Earth is the Terran homeworld.
In 2491 CE, William "Buck" Rogers discovers that Earth has changed drastically due to a cataclysmic event known as World War III, which 25th century Terrans refer to as the Holocaust (BR25: Film, "Awakening").
History
During the 20th century, the nations of Earth—notably the United States of America—began their forays into space exploration. The last of which is that of the Ranger 3 mission, piloted by William "Buck" Rogers, in the year 1987 CE. Following the failure of his mission, Earth's warring powers succumb into a global conflict some months after Rogers' departure. Known as World War III, this results in a holocaust that gripped the Earth and nearly rendered the planet virtually inhabitable (BR25: Film, "Awakening", "Testimony of a Traitor").
Over the span of five centuries, Earth begins the long process of rebuilding (BR25: Film, "Awakening"). Sometime during this time, various groups of humans spread out across the galaxy, resulting in numerous "lost colonies" that Searcher is tasked with searching for and reestablishing ties to in 2492 CE (BR25: "Time of the Hawk") following establishment of peace between Earth and the Draconian Empire (BR25: "Flight of the War Witch, Part II").
In the 25th century, Earth was one of the planets that had not been yet subjugated under the Draconian Empire, despite Earth's reliance on foodstuffs from planets outside of Sol (BR25: Film, "Awakening", "Planet of the Slave Girls").