Pluto, the Planet that is NOT a Planet

Rotating Face of Pluto taken by Hubble

Image Credit:

NASA, ESA, and M. Buie (Southwest Research Institute).

Pluto. Never in astronomy has there been a more controversial object that we have discovered in our Solar System, our galaxy, or even the Universe. When it was first discovered, it was thought to be the farthest planet in our Solar System. But as we learned more about it, we were able to determine that it is not, in fact, a planet.

But before we learn why Pluto is not a planet, let's get to know a few things about one of the farthest denizens of our Solar System. For one, it was actually discovered by accident, even though it was actively being searched for by astronomers. Astronomers had used the orbit of Neptune and found that its orbit did not match calculations of how Neptune should go around the Sun. They thought that there was an object orbiting outside Neptune's orbit that caused these anomalies and were looking for it.

Secondly, it has a very eccentric orbit. In fact, its orbit is so eccentric, that for portions of its year, Pluto is actually closer to the Sun than Neptune, but because of its highly inclined orbital plane, Neptune and Pluto are never in danger of colliding.

Pluto is tiny. It is smaller than the largest moons in our Solar System including our Moon. But despite its small size, it does have satellites orbiting around it, including the largest Plutonian satellite, Charon, though it would be more accurate to say that Pluto and Charon co-orbit the Sun.

Pluto in comparison to some moons of the Solar System

Image Credit:

NASA

It is an icy body, much like the objects in the Kuiper Belt and Oort Cloud. There is another Kuiper Belt objects that have a similar composition to Pluto, but it actually larger than it.