Definition of planet

The word "planet" means something that moves around the Sun and is big enough for its own gravity to make it round. It also has to be the biggest thing around its path so that smaller objects either crash into it or get pulled away by its gravity. This idea about what a planet is came about because astronomers found a big object called Eris in 2005. Eris is even bigger than Pluto, which used to be called a planet.

For a long time, people thought Pluto was a planet, but now it does not meet all the rules for being a planet. Only bodies that are big enough and clear their area around the Sun can be called planets. This new rule was made by the International Astronomical Union, the group that helps decide the names and rules for space things.

Even with this rule, some scientists still argue about what should count as a planet. They think that if something is round because of its own gravity, it should be called a planet, no matter where it is or what path it takes around the Sun. So, the idea of what a planet is can still change as we learn more about space.

History

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

The idea of what a planet is has changed a lot over time. In ancient Greece, people called the Earth the center of the universe. They used the term "wandering stars" for objects in the sky that moved, unlike the "fixed stars" that stayed in the same place. The Greeks knew five of these moving objects: Venus, Jupiter, Mercury, Mars, and Saturn. Later, both the Sun and the Moon were also sometimes called planets.

As time went on, our understanding changed. When the idea that the Earth moves around the Sun became accepted, Earth itself was added to the list of planets. This was a big shift in how people thought about our place in space.

IAU definition

Main article: IAU definition of planet

The definition of what a planet is has changed over time. In 2006, the International Astronomical Union made an official definition after discovering new objects in space. They decided that a planet must orbit the Sun, be round because of its own gravity, and have cleared its area of other objects.

Pluto didn’t meet all these rules, so it was placed in a new group called "dwarf planets." These are round and orbit the Sun but haven’t cleared their areas. Other objects like Ceres and Eris are also dwarf planets.

Acceptance of the IAU definition

Many scientists support the International Astronomical Union (IAU) definition of a planet. 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 decided not to call Pluto the ninth planet during a big renovation of his planetarium. He preferred to group planets by what they have in common instead of just counting them. This choice led to many letters and messages from children who disagreed with him. Later, in 2009, Tyson wrote a book about why Pluto was no longer called a planet.

Steven Soter explained in a magazine article that planets grow by collecting material from their area of space. The planets that grew big enough to collect almost everything around them are the ones we call planets today. Smaller objects, like those in the asteroid belt or the Kuiper belt, did not collect enough material to be considered planets. The difference in size between these groups makes it clear which objects should be called planets.

Ongoing controversies

Despite the International Astronomical Union's declaration, many people are still not convinced. They think the definition is confusing and not clear. Some, like Alan Stern, head of NASA's New Horizons mission to Pluto, think the decision was not made by enough astronomers to count for everyone.

One big question is about what it means for a planet to "clear its neighborhood." Alan Stern argues that since big planets like Earth, Mars, Jupiter, and Neptune don't completely clear their areas, they shouldn't all be called planets under the current definition. However, others explain that these planets still control the paths of smaller objects in their areas through their strong gravity.

Another point of debate is whether a planet needs to be round. The definition says planets should be large enough for their own gravity to make them round. But there isn't a clear line between round and not-round objects, and factors like what the object is made of and heat from the sun also play a role.