On our journey to learn more about the dwarf planets, the first one we come across is Ceres.
Before Ceres was discovered, astronomers were curious about the apparent gap seen between Mars and Jupiter. At the time, there was a hypothesis known as the Titus-Bode law, which basically noticed a pattern seen in the semi-major axes of the then-known seven planets. The hypothesis had recently gotten support because William Herschel discovered Uranus in 1781 near the predicted distance past Saturn. The hypothesis also predicted the existence of a planet between Mars and Jupiter located around 2.8 AU from the sun.
In 1801, Giuseppe Piazzi found Ceres more or less on accident. When he published his findings, he said Ceres looked like a comet but the object could be something more significant. Shortly after Ceres was confirmed, it was designated as a planet, along with other objects such as 2 Pallas, 3 Juno, and 4 Vesta; they held on to the planetary designation for about half a century, even as more and more objects were found orbiting in this same region. Ceres and it’s planetary palls were eventually reclassified as asteroids (just think, people feel like Pluto feels cheated – I’d hate to be in the Ceres camp).
As an interesting side note, in 1802, several astronomers were realizing that Ceres might be the first in a new class of objects. Herschel coined the term “asteroid” meaning “star-like” because he claimed “they resemble small stars so much as hardly to be distinguished from them, even by very good telescopes.” Because Ceres is the first such object to have been discovered, it was designated as 1 Ceres according to the modern system of asteroid numbering.
In the 2006 debate that eventually lead to a definition of ‘planet’, Ceres was back on the table. Ceres fulfills the first two definitions of planet since it’s circular and orbits the sun, but falls dismally short in the category of clearing its orbit – since it shares its orbit with thousands of asteroids. Thus, Ceres was reclassified again, this time, as a dwarf planet.