We have monitored photometrically the Y0 brown dwarf WISEP J173835.52 + 273258.9 ( W1738 ) at both near- and mid-infrared wavelengths .
This \lesssim 1 Gyr-old 400 K dwarf is at a distance of 8 pc and has a mass around 5 M _ { Jupiter } .
We observed W1738 using two near-infrared filters at \lambda \approx 1 \mu m , Y and J , on Gemini observatory , and two mid-infrared filters at \lambda \approx 4 \mu m , [ 3.6 ] and [ 4.5 ] , on the Spitzer observatory .
Twenty-four hours were spent on the source by Spitzer on each of June 30 and October 30 2013 UT .
Between these observations , around 5 hours were spent on the source by Gemini on each of July 17 and August 23 2013 UT .
The mid-infrared light curves show significant evolution between the two observations separated by four months .
We find that a double sinusoid can be fit to the [ 4.5 ] data , where one sinusoid has a period of 6.0 \pm 0.1 hours and the other a period of 3.0 \pm 0.1 hours .
The near-infrared observations suggest variability with a \sim 3.0 hour period , although only at a \lesssim 2 \sigma confidence level .
We interpret our results as showing that the Y dwarf has a 6.0 \pm 0.1 hour rotation period , with one or more large-scale surface features being the source of variability .
The peak-to-peak amplitude of the light curve at [ 4.5 ] is 3 % .
The amplitude of the near-infrared variability , if real , may be as high as 5 to 30 % .
Intriguingly , this size of variability and the wavelength dependence can be reproduced by atmospheric models that include patchy KCl and Na _ { 2 } S clouds and associated small changes in surface temperature .
The small number of large features , and the timescale for evolution of the features , is very similar to what is seen in the atmospheres of the solar system gas giants .