Multiepoch Stability in the Rotational Modulation of the Planetary-mass Companion Ross 458C

Abstract Spectroscopic variability is detected in brown dwarfs across all spectral types, suggesting atmospheric patchiness in brown dwarf atmospheres. We present JWST/NIRSpec time series observations of the T8.0 planetary-mass companion Ross 458C. We compared our derived spectral variability and light curve to those obtained using Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) observations more than 7.5 yr before and published by E. Manjavacas et al. We found that the light curve of Ross 458C is remarkably similar in the two epochs in terms of shape and variability amplitude potentially created by stable weather patterns. We measured a rotational period of 11.57 ± 0.09 hr, demonstrating that the previously reported HST/WFC3 period of 6.75 ± 1.58 hr is most likely a second-order harmonic produced by an unresolved double-peaked rotational modulation, likely due to the higher uncertainties of the HST/WFC3 light curve and the short time baseline of those observations. We measured the variability amplitude in the white light curve created using only the wavelength range covered by HST/WFC3 (1.10–1.63 μ m), obtaining a value of 1.47% ± 0.18%, which is consistent with that measured in the HST/WFC3 light curve.