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National Aeronautics and Space Administration, Science Mission Directorate. (2009). What is a Planet?. Retrieved , from Mission:Science website:


Science Mission Directorate. "What is a Planet?" Mission:Science. 2009. National Aeronautics and Space Administration.

What is the definition of a planet?

Image of the Greek Parthanon under a starry night sky.

Before telescopes, scientists relied on their naked eye and careful observations to catalog the night sky. The five planets easily visible with the naked eye have been observed for all of human history: Mercury, Venus, Mars, Jupiter and Saturn.

Technically, there was never a scientific definition of the term Planet before 2006. When the Greeks observed the sky thousands of years ago, they discovered objects that acted differently than stars. These points of light seemed to wander around the sky throughout the year. We get the term "planet" from the Greek word "Planetes" - meaning wanderer.

Telescopes add to our knowledge

In the 1600's scientists began to use telescopes to view our solar system. As technology got better, scientists discovered more planets orbiting our Sun such as Uranus in 1781, Neptune in 1846 and Pluto in 1930. Then, in 1991, advances in telescope technology enabled scientists to discover many more objects in a disk-shaped cloud beyond Pluto called the Kuiper (KYE per) Belt. These objects were classified as Kuiper Belt Objects (KBO) partly because they are smaller than Pluto. This classification of KBO seemed to work fine as long as objects weren't bigger than Pluto.

Photograph of Clyde Tombaugh with his telescope in the 1930's.

Clyde Tombaugh discovered Pluto in 1930. Pluto was added to the list of planets and was our 9th planet until August 2006.

Eris, the Goddess of Discord

It was the recent discovery of an object larger than Pluto within the Kuiper Belt that changed everything. Is this object, now named Eris, our 10th planet since it is larger than Pluto? This discovery and the naming of this new object prompted the IAU to discuss a scientific definition for the term planet. What if Eris is given planet status? Then our solar system could grow to dozens of planets as more and more Kuiper Belt Objects are discovered. Try remembering all those planet names. But if Eris is not a planet, then is Pluto still a planet?

The new definition – Part 1 and 2

Astronomers of the International Astronomical Union (IAU) voted on and passed the first scientific definition of a planet in August 2006. According to this new definition, an object must meet three criteria in order to be classified as a planet. First, it must orbit the Sun. Second, it must be big enough for gravity to squash it into a round ball.

Diagram showing the orbits of 10's of Kuiper Belt Objects.

Some scientists believe that if Pluto remains classified as a planet, then the dozens of Kuiper Belt Objects (KBO's) orbiting our Sun would also be classified as planets. Our solar system would have the 9 original planets, an additional 43 KBO's, and more as they are cataloged.

Part 3 – The Ongoing Debate

And third, it must have cleared other objects out of the way in its orbital neighborhood. To clear an orbit, a planet must be big enough to pull neighboring objects into the planet itself or sling-shot them around the planet and shoot them off into outer space. According to the IAU, Pluto does not meet this third requirement but is now in a new class of objects called "dwarf planets." It is this third part of the definition that has sparked debate.

Illustration of small asteroid-like object crashing into a larger planet object.

To distinguish a planet from a round asteroid in the asteroid belt, a planet must be massive enough to clear smaller objects – like asteroids – from their own orbit. The gravity of the planet would pull in smaller objects which would become part of the new planet.

The problem for Pluto

The problem for Pluto is the fact that its orbit is in the Kuiper Belt along with 43 other known Kuiper Belt Objects (KBOs). There are possibly billions of objects in the Kuiper Belt that have not been cataloged yet. Scientists have even found 8 KBOs between Neptune and Pluto. Some scientists view the new definition as unclear. Exactly how much does Pluto have to "clear" from its neighborhood to be considered a planet? And how much has Pluto already influenced its own neighborhood since the planet formed? These and other questions have been raised in response to the IAU's definition of a planet.

Consider this: Pluto crosses into Neptune's orbit, but Neptune is still classified a planet. This is because of the orbits of Pluto and Neptune and that they never get closer to each other than 17AU (AU=distance from Earth to the Sun). Pluto may cross orbits with many other Kuiper Belt Objects, but how close do these objects get to Pluto? How close to objects have to get to Pluto to be considered "in" Pluto's neighborhood?

Diagram of the planet orbits showing how Pluto's orbit is at an incline to all the other orbits, and shows how Pluto crosses into Neptune's orbit.

Diagram of the planet orbits in our solar system, including Pluto, distinctly shows the cross over of Neptune's and Pluto's orbits.

Journey to the edge

NASA's New Horizon spacecraft is speeding toward the edge of the solar system on its mission to Pluto. Launched in January 2006, it will not be until July 2015 that we will reach Pluto. It will swing past Jupiter for a gravity boost and scientific studies in February 2007, and reach Pluto and its moon, Charon, in July 2015. Then, as part of an extended mission, the spacecraft would head deeper into the Kuiper Belt to study one or more of the icy mini-worlds in that vast region, at least a billion miles beyond Neptune's orbit. Sending a spacecraft on this long journey will help us answer basic questions about the surface properties, geology, interior makeup and atmospheres on these bodies.

An artists representation of the New Horizon spacecraft approaching Pluto and Charon.

New Horizon’s Space Craft will reach Pluto in 2015.

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National Aeronautics and Space Administration, Science Mission Directorate. (2009). What is a Planet?. Retrieved , from Mission:Science website:


Science Mission Directorate. "What is a Planet?" Mission:Science. 2009. National Aeronautics and Space Administration.