How did astronomers determine that William Herschel’s new object was a planet rather than a comet?

When William Herschel first observed the object on March 13, 1781, he initially identified it as a "nebulous star" or a comet. However, astronomers were able to distinguish it as a planet rather than a comet through several key observations:12

• Physical Appearance: Unlike a comet, which typically displays a tail, the object was distinguished by a clearly visible disk when viewed through a telescope.2
• Orbital Path: Other astronomers analyzed the object's movement and demonstrated that it was traveling in a nearly circular orbit around the Sun, rather than the highly elliptical orbits characteristic of comets.12
• Motion: The object's slow motion over the course of several months further supported the conclusion that it was a planet orbiting well beyond Saturn.2

These findings confirmed the discovery of Uranus, the first planet to be discovered that had not been recognized in prehistoric times.34