Kepler-9 is a Sun-like G-type star (about 1.07 solar masses, 5,777 K, 1.02 solar radii), 628 light-years from Earth. Kepler-9 b and c were the first planets confirmed through transit timing variations, the gravitational drift of their transit clocks. Kepler-9 d is a planet with 1.6 Earth radii, with an orbital period of 1.6 days, at 0.027 AU from its star, and an equilibrium temperature near 2,026 K, discovered in 2010 via transit. Kepler-9 b is a 0.14-Jupiter-mass companion, with an orbital period of 19.2 days, at 0.143 AU from its star, on an eccentric orbit (e ≈ 0.06), discovered in 2010 via transit. Mutual gravitational tugs between the planets can slowly reshape their orbits, preserving clues to the system's assembly.