Pluto's demotion from the list of solar system planets grabbed front-page headlines in 2006. But the debate over the qualifications for planethood reaches back to the early 1800s when astronomers spotted an object called Ceres.
Guiseppe Piazzi at the Palermo Observatory spied Ceres in 1801 in a gap between Mars and Jupiter where a planet was expected to reside, based on the spacing of the known planets in the solar system. Known as the Titius-Bode Law, this prediction was named for the astronomers who had noticed in the 1760s and 1770s that the relative distances of the six known planets from the Sun fit a mathematical relationship. Although it is called a "law," it has no basis in physics.
According to Titius-Bode, a planet should exist between Mars and Jupiter. Astronomers had been hunting for this phantom planet since Uranus was discovered in 1781. Uranus, too, was at just the distance where the law predicted a planet should be.
Nestled between Mars and Jupiter, Ceres was in just the right spot, too. So astronomers called it a planet. Piazzi initially named the new planet Ceres Ferdinandea after the Roman goddess of the harvest and King Ferdinand IV of Naples and Sicily. Ferdinandea, however, was later dropped.
Ceres's discovery was just the beginning of a landslide of small bodies spotted between Mars and Jupiter. A year after Ceres was discovered, astronomers found another body between the two planets that was almost as bright as Ceres. Heinrich Olbers unexpectedly found the second body, which he called Pallas after Pallas Athena, an alternate name for the goddess Athena.
Many astronomers realized that neither Ceres nor Pallas fit the conventional idea of a planet because their disks were so small they could not be resolved through telescopes. Because of their star-like appearance, Sir William Herschel coined the term "asteroid" for such bodies, writing in 1802: "They resemble small stars so much as hardly to be distinguished from them, even by very good telescopes." Herschel, therefore, argued that Ceres and Pallas were different from other planets.
Most other astronomers, however, disagreed. These two new additions to the solar system were listed with the rest of the planets.
The gold rush of small bodies continued to pile up. Astronomers nabbed Juno in 1804 and Vesta in 1807. The growing list of small objects raised concern that the asteroids were debris from a planet that had somehow disintegrated. Nevertheless, Juno and Vesta joined Ceres and Pallas as planets.
By the 1820s, astronomers counted 11 planets in the solar system. Introductory astronomy texts of that time listed the planets as Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus.
Astronomers found another body, Astraea, near the end of 1845, almost 39 years after Vesta was spied. Three new objects were spotted in 1847. By the end of 1851, there were 15 known bodies between Mars and Jupiter.
Finally, astronomers realized that this large number of similar bodies all in orbit between Mars and Jupiter represented a new class of solar-system object. They called them asteroids, the name Herschel had coined 50 years earlier. Instead of listing them by distance from the Sun, as they did the planets, astronomers categorized them by their order of discovery. Astronomers today list about 100,000 known asteroids as large as 6 miles (10 kilometers) across located between Mars and Jupiter, a region now called the asteroid belt.
So Pluto's dismissal from the planetary ranks is not unique. Ceres, Vesta, and the other asteroids found in the 1800s, suffered the same indignity. In fact, their stories are similar. Astronomers began to question the planethood of Ceres, Vesta, and the other asteroids as they spotted more objects in the same region. Likewise, Pluto's planetary pedigree was put to the test when astronomers began finding other icy rocks in the planet's neighborhood, a region now called the Kuiper belt. In fact, one of the objects discovered, Eris, is even larger than Pluto. Ceres, Vesta, and Pluto also are among the most massive bodies in their respective regions, the asteroid belt and the Kuiper belt.
In 2006, the International Astronomical Union (IAU), an astronomers' professional society, attempted to settle the ongoing debate by adopting a new planet definition. According to the IAU, a planet is an object that orbits a star, is large enough to have settled into a round shape, and dominates its orbit. The newly adopted definition was bad news for Pluto but good news for Ceres. Pluto was demoted to a new category called dwarf planets, and Ceres was promoted to the same category. Dwarf planets share some, but not all, of a planet's characteristics. Both bodies are round like planets but do not clear out their orbits of debris.
The long debate on the definition of a planet is a textbook example of how scientific concepts are not etched in stone but continue to evolve with new discoveries.
The Dawn spacecraft traveling to Vesta and Ceres and the New Horizons satellite heading to Pluto may help clarify the debate by delivering close-up views of these objects.