With the utter dismay of some science nerds out there (Hello, Dr. Sheldon Cooper) and the rewriting of science textbooks anywhere, Pluto is no longer considered a planet. The question is: Why?
First, some background about Pluto.
Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X.
After a year of intense observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. His team settled on Pluto, the same name of the Roman god of the underworld. The Solar System now had 9 planets.
However, over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (or 55 times the distance of the Earth to the Sun).
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. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.
In addition, astronomers had been turning up larger and larger objects in the Kuiper Belt. In 2004, another object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Eris has approximately 25% more mass than Pluto and made of the same ice/rock mixture.
The concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union (IAU) in August 2006 in Prague, Czech Republic.
Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale, and a third would drop the number of planets down to 8, and Pluto would be out of the planet club.
In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarf planet”.