  In a transiting planetary system, the presence of a second planet will cause
the time interval between transits to vary. These transit timing variations
(TTV) are particularly large near mean-motion resonances and can be used to
infer the orbital elements of planets with masses that are too small to detect
by any other means. I present the results of a study of simulated data where I
show the potential that this planet detection technique has to detect and
characterize secondary planets in transiting systems. These results have
important ramifications for planetary transit searches since each transiting
system presents an opportunity for additional discoveries through a TTV
analysis. I present such an analysis for 13 transits of the HD 209458 system
that were observed with the Hubble Space Telescope. This analysis indicates
that a putative companion in a low-order, mean-motion resonance can be no
larger than the mass of the Earth and constitutes, to date, the most sensitive
probe for extrasolar planets that orbit main sequence stars. The presence or
absence of small planets in low-order, mean-motion resonances has implications
for theories of the formation and evolution of planetary systems. Since TTV is
most sensitive in these regimes, it should prove a valuable tool not only for
the detection of additional planets in transiting systems, but also as a way to
determine the dominant mechanisms of planet formation and the evolution of
planetary systems.
