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What Counts as a Planet?

There are eight planets in our solar system. If you went to school before the late '90s, you probably learned that there were nine planets. Why the discrepancy? Over the years, the definition of “planet” has changed a few times. Scientists just can’t seem to agree. So according to the modern guidelines, what counts as a planet?

It must orbit a star

Earth and Moon taken from 3.9 million miles away by the Galileo Spacecraft
Credit: NASA

Objects floating around in space must obey three rules to be officially classified as a planet. Rule number one is that it must orbit a star. In the case of our solar system, a planet has to orbit the sun. That disqualifies the moon and all other moons from being classified as planets since they orbit their respective planets.

There are interstellar objects that have been discovered that have been dubbed “rogue planets.” These planets don’t orbit anything and float around aimlessly through space. It has been estimated that billions of these rogue planets exist in the Milky Way galaxy alone. While they might be the same mass as a planet, they can’t be officially classified as a planet because they don’t orbit a star.

Planets are spherical

View of Earth curvature and lights at night seen from the International Space Station
Credit: NASA

The official rules state that a planet has to have enough gravity to force itself into a spherical shape. This rule is designed to exclude all the smaller objects that can be found at the edges of our solar system. Asteroids often orbit stars, but they are too small to have enough gravity to make themselves round.

The amount of gravity required to pull a planet into a spherical shape also means that it would have enough gravity to potentially have a moon. So, if a celestial body has a moon, chances are that it has enough gravity to be considered a planet.

Cleared its orbit

View of the north polar region of the Moon seen by NASA's Galileo Spacecraft
Credit: NASA

Another rule to eliminate smaller planets, and the rule that knocked out poor little Pluto from being an official planet, is that planets have to be bigger than anything else that crosses their orbit. Pluto has an elongated orbit around the sun. At certain times, it can be closer to the sun than Neptune. Since the two cross orbits, the larger, Neptune, is considered an official planet while Pluto is demoted to dwarf planet status.

There are several objects in the Kuiper Belt, a region beyond Neptune’s orbit filled with icy bodies, that are much larger than Pluto. They also don’t get planetary status because their orbits are filled with other objects.

Dwarf planets

View of dwarf planet Ceres' south polar region taken by NASA's Dawn Spacecraft
Credit: NASA

Dwarf planets have to follow all the same rules as “real” planets except for the last one: they don’t need to be the largest object in their orbit. If they did, they would have some serious competition.

Pluto, the most well-known, was de-classified from “planet” to “dwarf planet” in 2016. It has about the same surface area as Russia, and it’s still one of the largest dwarf planets. But just because it’s small, doesn’t mean that it can’t fulfill all the other planetary rules. Although it takes almost 250 years to make one lap of its orbit, Pluto still does orbit the sun and has enough gravity to pull itself into a sphere. It even has five of its own moons.

Experts believe there could be close to 50 dwarf planets in the solar system, but there are currently just five that we know of:

  • Ceres – discovered 1801
  • Pluto – discovered 1930
  • Eris – discovered 2003
  • Haumea – discovered 2003
  • Makemake – discovered 2005

Most dwarf planets exist beyond Neptune’s orbit in the Kuiper Belt. Ceres, however, is the only dwarf planet that exists in the inner part of the solar system. It’s the largest object in the asteroid belt between Mars and Jupiter. It’s so much larger and looks so different from the other asteroids that scientists awarded it the designation of dwarf planet in 2006. Despite being the largest object in the asteroid belt, it’s still 14 times smaller than Pluto and has about the same surface area as Argentina.

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