| Résumé | The properties of 2M1207b, a young (similar to 8 Myr) planet-mass companion, have lacked a satisfactory explanation for some time. The combination of low luminosity, red near-IR colors, and L-type near-IR spectrum (previously consistent with T(eff) similar to 1600 K) implies an abnormally small radius. Early explanations for the apparent underluminosity of 2M1207b invoked an edge-on disk or the remnant of a recent protoplanetary collision. The discovery of a second planet-mass object (HR8799b) with similar luminosity and colors as 2M1207b indicates that a third explanation, one of a purely atmospheric nature, is more likely. By including clouds, non-equilibrium chemistry, and low gravity, an atmosphere with effective temperature consistent with evolution cooling-track predictions is revealed. Consequently, 2M1207b, and others like it, requires no new physics to explain nor do they belong to a new class of objects. Instead they most likely represent the natural extension of cloudy substellar atmospheres down to low T(eff) and log(g). If this atmosphere only explanation for 2M1207b is correct, then very young planet-mass objects with near-IR spectra similar to field T dwarfs may be rare. |
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