55 Cancri: A Coplanar Planetary System that is Likely Misaligned with its Star

TL;DR

This study suggests that the 55 Cancri planetary system, despite being coplanar, is likely highly inclined relative to its star's spin axis due to gravitational perturbations, challenging assumptions about planetary alignment.

Contribution

The paper demonstrates through simulations that 55 Cancri's planets are likely misaligned with their star, influenced by a distant binary companion, a novel insight into planetary system orientations.

Findings

Most probable spin-orbit angle is ~50 degrees.

Approximately 30% chance of a retrograde configuration.

Potential verification via Rossiter-McLaughlin effect on planet 55 Cnc e.

Abstract

Although the 55 Cnc system contains multiple, closely packed planets that are presumably in a coplanar configuration, we use numerical simulations to demonstrate that they 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. Consequently, the parent star's spin axis and the planetary orbit normal likely diverged long ago. Because only the projected separation of the binary is known, we study this effect statistically, assuming an isotropic distribution for wide binary orbits. We find 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 verify these findings 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.