  Long-term orbital evolution of multi-planet systems under tidal dissipation
often converges to a stationary state, known as the tidal fixed point. The
fixed point is characterized by a lack of oscillations in the eccentricities
and apsidal alignment among the orbits. Quantitatively, the nature of the fixed
point is dictated by mutual interactions among the planets as well as
non-Keplerian effects. We show that if a roughly coplanar system hosts a hot,
sub-Saturn mass planet, and is tidally relaxed, separation of planet-planet
interactions and non-Keplerian effects in the equations of motion leads to a
direct determination of the true masses of the planets. Consequently, a
"snap-shot" observational determination of the orbital state resolves the
sin(I) degeneracy, and opens up a direct avenue towards identification of the
true lowest-mass exo-planets detected. We present an approximate, as well as a
general, mathematical framework for computation of the line of sight
inclination of secular systems, and apply our models illustratively to the 61
Vir system. We conclude by discussing the observability of planetary systems to
which our method is applicable and we set our analysis into a broader context
by presenting a current summary of the various possibilities for determining
the physical properties of planets from observations of their orbital states.
