The origin of the spin-axis orientations (obliquities) of the giant planets is a fundamental issue because if the obliquities resulted from tangential collisions with primordial Earth-sized protoplanets, then they are related to the masses of the largest planetesimals out of which the planets form^{1, 2, 3, 4, 5, 6, 7}. A problem with this mechanism, however, is that the orbital planes of regular satellites would probably be uncorrelated with the obliquities, contrary to observations^{6, 7}. Alternatively, they could have come from an external twist that affected the orientation of the Solar System plane²; but in this model, the outer planets must have formed too rapidly, before the event that produced the twist⁸. Moreover, the model cannot be quantitatively tested. Here I show that the present obliquities of the giant planets were probably achieved when Jupiter and Saturn crossed the 1:2 orbital resonance⁹ during a specific migration process: different migration scenarios cannot account for the large observed obliquities. The existence of the regular satellites of the giant planets does not represent a problem in this model because, although they formed soon after the planetary formation, they can follow the slow evolution of the equatorial plane it produces.

1. Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n (1900) La Plata, Argentina
2. Instituto Astrofísico de La Plata, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Paseo del Bosque s/n (1900) La Plata, Argentina

Correspondence to: Adrián Brunini^1,2 Correspondence and requests for materials should be addressed to the author (Email: abrunini@fcaglp.unlp.edu.ar).

Received 7 October 2005 | Accepted 2 January 2006

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