A New 24 μm Phase Curve for υ Andromedae b

We report the detection of 24 μm variations from the planet-hosting υ Andromedae system consistent with the orbital periodicity of the system's innermost planet, υ And b. We find a peak-to-valley phase curve amplitude of 0.00130 times the mean system flux. Using a simple model with two hemispheres of constant surface brightness and assuming a planetary radius of 1.3 R_J give a planetary temperature contrast of gsim900 K and an orbital inclination of gsim28°. We further report the largest phase offset yet observed for an extrasolar planet: the flux maximum occurs ~80° before phase 0.5. Such a large phase offset is difficult to reconcile with most current atmospheric circulation models. We improve on earlier observations of this system in several important ways: (1) observations of a flux calibrator star demonstrate the MIPS detector is stable to 10^-4 on long timescales, (2) we note that the background light varies systematically due to spacecraft operations, precluding use of this background as a flux calibrator (stellar flux measured above the background is not similarly affected), and (3) we calibrate for flux variability correlated with motion of the star on the MIPS detector. A reanalysis of our earlier observations of this system is consistent with our new result.