IAU definition of planet

The International Astronomical Union (IAU) decided what a planet is in August 2006. Two astronomers, Julio Ángel Fernández and Gonzalo Tancredi, helped create this definition.

According to the IAU, a planet in our Solar System must meet three rules. First, it must orbit the Sun. Second, it must be big enough to be almost round because of its own gravity, called hydrostatic equilibrium. Third, it must be the main gravitational force in its orbit area.

Because of this definition, there are eight known planets in our Solar System. Some astronomers thought the definition was too strict. They said it depended too much on where the object is located.

The IAU also separated planets from dwarf planets. Objects that orbit the Sun and are almost round but haven’t cleared their orbits, like Pluto, are called dwarf planets. They are a different group and not true planets. This definition helps scientists organize objects in space better.

Background

The story of how Pluto changed from being called a planet began long ago, with the discovery of Ceres in 1801. At first, people thought Ceres was a new planet, but they later found many similar objects and called them asteroids.

When Pluto was found in 1930 by Clyde Tombaugh, it was seen as the ninth planet. But in 1978, its moon Charon was discovered, showing Pluto was smaller than people thought.

In the 1990s, more objects far from the Sun were found, similar to Pluto. Some thought Pluto was just the biggest of these. By 2006, with the discovery of objects like Eris and Sedna, it became clear that many more such objects might be found. This led to a need for a better way to decide what is a planet.

Proposals

First draft proposal

The International Astronomical Union (IAU) shared an early idea for defining planets on August 16, 2006. This idea said a planet should be round because of its own gravity and orbit a star, but not be a star itself or a moon.

This idea would have made three more objects planets beside the nine already known: Ceres, which used to be called a planet but then was called an asteroid; Charon, a moon of Pluto; and Eris, far out in space. Twelve more objects could maybe become planets too, if we learned more about them.

Alternative proposal

Some astronomers thought of another way on August 18, 2006. They said a planet should be the biggest object where it orbits, be round from its gravity, and not make energy like stars do. Under this idea, only the eight planets we know now would stay planets, and Pluto would become a "dwarf planet."

Revised draft proposal

On August 22, 2006, the idea changed a little. They decided not to call Pluto-like planets “plutons” because that word is used in geology and could confuse people. They also changed the rule for pairs of orbiting worlds so it wouldn’t change during their orbit.

Final draft proposal

The last idea, shared on August 24, 2006, said a planet must orbit the Sun, be round from its gravity, and be the biggest thing near its path. A “dwarf planet” would be round and orbit the Sun but not be the biggest nearby. Everything else would be called a “Small Solar System Body.”

During a big meeting, most astronomers voted for this idea. They agreed that Pluto would be a dwarf planet, starting a new group of objects past Neptune.

Final definition

In August 2006, scientists from the International Astronomical Union decided what we call a planet. They said a planet must do three things: orbit the Sun, be big enough to be round, and be the only big object in its path around the Sun.

Objects that are round and orbit the Sun but share their space with other objects are called "dwarf planets." This means they are not big enough to be planets. Pluto, for example, is a dwarf planet. So, while it looks like a planet, it shares its area with other objects beyond Neptune and does not meet all the rules to be called a planet.

Closing issues

Alan Stern, the lead scientist on NASA's robotic mission to Pluto, said that Earth, Mars, Jupiter, and Neptune might not have cleared their paths around the Sun, just like Pluto. For example, Earth has many near-Earth asteroids, and Jupiter has many Trojan asteroids moving along its path.

Other astronomers think these planets control the paths of objects near them. Jupiter's gravity holds the Trojan asteroids in place, and Neptune's gravity affects Pluto and other objects in the Kuiper belt, called plutinos. On June 11, 2008, the IAU decided to call dwarf planets with paths beyond Neptune "plutoids". This group includes Pluto, Haumea, Makemake, and Eris.

Some parts of this idea are hard to use for planets around other stars, because we often cannot tell if they have "cleared their orbit". The IAU suggested a different idea for such planets in 2003.

Impact

The change in how we define planets had big effects on books, toys, and culture. Many books about space were printed after 2006 with the new definition.

The change in Pluto’s status inspired many songs and music. Artists wrote songs about Pluto’s new status. The idea of Pluto’s change was used in some songs as a way to talk about feeling less important.