IAU definition of planet

The International Astronomical Union (IAU) established a formal definition of what a planet is in August 2006. This definition was created by Uruguayan astronomers Julio Ángel Fernández and Gonzalo Tancredi. According to the IAU, in our Solar System, a planet must meet three specific conditions. First, it must orbit the Sun. Second, it must be big enough to have a nearly round shape due to its own gravity, which is called hydrostatic equilibrium. Third, it must have "cleared the neighbourhood" around its orbit, meaning it is the dominant gravitational force in its path.

Because of this definition, there are officially eight known planets in our Solar System. However, this decision was not without controversy. Some astronomers argued that the definition was too strict, especially because it depends on where the object is located. For example, a body similar in size to Mars found far away in the inner Oort cloud might not be able to clear its orbit and thus wouldn’t qualify as a planet. Interestingly, even though Mercury isn’t perfectly round, it is still considered a planet under this definition.

The IAU also made a clear distinction between planets and dwarf planets. Objects that orbit the Sun and are nearly round but haven’t cleared their orbits, like Pluto, are classified as dwarf planets. These are considered a separate group and not true planets. This definition helps scientists categorize objects in space more clearly and continues to guide how we understand and talk about planets both in our Solar System and around other stars.

Background

The story of how Pluto changed from being called a planet to something else began long ago, with the discovery of Ceres in 1801. At first, people thought Ceres was a new planet, but soon found many similar objects, which led to calling 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 much smaller than 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 was 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, a group of scientists called the International Astronomical Union made an important decision about what we call a planet. They said a planet must do three things: orbit the Sun, be big enough to pull itself into a round shape, and be the main object in its path around the Sun, with no other big objects nearby.

Objects that are round and orbit the Sun but share their space with other objects are called "dwarf planets." This means they are not quite 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 therefore 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, argued that Earth, Mars, Jupiter, and Neptune might not have fully cleared their orbital zones, similar to Pluto. For example, Earth has many near-Earth asteroids, and Jupiter has numerous Trojan asteroids sharing its path.

Other astronomers believe that these planets do control the orbits of nearby objects. Jupiter's gravity keeps 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 orbits beyond Neptune "plutoids". This group includes Pluto, Haumea, Makemake, and Eris.

Some parts of this definition 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 definition for such planets in 2003.

Impact

The decision to change 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 defending Pluto as a planet, mourning its reclassification, or using the idea of Pluto’s demotion as a metaphor. For example, some songs were written from Pluto’s perspective, asking why it was changed to a dwarf planet. The term “plutoed” became popular to describe being downgraded or devalued, much like Pluto was when it lost its planet status.