Observations of extrasolar planets were not projected to be a substantial 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-sized planet, and the range of its 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. Its 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 Space Telescope spectroscopy at shorter wavelengths. Its capability for long continuous observing sequences enabled searches for new transiting planets around cool stars and helped to define the architectures of planetary systems such as 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.

在构思和设计斯皮策太空望远镜时，太阳系外行星的观测并不会成为其主要任务。尽管如此，斯皮策还是第一个探测热木星大小行星的热辐射的设施，其系外行星研究的范围不断扩大，涵盖了过境行星，微透镜，褐矮星以及直接成像搜索和天文测量。Spitzer使用相位曲线来测量热木星的热量的纵向分布以及与时间有关的热量。它的次日食观测强烈地限制了日热辐射光谱和热木星的相应大气成分，日食的时间被用于研究轨道动力学。Spitzer对诸如甲烷和一氧化碳之类的碳基分子的敏感性是研究过渡系外行星的大气成分以及棕矮星成像光谱学的关键，并补充了在较短波长下的哈勃太空望远镜光谱学。它具有长时间连续观测序列的能力，可以搜索围绕冷恒星的新的过时行星，并有助于定义TRAPPIST-1等行星系统的体系结构。Spitzer使用微透镜视差在大轨道距离上测量了小行星的质量。Spitzer观测到的棕色小矮星探测了它们的温度，质量和天气模式。Spitzer的影像和天文测量技术被用于发现新的行星状棕矮星，并测量许多其他行星的距离和空间密度。