Ever since you were six years old, you have known how many planets there are: Nine. You can probably name most of them, hopefully in order, and the count always ends with the tiniest of them all — Pluto.
So you might be surprised to learn that, as of this month, there are only eight! That is the recent decision of 2500 astronomers at the General Assembly of the International Astronomical Union (IAU) in Prague.
The question of whether Pluto should be defined as a planet has been nagging these scientists for decades. After all, Pluto is smaller than our own Moon and travels on a wild, sideways path compared to the rest of the planets.
According to the definition proposed by the IAU, an object can only be called a planet if it meets three conditions: 1) It has to have cleared the neighbourhood around its orbit; 2) It has to be round; and 3) It has to be orbiting a star.
Did You Know?
The IAU has specified 3 conditions that a celestial body must meet to be considered an official planet.
Stars are very, very massive. This does not necessarily mean stars are large (although our own Sun is 100 times larger than Earth). It just means that stars have a lot of mass, and things with a lot of mass exert large gravitational forces on other objects.
Gravitational forces work by bending space. Think of a star as a bowling ball that is stretching down a trampoline to create a "dimple" on the elastic surface. If you rolled a tennis ball across the trampoline, it would get caught in the bend and start rolling around the bowling ball. In the same way, planets orbit around a star and are guided by the star's gravitational forces. Pluto definitely "rolls" around the sun, even if it takes more than 200 years to make it around once. So, that means Pluto meets Condition #3.
Did You Know?
Albert Einstein proposed the space-time theory, in which space & time are woven together like a fabric. A massive object, such as a star, bends this fabric. Gravity, according to Einstein, is the movement of objects (i.e. planets) along the curvaceous lines of bent fabric (see Figure 3).
All objects exert some gravitational force, depending on how much mass they have. If an object has enough mass to bend space to the point that a lot of particles form a roughly spherical shape out of the matter in the object, it has the right shape to be considered a planet. Pluto is undoubtedly round, which means it satisfies Condition #2 that was set by the IAU.
In fact, Pluto meets all of the requirements for a planet except Condition #1: clearing the neighbourhood around its orbit. Over time, a bona fide planet will eventually clear away everything in its orbit. Unfortunately, Pluto has a rather large object still crossing its path every 248 years. It is called Neptune. So, Pluto does not count as a planet.
Did You Know?
Now that Pluto is no longer classified as a planet, Neptune is officially the furthest planet from the sun.
As a consolation, the IAU has classified Pluto as a 'dwarf planet', along with two other objects called 'Ceres' and, for now, 'Xena'. They also named Pluto as the prototype for a new category of trans-Neptunian objects (TNOs), which are distant solar system objects that are found past the planet Neptune. TNOs hang out three billion kilometers from the sun in a vast area called the 'Kuiper Belt'.
Astronomers have got their eyes (ahem, telescopes) on at least 12 more dwarf planets. So consider yourself lucky...the next generation might have to memorize 21 names instead of only nine.
International Astronomical Union
Morehead Planetarium and Science Centre
Current News Articles About Pluto
Arthur Churchyard is filling his mind with an Arts and Science degree at the University of Guelph. Since he can never decide which science he likes best, he consoles himself by writing about all of them. Arthur interviews Guelph researchers and publishes articles about their work in different Canadian magazines and newspapers as part of a group called Students Promoting Awareness of Research and Knowledge (SPARK).