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Pluto does not meet all three criteria that now define a planet. A planet must orbit around the Sun, have sufficient mass to maintain a nearly round shape, and it must have cleared the neighborhood around its orbit.
Pluto has not cleared the neighboorhood around its orbit. It has been reclassified as a "dwarf planet". More >>

en.wikipedia.org/wiki/Pluto

Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered. Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. ..... This alone is not enough to protect Pluto; perturbations from the planets ...

wonderopolis.org/wonder/why-is-pluto-no-longer-a-planet

Because it does not follow this rule, Pluto is no longer considered a planet. ... Dwarf planets, like Pluto, have not yet cleared the neighborhoods of their orbits ...

www.universetoday.com/13573/why-pluto-is-no-longer-a-planet

Oct 16, 2016 ... Let's find out why Pluto is no longer considered a planet. Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in ...

www.bbc.com/news/science-environment-33462184

Jul 13, 2015 ... In 2006, Pluto was demoted from planet status. Why was it removed from the planet club?

www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-pluto-k4.html

Pluto is not very big. It is only half as wide as the United States. Pluto is smaller than Earth's moon. This dwarf planet takes 248 Earth years to go around the sun.

science.howstuffworks.com/pluto-planet.htm

Aug 24, 2006 ... Pluto, shown as the outermost ring in both illustrations, has the most irregular orbit of all of the objects previously considered planets.

news.nationalgeographic.com/news/2006/08/060824-pluto-planet.html

Aug 24, 2006 ... Pluto is no longer a planet, according to a new official definition. Instead the icy sphere will be considered one of more than 40 "dwarf planets."

www.planetsforkids.org/why-is-pluto-no-longer-a-planet.html

Pluto used to be known as the smallest planet in our solar system, and the furthest planet from the sun. Pluto was discovered in 1930 by Clyde Tombaugh, ...

www.physics.org/article-questions.asp?id=49

When is a planet not a planet? And what is it about Pluto that has got astronomers fighting? Big Bang Blogs examines the sorry fate of the solar system's outcast.