A non-satellite body fulfilling only the first two of these criteria is classified as a dwarf planet. According to the IAU, planets and dwarf planets are two classes of objects. A non-satellite body fulfilling only the first criterion is termed a small Solar System body, an alternate proposal included dwarf planets as a subcategory of planets, but IAU members voted against this proposal. The definition was a one, and has drawn both support and criticism from different astronomers, but has remained in use. According to this definition, there are eight planets in the Solar System. The definition distinguishes planets from smaller bodies and is not useful outside the Solar System, extrasolar planets, or exoplanets, are covered separately under a complementary 2003 draft guideline for the definition of planets, which distinguishes them from dwarf stars, which are larger. Before the discoveries of the early 21st century, astronomers had no real need for a definition for planets. With the discovery of Pluto in 1930, astronomers considered the Solar System to have nine planets, along with thousands of bodies such as asteroids. Pluto was thought to be larger than Mercury, in 1978, the discovery of Plutos moon Charon radically changed this picture. By measuring Charons orbital period, astronomers could accurately calculate Plutos mass for the first time, in the 1990s, astronomers began finding other objects at least as far away as Pluto, now known as Kuiper Belt objects, or KBOs. Many of these some of Plutos key orbital characteristics and are now called plutinos. Pluto came to be seen as the largest member of a new class of objects, Plutos eccentric and inclined orbit, while very unusual for a planet in the Solar System, fits in well with the other KBOs. New York Citys newly renovated Hayden Planetarium did not include Pluto in its exhibit of the planets when it reopened as the Rose Center for Earth, astronomers also knew that more objects as large as Pluto would be discovered, and the number of planets would start growing quickly. They were also concerned about the classification of planets in other planetary systems, in 2006, the matter came to a head with the first measurement of the size of 2003 UB313. That measurement had showed Eris to appear to be larger than Pluto. The process of new discoveries spurring a contentious refinement of Plutos categorization echoed a debate in the 19th century that began with the discovery of Ceres on January 1,1801, astronomers immediately declared the tiny object to be the missing planet between Mars and Jupiter. Within four years, however, the discovery of two objects with comparable sizes and orbits had cast doubt on this new thinking. By 1851, the number of planets had grown to 23, astronomers began cataloguing them separately and began calling them asteroids instead of planets
The original proposal would have immediately added three planets, shown here in a size comparison to Earth. Leftmost is Pluto (shown in lieu of Eris, which is about the same size), then Charon, Ceres, and Earth
The twelve "candidate planets" that were possibilities for inclusion under the originally proposed definition. Note that all but the last three are trans-Neptunian objects. The smallest three (Vesta, Pallas, Hygeia) are in the asteroid belt.
Satirical protest demonstration against the "demotion" of Pluto
Plenary session of the IAU General Assembly on August 24, 2006. Votes were cast by raising yellow cards.