledge the financial support of the SNSF. H.M.C. gratefully acknowledges support as a CHEOPS Fellow from the SNSF National Centre of Competence in Research ‘PlanetS’. G.W.H. acknowledges long-term support from Tennessee State University and the State of Tennessee through its Centers of Excellence programme. X.B. and X.D. acknowledge the support of CNRS/PNP (Programme national de planétologie). X.B. acknowledges funding from the European Research Council under the ERC Grant Agreement number 337591-ExTrA. We thank C. A. Watson for calculating the convective mass of GJ 436, H. Knutson for facilitating the determination of the stellar rotation period, J.-B. Delisle for discussing the system geometry, and the Telescopio Nazionale Galileo staff for the service observation.