  We consider the issue of excessive TTV noise observed for the exoplanet HD
189733 b. Trying to explain it through the host star photospheric activity, we
model the stellar surface brightness as a random field, then characterize
statistical properties of the resulting transit signal perturbation and compute
individual corrections to transit timings uncertainties. We find that possible
effect of the photospheric brightness field can explain only a minor portion
($\sim 10$ s) of the observed ($\sim 70$ s) TTV excess of HD 189733, suggesting
that the rest should be attributed to other sources. Regarding the photospheric
pattern, we place an upper limit of $\sim 0.01$ on the combination
$\varepsilon_{\rm cell} r_{\rm cell}$, where $\varepsilon_{\rm cell}$ is the
relative magnitude of brightness variations, and $r_{\rm cell}$ is the
geometric cellularity scale (relative to star radius).
