  Using new data from the K2 mission, we show that WASP-47, a previously known
hot Jupiter host, also hosts two additional transiting planets: a Neptune-sized
outer planet and a super-Earth inner companion. We measure planetary properties
from the K2 light curve and detect transit timing variations, confirming the
planetary nature of the outer planet. We performed a large number of numerical
simulations to study the dynamical stability of the system and to find the
theoretically expected transit timing variations (TTVs). The theoretically
predicted TTVs are in good agreement with those observed, and we use the TTVs
to determine the masses of two planets, and place a limit on the third. The
WASP-47 planetary system is important because companion planets can both be
inferred by TTVs and are also detected directly through transit observations.
The depth of the hot Jupiter's transits make ground-based TTV measurements
possible, and the brightness of the host star makes it amenable for precise
radial velocity measurements. The system serves as a Rosetta Stone for
understanding TTVs as a planet detection technique.

Earth and Planetary Astrophysics