Definition of planet

The International Astronomical Union has rules to decide what is a planet in our Solar System.

A planet must:

• Orbit the Sun
• Be big enough for its own gravity to make it round
• Have a clear path around the Sun without too many other objects nearby

The idea of what a planet is has changed over time. Long ago, ancient Greeks called any moving object in the sky a “wandering star.”

In 2006, the International Astronomical Union made new rules. They had to decide what to call objects like Eris. This led to Pluto being reclassified, as it didn’t meet all the new rules. Some scientists agree with this, while others have different ideas about what should count as a planet.

History

See also: Geocentric model, Heliocentrism, Celestial spheres, and Classical planet

The idea of what a planet is has changed a lot over time. Long ago, people in ancient Greece called any moving object in the sky a "wandering star." They knew of five main objects that moved: Venus, Jupiter, Mercury, Mars, and Saturn. Sometimes they also thought about the Sun and the Moon, but usually they kept them separate.

As time passed, we learned more. When Nicolaus Copernicus said that the Earth moves around the Sun, Earth became a planet too. Later, we found more planets, like Uranus in 1781 and Neptune in 1846. Each new discovery helped us understand what a planet is.

IAU definition

Main article: IAU definition of planet

When the object Eris was found, astronomers needed a clear way to say what a planet is. They made new rules in 2006.

A planet must go around the Sun, be big enough for its gravity to make it almost round, and be the main thing in its path around the Sun. A dwarf planet also goes around the Sun and is round, but shares its path with other objects. The first dwarf planet is Pluto. Other objects that go around the Sun but do not follow these rules are called small Solar System bodies.

Acceptance of the IAU definition

Many scientists agree with the way the International Astronomical Union (IAU) defines what a planet is. Important supporters include Mike Brown, who discovered the object Eris; Steven Soter, a professor at the American Museum of Natural History; and Neil deGrasse Tyson, director of the Hayden Planetarium.

In the early 2000s, Neil deGrasse Tyson chose not to call Pluto a planet at his planetarium during a big renovation. He wanted to group planets by what they share instead of just counting them. This decision got many letters and messages from people, especially children, who did not agree with him. Later, Tyson wrote a book about why Pluto was reclassified.

Steven Soter explained that planets form from dust and material around the Sun. Bigger objects pull in more material and grow much larger than smaller objects. For example, Mars is much bigger than all the other objects in its path around the Sun. In contrast, Ceres — the largest object in the asteroid belt — is only a small part of the material there. This big difference helps scientists decide which objects are planets and which are not.

Ongoing controversies

Even though we have ways to describe what a planet is, some people still don't agree. Many think the rules are too strict or confusing. For example, some scientists, like Alan Stern from NASA, think that Pluto should still be called a planet. He believes the decision to change Pluto’s status didn’t include enough astronomers, so it might not reflect everyone’s view. Even without this debate, there are still unclear parts in the definition.

One big point of discussion is about how much a planet needs to "clear its neighborhood." This means how much it controls the area around its path as it moves around the Sun. Some scientists argue that big planets like Earth, Mars, Jupiter, and Neptune haven’t completely cleared their areas of smaller space rocks, so they might not fit the current rules. Others say these planets do control the space around them, even if there are still small objects left.

Another idea is that planets need to be big enough for their own gravity to make them round. This is called being in “hydrostatic equilibrium.” But there isn’t a clear point where an object becomes round enough to be a planet. Also, some objects change shape depending on what they’re made of or how hot they are. Because of this, the rule about being round isn’t always easy to use.

There’s also debate about whether moons should be called planets. Some moons are big enough and have features like geological activity that are similar to planets. However, most scientists still classify them separately.

Finally, finding planets around other stars has added more questions. Some of these objects are almost as big as stars, and it’s hard to know whether they should be called planets or stars. Scientists keep working on better definitions to explain all these different space objects.