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Current observations indicate that the planet formation process oftenproduces multiple planet systems with nearly circular orbits, regular spacing,a narrow range of inclination angles, and similar planetary masses of order$m_{\rm p}\sim10M_\oplus$. Motivated by the observational sample, this paperdetermines the tidal equilibrium states for this class of extrasolar planetarysystems. We start by considering two planet systems with fixed orbital spacingand variable mass ratios. The basic conjecture explored in this paper is thatthe planet formation process will act to distribute planetary masses in orderto achieve a minimum energy state. The resulting minimum energy configuration--- subject to the constraint of constant angular momentum --- corresponds tocircular orbits confined to a plane, with nearly equal planetary masses (asobserved). We then generalize the treatment to include multiple planet systems,where each adjacent pair of planets attains its (local) tidal equilibriumstate. The properties of observed planetary systems are close to those expectedfrom this pairwise equilibrium configuration. In contrast, observed systems donot reside in a global minimum energy state. Both the equilibrium states ofthis paper and observed multi-planet systems, with planets of nearly equal masson regularly spaced orbits, have an effective surface density of the form$\sigma\propto r^{-2}$, much steeper than most disk models.

Pairwise Tidal Equilibrium States and the Architecture of Extrasolar Planetary Systems