  Kepler-9, discovered by Holman et al. 2010, was the first system with
multiple confirmed transiting planets and the first system to clearly show
long-anticipated transit timing variations (TTVs). We review the historical
circumstances behind the discovery and characterization of these planets and
the publication of Holman et al. 2010. It was the first major novel exoplanet
discovery of the Kepler Space Telescope mission. The Kepler pipeline identified
two Saturn-radius candidates (called Kepler Objects of Interest or KOIs):
KOI-377.01 with a 19-day period and KOI-377.02 with a 39-day period. Even with
only 9 transits for KOI-377.01 and 6 of KOI-377.02, the transit times were
completely inconsistent with a linear ephemeris and showed strongly
anti-correlated variations in transit times. Holman et al. 2010 were able to
readily show that these objects were planetary mass, confirming them as bona
fide planets Kepler-9b and Kepler-9c. As a multi-transiting system exhibiting
strong TTVs, the relative planetary properties (e.g., mass ratio, radius ratio)
were strongly constrained, opening a new chapter in comparative planetology.
KOI-377.03, a small planet with a 1.5-day period, was not initially discovered
by the Kepler pipeline, but was identified during the analysis of the other
planets and was later confirmed as Kepler-9d through the BLENDER technique by
Torres et al. 2011. Holman et al. 2010 included significant dynamical analysis
to characterize Kepler-9's particular TTVs: planets near resonance show large
amplitude anti-correlated TTVs with a period corresponding to the rotation of
the line of conjunctions and an additional "chopping" signal due to the
changing positions of the planets. We also review the updated properties of
this system and propose ideas for future investigations.
