This is true; Pluto was reclassified as a dwarf planet in 2006. There are several reasons for this reclassification, and for why Pluto was originally considered a planet.
Pluto was considered a planet because;
- It was discovered at a time when many astronomers were predicting and looking for new planets in the solar system. Pluto was thought to be the anticipated "Planet X", whose existence was hypothesized as an explanation for the disruptions in Neptune's orbit.
- Planets did not have a formal designation; it was generally "common sense".
- The astronomical equipment and measurements being used were, for a long time, unable to make the sort of observations which later came to challenge Pluto's planethood.
Dissent regarding Pluto's status began when it became clear that Pluto was unlike any other planet; it is significantly smaller than any other planet, and its orbit is far more eccentric and inclined relative to the solar plane. On the other hand, Pluto has moons; this was cited as evidence in its favor, until other small bodies, including asteroids, were found to have moons as well. In recent years, improvements in equipment allowed the observation and measurement of a number of small bodies, particularly the "planet" Eris. Eris is larger than Pluto, has a moon, and is even farther from the sun. This discovery forced a re-evaluation of Pluto's planethood; the option was to either admit Eris as the tenth planet, or to change the qualifications required in order to be considered a planet.
The International Astronomical Union (IAU) which is responsible for the classification and naming of all astronomical data, convened to discuss this issue, and resolved to reclassify Pluto and Eris under the new term "dwarf planet". A standard planet has met three criteria;
- It orbits the sun
- Its own gravity causes it to be round
- It has "cleared its neighborhood" - that is to say, it is the dominant gravitational influence, other than the sun
A dwarf planet orbits the sun and is round, but has not cleared its neighborhood. This is evidenced via the fact that Pluto and Eris both approach Neptune's orbit at their perihelion; it has even been suggested that Pluto, at least, may have once been a moon of Neptune.