2011ApJ...726...71B

We evaluate the orbital evolution and several plausible origin scenarios for the mutually inclined orbits of υ And c and d. These two planets have orbital elements that oscillate with large amplitudes and lie close to the stability boundary. This configuration, and in particular the observed mutual inclination, demands an explanation. The planetary system may be influenced by a nearby low-mass star, υ And B, which could perturb the planetary orbits, but we find it cannot modify two coplanar orbits into the observed mutual inclination of 30°. However, it could incite ejections or collisions between planetary companions that subsequently raise the mutual inclination to >30°. Our simulated systems with large mutual inclinations tend to be further from the stability boundary than υ And, but we are able to produce similar systems. We conclude that scattering is a plausible mechanism to explain the observed orbits of υ And c and d, but we cannot determine whether the scattering was caused by instabilities among the planets themselves or by perturbations from υ And B. We also develop a procedure to quantitatively compare numerous properties of the observed system to our numerical models. Although we only implement this procedure to υ And, it may be applied to any exoplanetary system.