Pluto, probably the most favorite planet among kids from the 90s (because of Pluto, the cartoon character maybe?), is one of the many thousands of objects found in the "Kuiper belt" that orbit the sun farther than the planet Neptune. These are called trans-neptunian objects.

Pluto was recognized as a dwarf planet, after the discoveries of many more trans-neptunian objects. Slowly, planetariums started leaving out Pluto and made models of the solar system with only 8 planets leading to controversies. In 2006 or so, after much debate, Pluto was demoted to a "dwarf planet". Neil deGrasse Tyson, director of the Hayden planetarium, was very involved in this.

The reason for its demotion was that, it did not satisfy the one of the three conditions that defined a planet, according to IAU (Internation Astronomical Union): a planet should have cleared its neighborhood.

A star is something that produces energy (light and heat) due to a mechanism called nuclear fusion, which is what makes our sun shine everyday. Hydrogen in a star fuses together to form other elements in the periodic table and in the process releases a lot of energy in the form of heat and light.

Pluto is not a star.

Different people argue about what makes a planet or not. Pluto is large enough that its gravity makes it round, and it probably has different layers of material inside of it. These things are planet-like. Pluto has not swept the other things in its orbit away, though, which is a different meaning of the word "planet". As a result, people argue about whether Pluto is a planet or not.

Ultimately, whether something is a planet or not is something we call it. Pluto is what it is, and it doesn't matter whether we call it a planet, a dwarf planet, a comet, or something else.

Nothing about Pluto has changed but it is no longer considered a planet because we changed how we define a planet. We changed the definition of planet so that it would be more useful for scientists to tell different objects in space apart.

Planets are now defined as large round objects that orbit around stars in a predictable way, and they need to clear the area around their path of other objects. Our solar system has eight planets but there are many other objects around that don’t count as planets. For example, the asteroid belt contains lots of big rocks but never condensed into a single planet.

The Kuiper belt, past the orbit of Neptune, contains many large objects that might count as planets if they weren’t so close together. Pluto is one of these objects. It was first object in the Kuiper belt that we saw and it has a regular (but strange) orbit. We called it a planet until 2006. As our telescopes and sensors advanced, we found many more “planets” near Pluto and one of them is even bigger (Eris). Since Pluto doesn’t clear the space around it as it orbits, we no longer consider Pluto a planet – instead, it is considered part of the Kuiper belt.