Why is Pluto no longer a planet?

When I was a kid, I’m always fascinated with the stars, the sun, comets, and the universe. At my early years, I already know some astrological terms that are not common to my age, may be just because my mom is a science teacher and she used to bring me to her classes while she discusses the solar system, the space and the universe. And at my young mind I already know our 9 different planets and their arrangement according to their distance to our sun. That’s why I was shocked when I heard that Pluto has been dropped as a planet so I try to figure out what happened and the reasons Pluto has been thrown out of their club.

As I was preparing into a sabbatical mode, searching for an answer regarding Pluto’s demotion. Another interesting and surprising information I got is that there was no official definition of a planet before, they just came about it recently after numerous findings of similar ice/rock mixtures like Pluto and even more massive (Whoa! there goes my six years of elementary schooling). 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), which was held from August 14 to August 25, 2006 in Prague, Czech Republic.

Lets go back in time during the discovery of Pluto to sequence the events that leads to the realization that it is not really a planet. Long before, it has been predicted by astronomers that there would be a 9th planet in the solar system and they called it the Planet X. It was 1930 when Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona finally discovered an object in the right orbit and declared that he had discovered Planet X. They called it Pluto, a name suggested by an 11-year old school girl in Oxford England (it wasn’t named after the Disney character, but the Roman god of the underworld). Astronomers were not sure about its mass until the discovery of its largest Moon, Charon in 1978. By knowing its mass they could accurately gauge its size. The most accurate measurement gives the size of Pluto at 2,400km (1,500 miles) across which is small even compared to Mercury, which is only 4,880km (3,032 miles) across. Some scientists are already skeptical if Pluto is really a planet due to its size.

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 (55 times the distance of the Earth to the Sun). Astronomers realized that it was only a matter of time before an object larger than Pluto was to be discovered in the Kuiper Belt. And in 2005, Mike Brown and his team dropped the bombshell. They had discovered an object, further out than the orbit of Pluto. Officially named 2003 UB313, the object was later named as Eris. It has approximately 25% more mass than Pluto. With Eris being larger, made of same ice/rock mixture and more massive than Pluto, the concept of nine planets in the Solar System began to fall apart and astronomers decided to make a final decision about the definition of a planet.

Now, an object to be a planet must meet these 3 requirements as defined by IAU
1.) It needs to be in orbit around the sun. (Checked)
2.) It needs to have enough gravity to pull itself together into a spherical shape. (Checked)
3.) It needs to dominate the neighborhood around its orbit. (This is the problem).

Pluto has been demoted because it does not dominate its neighborhood. Charon, its large “moon”, is about half the size of Pluto, while all the true planets are far larger than its moons. In Addition, bodies that dominate their neighborhoods, “sweep up” asteroids, comets and other debris, clearing a path along their orbits. By contrast, Pluto’s orbit is somewhat untidy.

Eight major planets managed to satisfy the definition, but not Pluto. The definition causes some semantic atrocities between some Astronomers. Since Pluto didn’t meet the 3rd criteria, they decided to tag it as a dwarf planet. So Pluto is a dwarf planet. There are many objects with similar size and mass to Pluto jostling around in its orbit and until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.

=)

References:
http://www.universetoday.com/2008/04/10/why-pluto-is-no-longer-a-planet/
http://news.nationalgeographic.com/news/2006/08/060824-pluto-planet.html/
http://curious.astro.cornell.edu/question.php?number=624