Discovery of Rotational Modulations in the Planetary-Mass Companion 2M1207b: Intermediate Rotation Period and Heterogeneous Clouds in a Low Gravity Atmosphere

TL;DR

This study reports the first detection of rotational modulations in a planetary-mass companion, 2M1207b, revealing an intermediate rotation period and heterogeneous cloud structures in its low-gravity atmosphere through high-precision, time-resolved photometry.

Contribution

It presents the first measurement of a rotation period for a directly imaged exoplanetary-mass companion using space-based photometry.

Findings

Detected rotational modulations with a period of approximately 10.7 hours.

Found amplitude ratios in different bands similar to field brown dwarfs.

Provided the first direct measurement of rotation period for an exoplanetary-mass object.

Abstract

Rotational modulations of brown dwarfs have recently provided powerful constraints on the properties of ultra-cool atmospheres, including longitudinal and vertical cloud structures and cloud evolution. Furthermore, periodic light curves directly probe the rotational periods of ultra-cool objects. We present here, for the first time, time-resolved high-precision photometric measurements of a planetary-mass companion, 2M1207b. We observed the binary system with HST/WFC3 in two bands and with two spacecraft roll angles. Using point spread function-based photometry, we reach a nearly photon-noise limited accuracy for both the primary and the secondary. While the primary is consistent with a flat light curve, the secondary shows modulations that are clearly detected in the combined light curve as well as in different subsets of the data. The amplitudes are 1.36% in the F125W and 0.78% in the F160W filters, respectively. By fitting sine waves to the light curves, we find a consistent period of $10.7^{+1.2}_{-0.6}$ hours and similar phases in both bands. The J- and H-band amplitude ratio of 2M1207b is very similar to a field brown dwarf that has identical spectral type but different J-H color. Importantly, our study also measures, for the first time, the rotation period for a directly imaged extra-solar planetary-mass companion.