We evaluate the orbital evolution and several plausible origins scenarios for
the mutually inclined orbits of Upsilon Andromedae 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, Upsilon And B, which could perturb the planetary
orbits, but we find it cannot modify two coplanar orbits into the observed
mutual inclination of ~30 deg. However, it could incite ejections or collisions
between planetary companions that subsequently raise the mutual inclination to
>30 deg. Our simulated systems with large mutual inclinations tend to be
further from the stability boundary than Upsilon And, but we are able to
produce similar systems. We conclude that scattering is a plausible mechanism
to explain the observed orbits of Upsilon And c and d, but we cannot determine
whether the scattering was caused by instabilities among the planets themselves
or by perturbations from Upsilon 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 Upsilon And, it
may be applied to any exoplanetary system.Comment: 19 pages, 5 figures, accepted to Astrophysical Journa
