  Since the discovery of short-period exoplanets a decade ago, photometric
surveys have been recognized as a feasible method to detect transiting hot
Jupiters. Many transit surveys are now under way, with instruments ranging from
10-cm cameras to the Hubble Space Telescope. However, the results of these
surveys have been much below the expected capacity, estimated in the dozens of
detections per year. One of the reasons is the presence of systematics (``red
noise'') in photometric time series. In general, yield predictions assume
uncorrelated noise (``white noise''). In this paper, we show that the effect of
red noise on the detection threshold and the expected yields cannot be
neglected in typical ground-based surveys. We develop a simple method to
determine the effect of red noise on photometric planetary transit detections.
This method can be applied to determine detection thresholds for transit
surveys. We show that the detection threshold in the presence of systematics
can be much higher than with the assumption of white noise, and obeys a
different dependence on magnitude, orbital period and the parameters of the
survey. Our method can also be used to estimate the significance level of a
planetary transit candidate (to select promising candidates for spectroscopic
follow-up). We apply our method to the OGLE planetary transit search, and show
that it provides a reliable description of the actual detectionthreshold with
real correlated noise. We point out in what way the presence of red noise could
be at least partly responsible for the dearth of transiting planet detections
from existing surveys, and examine some possible adaptations in survey planning
and strategy. Finally, we estimate the photometric stability necessary to the
detection of transiting ``hot Neptunes''.
