In this paper we study the full sample of known Spitzer [3.6 µm] and J-band variable brown dwarfs. We calculate the rotational velocities, v sin i, of 16 variable brown dwarfs using archival Keck NIRSPEC data and compute the inclination angles of 19 variable brown dwarfs. The results obtained show that all objects in the sample with mid-IR variability detections are inclined at an angle > 20• , while all objects in the sample displaying J-band variability have an inclination angle > 35• . J-band variability appears to be more affected by inclination than Spitzer [3.6 µm] variability, and is strongly attenuated at lower inclinations. Since J-band observations probe deeper into the atmosphere than mid-IR observations, this effect may be due to the increased atmospheric path length of J-band flux at lower inclinations. We find a statistically significant correlation between the colour anomaly and inclination of our sample, where field objects viewed equator-on appear redder than objects viewed at lower inclinations. Considering the full sample of known variable L, T and Y spectral type objects in the literature, we find that the variability properties of the two bands display notably different trends, due to both intrinsic differences between bands and the sensitivity of ground-based versus spacebased searches. However, in both bands we find that variability amplitude may reach a maximum at ∼ 7 − 9 hr periods. Finally, we find a strong correlation between colour anomaly and variability amplitude for both the J-band and mid-IR variability detections, where redder objects display higher variability amplitudes.

We present simultaneous Hubble Space Telescope (HST) WFC3+Spitzer IRAC variability monitoring for the highly variable young (∼20 Myr) planetary-mass object PSO J318.5−22. Our simultaneous HST + Spitzer observations covered approximately two rotation periods with Spitzer and most of a rotation period with the HST. We derive a period of 8.6±0.1 hr from the Spitzer light curve. Combining this period with the measured v i sin for this object, we find an inclination of 56°.2±8°. 1. We measure peak-to-trough variability amplitudes of 3.4%±0.1% for Spitzer Channel 2 and 4.4%-5.8% (typical 68% confidence errors of ∼0.3%) in the near-IR bands (1.07-1.67 μm) covered by the WFC3 G141 prism-the mid-IR variability amplitude for PSO J318.5−22 is one of the highest variability amplitudes measured in the mid-IR for any brown dwarf or planetary-mass object. Additionally, we detect phase offsets ranging from 200°to 210°(typical error of ∼4°) between synthesized near-IR light curves and the Spitzer mid-IR light curve, likely indicating depth-dependent longitudinal atmospheric structure in this atmosphere. The detection of similar variability amplitudes in wide spectral bands relative to absorption features suggests that the driver of the variability may be inhomogeneous clouds (perhaps a patchy haze layer over thick clouds), as opposed to hot spots or compositional inhomogeneities at the top-of-atmosphere level.

As part of our ongoing NTT SoFI survey for variability in young free-floating planets and low mass brown dwarfs, we detect significant variability in the young, free-floating planetary mass object PSO J318.5-22, likely due to rotational modulation of inhomogeneous cloud cover. A member of the 23±3 Myr β Pic moving group, PSO J318.5-22 has T eff = 1160 +30 −40 K and a mass estimate of 8.3±0.5 M Jup for a 23±3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and β Pic b, the two known exoplanet companions in the β Pic moving group. With variability amplitudes from 7-10% in J S at two separate epochs over 3-5 hour observations, we constrain the rotational period of this object to >5 hours. In K S , we marginally detect a variability trend of up to 3% over a 3 hour observation. This is the first detection of weather on an extrasolar planetary mass object. Among L dwarfs surveyed at high-photometric precision (<3%) this is the highest amplitude variability detection. Given the low surface gravity of this object, the high amplitude preliminarily suggests that such objects may be more variable than their high mass counterparts, although observations of a larger sample is necessary to confirm this. Measuring similar variability for directly imaged planetary companions is possible with instruments such as SPHERE and GPI and will provide important constraints on formation.Measuring variability at multiple wavelengths can help constrain cloud structure.1 Based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme ID 095.C-0590