Measurements of photometric variability at different wavelengths provide insights into the vertical cloud structure of brown dwarfs and planetary-mass objects .
In seven Hubble Space Telescope consecutive orbits , spanning \sim 10 h of observing time , we obtained time-resolved spectroscopy of the planetary-mass T8-dwarf Ross 458 C using the near-infrared Wide Field Camera 3 .
We found spectrophotometric variability with a peak-to-peak signal of 2.62 \pm 0.02 % ( in the 1.10-1.60 \mu m white light curve ) .
Using three different methods , we estimated a rotational period of 6.75 \pm 1.58 h for the white light curve , and similar periods for narrow J - and H - band light curves .
Sine wave fits to the narrow J - and H -band light curves suggest a tentative phase shift between the light curves with wavelength when we allow different periods between both light curves .
If confirmed , this phase shift may be similar to the phase shift detected earlier for the T6.5 spectral type 2MASS J22282889–310262 .
We find that , in contrast with 2M2228 , the variability of Ross 458C shows evidence for a color trend within the narrow J -band , but gray variations in the narrow H -band .
The spectral time-resolved variability of Ross 458C might be potentially due to heterogeneous sulfide clouds in the atmosphere of the object .
Our discovery extends the study of spectral modulations of condensate clouds to the coolest T dwarfs , planetary-mass companions .