We present simultaneous HST WFC3 + Spitzer IRAC variability monitoring for the highly-variable young ( \sim 20 Myr ) planetary-mass object PSO J318.5-22 .
Our simultaneous HST + Spitzer observations covered \sim 2 rotation periods with Spitzer and most of a rotation period with HST .
We derive a period of 8.6 \pm 0.1 hours from the Spitzer lightcurve .
Combining this period with the measured vsini for this object , we find an inclination of 56.2 \pm 8.1 ^ { \circ } .
We measure peak-to-trough variability amplitudes of 3.4 \pm 0.1 \% for Spitzer Channel 2 and 4.4 - 5.8 \% ( typical 68 \% confidence errors of \sim 0.3 \% ) in the near-IR bands ( 1.07-1.67 \mu m ) covered by the WFC3 G141 prism – the mid-IR variability amplitude for PSO J318.5-22 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 200–210 ^ { \circ } ( typical error of \sim 4 ^ { \circ } ) between synthesized near-IR lightcurves and the Spitzer mid-IR lightcurve , 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 .