  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$\pm$3
Myr $\beta$ Pic moving group, PSO J318.5-22 has T$_\mathrm{eff}$ =
1160$^{+30}_{-40}$ K and a mass estimate of 8.3$\pm$0.5 M$_{Jup}$ for a
23$\pm$3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and
$\beta$ Pic b, the two known exoplanet companions in the $\beta$ 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.
