Direct Detection of CO in CI Tau b: Support for Hot Start Formation

We analyze high resolution IR spectra of CI Tau, the host star of one of the few young planet candidates amenable to such observations. We confirm the planet's existence with a direct detection of CO in the planet's atmosphere. We determine a mass of 11.6 Mjup based on the amplitude of the planet's radial velocity variations. We estimate the planet's flux contrast with its host star to obtain an absolute magnitude estimate for the planet of 8.17 in the K band. This magnitude implies the planet formed via a "hot start" formation mechanism. This makes CI Tau b the youngest confirmed exoplanet as well as the first exoplanet around a T Tauri star with a directly determined, model-independent, dynamical mass.