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As part of our ongoing NTT SoFI survey for variability in young free-floatingplanets and low mass brown dwarfs, we detect significant variability in theyoung, free-floating planetary mass object PSO J318.5-22, likely due torotational modulation of inhomogeneous cloud cover. A member of the 23$\pm$3Myr $\beta$ 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 a23$\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 movinggroup. With variability amplitudes from 7-10$\%$ in J$_{S}$ at two separateepochs over 3-5 hour observations, we constrain the rotational period of thisobject to $>$5 hours. In K$_{S}$, we marginally detect a variability trend ofup to 3$\%$ over a 3 hour observation. This is the first detection of weatheron an extrasolar planetary mass object. Among L dwarfs surveyed athigh-photometric precision ($<$3$\%$) this is the highest amplitude variabilitydetection. Given the low surface gravity of this object, the high amplitudepreliminarily suggests that such objects may be more variable than their highmass counterparts, although observations of a larger sample is necessary toconfirm this. Measuring similar variability for directly imaged planetarycompanions is possible with instruments such as SPHERE and GPI and will provideimportant constraints on formation. Measuring variability at multiplewavelengths can help constrain cloud structure.

Variability in a Young, L/T Transition Planetary-Mass Object