  Both ground and space-based transit observatories are poised to significantly
increase the number of known transiting planets and the number of precisely
measured transit times. The variation in a planet's transit times may be used
to infer the presence of additional planets. Deducing the masses and orbital
parameters of such planets from transit time variations (TTVs) alone is a rich
and increasingly relevant dynamical problem. In this work, we evaluate the
extent of the degeneracies in this process, systematically explore the
dependence of TTV signals on several parameters and provide phase space plots
that could aid observers in planning future observations. Our explorations are
focused on a likely-to-be prevalent situation: a known transiting short-period
Neptune or Jupiter-sized planet and a suspected external low-mass perturber on
a nearly-coplanar orbit. Through approximately 10^7 N-body simulations, we
demonstrate how TTV signal amplitudes may vary by orders of magnitude due to
slight variations in any one orbital parameter (0.001 AU in semimajor axis,
0.005 in eccentricity, or a few degrees in orbital angles), and quantify the
number of consecutive transit observations necessary in order to obtain a
reasonable opportunity to characterize the unseen planet (approximately greater
or equal to 50 observations). Planets in or near period commensurabilities of
the form p:q, where p < 21 and q < 4, produce distinct TTV signatures,
regardless of whether the planets are actually locked in a mean motion
resonance. We distinguish these systems from the secular systems in our
explorations. Additionally, we find that computing the autocorrelation function
of a TTV signal can provide a useful diagnostic for identifying possible orbits
for additional planets and suggest that this method could aid integration of
TTV signals in future studies of particular exosystems.
