Fast-evolving weather for the coolest of our two new substellar neighbours

Abstract

We present the results of intense photometric monitoring in the near-infrared (~0.9 μm) with the TRAPPIST robotic telescope of the newly discovered binary brown dwarf WISE J104915.57-531906.1, the third closest system to the Sun at a distance of only 2 pc. Our twelve nights of time-series photometry reveal a quasi-periodic (P = 4.87 ± 0.01h) variability with a maximum peak-peak amplitude of ~11% and strong night-to-night evolution. We attribute this variability to the rotational modulation of fast-evolving weather patterns in the atmosphere of the coolest component (~T1-type) of the binary. No periodic signal is detected for the hottest component (~L8-type). For both brown dwarfs, our data allow us to firmly discard any unique transit during our observations for planets ≥2 R⊕. For orbital periods smaller than ~9.5 h, transiting planets are excluded down to an Earth-size.