What Is #2: A Planet

For the first post of this new 'What Is' series, I explained what makes a star a star. It only seems fitting today, for the second post in this series to define a planet. Most of us are familiar with the concept of a planet, but I'm guessing most people don't know much more than a planet is a large object orbiting the Sun. And yes, that describes a planet, but there's more to a planet than being a large body that orbits the Sun. For starters, what is meant by large body and secondly, what is meant by orbiting the Sun?

Mars taken by the Rosetta spacecraft. ESA - European Space Agency & Max-Planck Institute for Solar System Research for OSIRIS Team 

If you were born before 2006, but after 1930, you grew up knowing Pluto as the ninth planet in the Solar System. But Pluto was always a bit odd with some characteristics markedly different from the other planets. The orbit was more inclined and more elliptical than the other planets and it was the smallest planet. If you were born before 1930, Pluto was yet to be discovered. If you were born after 2006, Pluto was already moved from the planet category, to a new category defined by the International Astronomical Union, dwarf planets. So what makes a planet a planet? There are three criteria.

1. A planet directly orbits the Sun. Earth's Moon indirectly orbits the Sun. This means that it does move around the Sun, but only as it orbits Earth. The Moon directly orbits Earth. Therefore the Moon fails this criteria. Pluto, however, satisfies this requirement. Pluto directly orbits the Sun.

2. A planet must be large enough, with enough gravity, to be spherical in shape. Objects that are small do not have enough gravity to form into a sphere. They tend to be randomly shaped based on how the material came together as they formed. Pluto, despite being the smallest planet before 2006, was large enough to be a sphere. There are several non-planetary objects in the solar system that are spherical in shape.

3. A planet must have cleared its area of neighboring debris. This is where Ceres, a very large, spherical asteroid in the asteroid belt fails as a planet. Ceres has not cleared the asteroid belt of debris and is therefore a dwarf planet. Objects that satisfy the first two requirements but fail this third requirement are called dwarf planets. Pluto has not cleared its area of debris. Astronomers have discovered many objects near the orbit of Pluto that Pluto has not cleared. In fact, it was the discovery of these objects that led astronomers to re-think the definition of a planet.

So there you have it. A planet is a large object that orbits the Sun, but it must directly orbit the Sun, be large enough to be a sphere, and be large enough to clear its area of debris. In our solar system, only eight objects satisfy these requirements: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.