  We present results from Spitzer Space Telescope observations of the
mid-infrared phase variations of three short-period extrasolar planetary
systems: HD 209458, HD 179949 and 51 Peg. We gathered IRAC images in multiple
wavebands at eight phases of each planet's orbit. We find the uncertainty in
relative photometry from one epoch to the next to be significantly larger than
the photon counting error at 3.6 micron and 4.5 micron. We are able to place
2-sigma upper limits of only 2% on the phase variations at these wavelengths.
At 8 micron the epoch-to-epoch systematic uncertainty is comparable to the
photon counting noise and we detect a phase function for HD 179949 which is in
phase with the planet's orbit and with a relative peak-to-trough amplitude of
0.00141(33). Assuming that HD 179949b has a radius R_J < R_p < 1.2R_J, it must
recirculate less than 21% of incident stellar energy to its night side at the
1-sigma level (where 50% signifies full recirculation). If the planet has a
small Bond albedo, it must have a mass less than 2.4 M_J (1-sigma). We do not
detect phase variations for the other two systems but we do place the following
2-sigma upper limits: 0.0007 for 51 Peg, and 0.0015 for HD 209458. Due to its
edge-on configuration, the upper limit for HD 209458 translates, with
appropriate assumptions about Bond albedo, into a lower limit on the
recirculation occuring in the planet's atmosphere. HD 209458b must recirculate
at least 32% of incident stellar energy to its night side, at the 1-sigma
level, which is consistent with other constraints on recirculation from the
depth of secondary eclipse depth at 8 micron and the low optical albedo. These
data indicate that different Hot Jupiter planets may experience different
recirculation efficiencies.
