  We report strong cosmological constraints on the Brans-Dicke (BD) theory of
gravity using Cosmic Microwave Background data from Planck.We consider two
types of models. First, the initial condition of the scalar field is fixed to
give the same effective gravitational strength $G_{eff}$ today as the one
measured on the Earth, $G_N$. In this case the BD parameter $\omega$ is
constrained to $\omega > 692$ at the $99\%$ confidence level, an order of
magnitude improvement over previous constraints.In the second type the initial
condition for the scalar is a free parameter leading to a somewhat stronger
constraint of $\omega > 890$ while $G_{eff}$ is constrained to $0.981
<\frac{G_{eff}}{G_N} <1.285$ at the same confidence level. We argue that these
constraints have greater validity than for the BD theory and are valid for any
Horndeski theory, the most general second-order scalar-tensor theory, which
approximates BD on cosmological scales. In this sense, our constraints place
strong limits on possible modifications of gravity that might explain cosmic
acceleration.
