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

Abstract

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 ≥10 10 g/s of the observed expanded exosphere of HD 209458b (Vidal-Madjar et al. 2003) and will be an ionized part of the estimated upper energy-limited neutral hydrogen loss rates of about 10 12 g/s (Lammer et al. 2003a).

Why an intrinsic magnetic field does not protect a planet against atmospheric escape

“…For a magnetised planet, r c is found by tracing a magnetic field line from the subsolar MP towards the planet. Assuming pressure balance between the solar wind dynamic pressure and the magnetic pressure of the magnetosphere, the standoff distance of the MP can be estimated by Voigt (1981), Grießmeier et al (2004) …”

Variability of solar/stellar activity and magnetic field and its influence on planetary atmosphere evolution

“…These parameters for a Sun-like star can be estimated as a function of the age from the observations discussed above, together with an empirical model for an age-dependence of the solar/stellar wind velocity (Newkirk Jr., 1980;Grießmeier et al, 2004Grießmeier et al, , 2007Lichtenegger et al, 2010) …”

Determining the mass loss limit for close-in exoplanets: what can we learn from transit observations?

“…Taking into account that tidal-locking of short periodic exoplanets may result in weaker intrinsic magnetic moments, as compared to fast rotating Jupiter-class exoplanets at larger orbital distances (Grießmeier et al 2004), Khodachenko et al (2007) found that the encountering of dense stellar wind and CME plasma may compress the magnetosphere and force the magnetospheric standoff location to move down to the heights where ionization and ion pick-up of a planetary neutral atmosphere by the CME plasma flow takes place. In our Solar System, it is known that magnetosphere-like structures are found on all planets regardless of whether the planet has an intrinsic global magnetic field or not.…”

“…This non-thermal mass loss process depends Article published by EDP Sciences on the strengths of the intrinsic magnetic moments. Grießmeier et al (2004) estimated for slow rotating, tidally locked "Hot Jupiters" that their magnetic moments should be in the range between 0.005-0.1 M Jup . From this study one can expect that weakly magnetized short periodic gas giants, which experience a Venus-like stellar plasma-atmosphere interaction, should exist.…”