Herschel was convinced that not only the Moon but also the planets and their satellites are inhabited. For example, in a paper on Mars, he repeatedly mentioned its inhabitants, and in another paper he casually referred to “the inhabitants of the satellites of Jupiter, Saturn, and the Georgian planet [Uranus].”34 Moreover, when one finds that after the discovery of the asteroids Ceres and Pallas, Herschel reported his observations that they “have an atmosphere of considerable extent,”35 one suspects that his passion for extraterrestrials may have influenced this spurious observation.

Herschel believed, at least during the 1780s, that stars are suns surrounded by inhabited planets. For example, in a 1789 paper, he stated that stars are suns and commented that each sun “is probably of as much consequence to a system of planets, satellites, and comets, as our own sun….”36

Herschel was very interested in variable stars. In a 1779 manuscript, for example, Herschel attempted to explain the variability of the star Mira Ceti by associating a giant planet with it, the planet eclipsing the star. Moreover, in a 1783 publication, Herschel justified his observations of the variable star Algol by claiming that such observations could serve to verify the existence of “a plurality of solar and planetary systems.”37 Although admitting in a 1791 paper that such extra-Solar System planets “can never be perceived by us,”38 he nonetheless stated in 1795 that:

[S]ince stars appear to be suns, and suns, according to the common opinion, are bodies that serve to enlighten, warm, and sustain a system of planets, we may have an idea of numberless globes that serve for the habitation of living creatures.39