What Is a Planet

A planet is a large celestial object that orbits a star. Scientists use a precise definition created by the International Astronomical Union (IAU) in 2006. This definition includes three specific criteria that an object must meet to be called a planet.

The first criterion states that a planet must orbit the Sun. This means the object travels around the Sun in a regular path. The second criterion requires that a planet must be round in shape. Gravity pulls the object's material toward its center, creating a spherical form. Scientists explain that an object needs enough mass for gravity to shape it this way. The third criterion is the most complex. A planet must have cleared its orbital neighborhood of other debris. Evidence shows that planets are massive enough to push away or absorb smaller objects near their orbital paths.

These three criteria help scientists classify objects in our solar system. When astronomers discovered many small, icy objects beyond Neptune, they needed to decide how to classify them. Pluto, once considered the ninth planet, became the center of scientific debate. Observations revealed that Pluto shares its orbital region with thousands of similar objects in the Kuiper Belt. Pluto has not cleared its orbital neighborhood. In 2006, the IAU reclassified Pluto as a dwarf planet. Dwarf planets meet the first two criteria but fail the third one.

This reclassification matters because scientific definitions serve as tools for understanding. Definitions help scientists organize information and communicate clearly about discoveries. The planet definition allows astronomers to distinguish between different types of objects. Our solar system now has eight recognized planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Understanding what makes a planet helps us study how solar systems form and change over time.

Interesting Fact: Scientists have discovered thousands of planets orbiting stars beyond our Sun. These exoplanets may not orbit our Sun, so astronomers use modified criteria to classify them.