55 Cancri: A Coplanar Planetary System that is Likely Misaligned with its Star
Résumé
Most mechanisms invoked to explain the high spin-orbit angles observed for some transiting planets require them to pass through a phase of extremely high orbital eccentricity. This would seem to preclude high spin-orbit angles in systems with multiple, coplanar planets on circular orbits, since these mechanisms would produce planet-planet scattering. Although the 55 Cnc system contains such well-ordered planets, we use numerical simulations to demonstrate that they too are likely to be highly inclined to their parent star's spin axis. Due to perturbations from its distant binary companion, this planetary system precesses like a rigid body about its parent star (without exciting the planets' eccentricities). Consequently, the parent star's spin axis and the planetary orbit normal likely diverged long ago. We predict that the most likely projected spin-orbit angle is 50 degrees, with a 30% chance of a retrograde configuration. Transit observations of the innermost planet - 55 Cnc e - may be used to test these predictions via the Rossiter-McLaughlin effect. 55 Cancri may thus represent a new class of planetary systems with well- ordered, coplanar orbits that are inclined with respect to the stellar equator. This work was funded by a CITA National Fellowship and Canada's NSERC. SNR thanks the CNRS's PNP program and the NASA Astrobiology Institute's Virtual Planetary Laboratory team.