With the discovery of Y dwarfs by the WISE mission , the population of field brown dwarfs now extends to objects with temperatures comparable to those of Solar System planets .
To investigate the atmospheres of these newly identified brown dwarfs , we have conducted a pilot study monitoring an initial sample of three late T-dwarfs ( T6.5 , T8 and T8.5 ) and one Y-dwarf ( Y0 ) for infrared photometric variability at multiple epochs .
With J -band imaging , each target was observed for a period of 1.0 h to 4.5 h per epoch , which covers a significant fraction of the expected rotational period .
These measurements represent the first photometric monitoring for these targets .
For three of the four targets ( 2M1047 , Ross 458C and WISE0458 ) , multi-epoch monitoring was performed , with the time span between epochs ranging from a few hours to \sim 2 years .
During the first epoch , the T8.5 target WISE0458 exhibited variations with a remarkable min-to-max amplitude of 13 per cent , while the second epoch light curve taken \sim 2 years later did not note any variability to a 3 per cent upper limit .
With an effective temperature of \sim 600 K , WISE0458 is the coldest variable brown dwarf published to-date , and combined with its high and variable amplitude makes it a fascinating target for detailed follow-up .
The three remaining targets showed no significant variations , with a photometric precision between 0.8 and 20.0 per cent , depending on the target brightness .
Combining the new results with previous multi-epoch observations of brown dwarfs with spectral types of T5 or later , the currently identified variables have locations on the colour-colour diagram better matched by theoretical models incorporating cloud opacities rather than cloud-free atmospheres .
This preliminary result requires further study to determine if there is a definitive link between variability among late-T dwarfs and their location on the colour-colour diagram .