Why did William Herschel first think Uranus might be a comet?

When William Herschel first observed the object that would become known as Uranus on March 13, 1781, he identified it as a "curious either nebulous star or perhaps a comet" because it appeared as a distinct, visible disk that set it apart from the surrounding stars.12

At the time, discovering a new comet was not an unusual occurrence for astronomers. Herschel initially convinced himself that he had indeed found a comet. However, within months, other astronomers were able to track the object's movement and demonstrate that it was traveling in a nearly circular orbit around the Sun, which led to the conclusion that it was a planet rather than a comet.12