The effect of tidal locking on the magnetospheric and atmospheric evolution of “Hot Jupiters”

¹ Institut für Theoretische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany e-mail: j-m.griessmeier@tu-bs.de; u.motschmann@tu-bs.de
² Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Mendelssohnstraße 3, 38106 Braunschweig, Germany e-mail: a.stadelmann@tu-bs.de
³ Institute for Theoretical Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: penzt@stud.uni-graz.at
⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria e-mail: helmut.lammer@oeaw.ac.at; helfried.biernat@oeaw.ac.at
⁵ Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir, km 4 28850 Torrejon de Ardoz, Madrid, Spain e-mail: selsisf@inta.es; selsis@obs.u-bordeaux1.fr
⁶ Institut de Ciències de l'Espai (CSIC-IEEC), c/Gran Capità, 2-4, 08034 Barcelona, Spain e-mail: iribas@ieec.fcr.es
⁷ Departament d' Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain
⁸ Department of Astronomy and Astrophysics, Villanova University, Villanova, PA 19085, USA e-mail: edward.guinan@villanova.edu
⁹ Department for Astronomy, University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria e-mail: weiss@astro.univie.ac.at

Received: 14 November 2003
Accepted: 14 June 2004

Abstract

We study the interaction between the planetary magnetosphere and atmosphere of the close-in extrasolar planets HD 209458b and OGLE-TR-56b with the stellar wind during the evolution of their host stars. Recent astrophysical observations of solar-like stars indicate that the radiation and particle environments of young stars are orders of magnitudes larger than for stars with ages comparable to the sun (~4.6 Gyr). We model the interaction for the present and for early evolutionary stages, showing that it is possible that “Hot Jupiters” have an ionosphere-stellar wind interaction like Venus. Our study suggests that the internal magnetic field of exoplanets orbiting close to their host stars may be very weak due to tidal locking. The magnetic moments can be less than one tenth of the value presently observed for the rapidly rotating planet Jupiter. We find that the stronger stellar wind of younger solar-type stars compresses the magnetosphere to a standoff distance at which the ionized part of the upper atmosphere, hydrodynamically expanded by the XUV-flux, builds an obstacle. Because of a much larger stellar wind particle flux during the first ~0.5 Gyr after the host stars arrived on the Zero-Age-Main-Sequence, “Hot Jupiters” may have not been protected by their intrinsic magnetic fields, even if one neglects the effect of tidal locking. In such a case, the unshielded upper atmosphere will be affected by different ionization and non-thermal ion loss processes. This contributes to the estimated neutral hydrogen loss rates of about ≥ of the observed expanded exosphere of HD 209458b [CITE] and will be an ionized part of the estimated upper energy-limited neutral hydrogen loss rates of about [CITE].