Observations of extrasolar planets were not projected to be a significant part of the Spitzer Space Telescopes 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 Spitzers 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. Spitzers 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. Spitzers 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. Spitzers 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.
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