  Observations of extrasolar planets were not projected to be a significant
part of the Spitzer Space Telescope's mission when it was conceived and
designed. Nevertheless, Spitzer was the first facility to detect thermal
emission from a hot Jupiter, and the range of Spitzer's exoplanetary
investigations grew to encompass transiting planets, microlensing, brown
dwarfs, and direct imaging searches and astrometry. Spitzer used phase curves
to measure the longitudinal distribution of heat as well as time-dependent
heating on hot Jupiters. Spitzer's secondary eclipse observations strongly
constrained the dayside thermal emission spectra and corresponding atmospheric
compositions of hot Jupiters, and the timings of eclipses were used for studies
of orbital dynamics. Spitzer's sensitivity to carbon-based molecules such as
methane and carbon monoxide was key to atmospheric composition studies of
transiting exoplanets as well as imaging spectroscopy of brown dwarfs, and
complemented Hubble spectroscopy at shorter wavelengths. Spitzer's capability
for long continuous observing sequences enabled searches for new transiting
planets around cool stars, and helped to define the architectures of planetary
systems like TRAPPIST-1. Spitzer measured masses for small planets at large
orbital distances using microlensing parallax. Spitzer observations of brown
dwarfs probed their temperatures, masses, and weather patterns. Imaging and
astrometry from Spitzer was used to discover new planetary mass brown dwarfs
and to measure distances and space densities of many others.
