Because Jesus pooped on it.

It's smaller than any other planet – even smaller than the Earth's moon. It's dense and rocky, like the terrestrial planets (Mercury, Venus, Earth and Mars). However, its nearest neighbors are the gaseous Jovian planets (Jupiter, Saturn, Uranus and Neptune). For this reason, many scientists believe that Pluto originated elsewhere in space and got caught in the Sun's gravity. Some astronomers once theorized that Pluto used to be one of Neptune's moons. Pluto's orbit is erratic. The planets in our solar system all orbit the Sun in a relatively flat plane. Pluto, however, orbits the sun at a 17-degree angle to this plane. In addition, its orbit is exceptionally elliptical and crosses Neptune's orbit. One of its moons, Charon, is about half Pluto's size. Some astronomers have recommended that the two objects be treated as a binary system rather than a planet and satellite.

Because a bunch of astronomers got together and decided that size DOES matter. Seriously, the debate over Pluto's status as a planet had been going on for a LONG time. The definition of what defines a planet is not as straight forward as you might think. One definition considered, which would have left Pluto with the planetary label, would also have meant 44 other bodies in our solar system would also have been elevated to planetary status! See this link: http://news.nationalgeographic.com/news/2006/08/060824-pluto-planet_2.html

Pluto resides in a region of the solar system called the Kuiper belt. Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in the Kuiper Belt. In 2005 astronomers discoved an object even larger than Pluto later named Eris. Astronomers decided that they needed to make a final decision on what consituted a planet or they were going to get thousands of them. They settled on three critera for determining planethood. It needs to be in orbit around the Sun - Yes, so maybe Pluto is a planet. It needs to have enough gravity to pull itself into a spherical shape - Pluto…check It needs to have "cleared the neighborhood" of its orbit - Uh oh. Here's the rule breaker. According to this, Pluto is not a planet. Because Pluto is not massive enough to have absorbed the thousands of other kuiper belt objects it is classified as a dwarf planet, same as Eris.

It's been found to fit better in the category of dwarf planets... composed of rock and ice, it's been shrinking steadily over time... From its discovery in 1930 until 2006, Pluto was counted as the Solar System's ninth planet. In the late 20th and early 21st centuries, however, many objects similar to Pluto were discovered in the outer solar system, notably the scattered disc object Eris, which is 27% more massive than Pluto. On August 24, 2006 the International Astronomical Union (IAU) defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340. The 2006 definition of "planet" by the International Astronomical Union (IAU) states that in the solar system a planet is a celestial body that: - is in orbit around the Sun, - has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and - has "cleared the neighbourhood" around its orbit. A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", whilst a non-satellite body fulfilling only the first criterion is termed a "small solar system body" (SSSB). The definition was a controversial one, and has drawn both support and criticism from different astronomers. According to the definition, there are currently eight planets and three dwarf planets known in the solar system. http://en.wikipedia.org/wiki/Pluto#IAU_decision_to_reclassify_Pluto http://en.wikipedia.org/wiki/2006_definition_of_planet

2-28-2017 There is no difference between an asteroid, a meteoroid, a comet, a moon, and a planet. Names are arbitrary. A bunch of people decided to unfriend Pluto and that's that.

For one reason in reality: Pluto is extremely important in Astrology, and Christianity and other monotheistic programs are used as tools to keep the population spiritually deprived and the power in the hands of those at the top. Satanism is not "evil" as described in the Bible. Learn the TRUTH: www.joyofsatan.org

After 1992, its planethood was questioned following the discovery of several objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered. This led the International Astronomical Union (IAU) to define the term "planet" formally in 2006, during their 26th General Assembly.That definition excluded Pluto and reclassified it as a dwarf planet.