  In the coming decades, research in extrasolar planets aims to advance two
goals: 1) detecting and characterizing low-mass planets increasingly similar to
the Earth, and 2) improving our understanding of planet formation. We present a
new planet detection method that is capable of making large advances towards
both of these objectives and describe a modest network of telescopes that is
able to make the requisite observations. In a system where a known planet
transits its host star, a second planet in that system will cause the time
between transits to vary. These transit timing variations can be used to infer
the orbital elements and mass of the perturbing planet even if it has a mass
that is smaller than the mass of the Earth. This detection technique
complements other techniques because it is most sensitive in mean-motion
resonances where, due to degeneracies, other techniques have reduced
sensitivity. Small ground-based observatories have already exceeded the
photometric precision necessary to detect sub-Earth mass planets. However, TTV
planet searches are currently limited by the relatively small number of
high-precision transit data and insufficient observing time on existing
telescopes. These issues will be compounded as the number of known transiting
planets suitable for TTV study will increase substantially in the near future.
A relatively modest investment in a ground-based network of small ($\sim 0.5
{\rm m}$ telescopes could provide the needed coverage and so dramatically
increase the effectiveness of transit timing observations.
