A search of the Hipparcos satellite photometry data for the star HD 209458
reveals evidence for a planetary transit signature consistent with the plan
etary properties reported by Henry et al. and Charbonneau et al. and allows
further refinement of the planet's orbital period. The long time baseline
(about 2926 days of 830 periods) from the best Hipparcos transit-like event
to the latest transit reported by Henry et al, for the night of 1999 Novem
ber 15 (UT) allows for an orbital period determination of 3.524736 days wit
h an uncertainty of 0.000045 days (3.9 s). The transit events observed by C
harbonneau et al. fall at the interim times expected to within the errors o
f this newly derived period. A series of statistical tests was performed to
assess the likelihood of these events occurring by chance. This was crucia
l given the ill-conditioned problem presented by the sparse sampling of the
light curve and the non-Gaussian distribution of the points. Monte Carlo s
imulations using bootstrap methods with the actual Hipparcos HD 209358 data
set indicate that the transit-like signals of the depth observed would onl
y be produced by chance in 21 out of 1 million trials. The transit duration
s and depths obtained from the Hipparcos data are also consistent with thos
e determined by Charbonneau et al. and Henry et al. within the limitations
of the sampling intervals and photometric precision of the Hipparcos data.