The Young Planet-mass Object 2M1207b: a cool, cloudy, and methane-poor atmosphere

TL;DR

This paper explains the properties of the young planet-mass object 2M1207b through atmospheric models including clouds and chemistry, showing it fits within known substellar atmospheres without requiring new physics.

Contribution

It demonstrates that 2M1207b's observed features can be explained by atmospheric effects like clouds and chemistry, negating the need for exotic explanations or new physics.

Findings

Atmospheric models with clouds and non-equilibrium chemistry match observations.

2M1207b's properties are consistent with known substellar atmospheres.

Young planet-mass objects with similar spectra may be rare.

Abstract

The properties of 2M1207b, a young (~ 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 Teff ~ 1600K) 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 Teff 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.