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What is a Planet?
Illustration showing dwarf planets and the Moon compared to Earth.
A size comparison of dwarf planets Eris, Pluto and Pluto's moon Charon, Earth's Moon and Earth.

Science is a dynamic process of questioning, hypothesizing, discovering and changing previous ideas based on what is learned. Scientific ideas are developed through reasoning and tested against observations. Scientists assess and question each other's work in a critical process called peer review.

Our understanding about the universe and our place in it has changed over time. New information can cause us to rethink what we know and reevaluate how we classify objects in order to better understand them. New ideas and perspectives can come from questioning a theory or seeing where a classification breaks down.

Defining the term planet is important, because such definitions reflect our understanding of the origins, architecture, and evolution of our solar system. Over time, the objects thought to be planets have changed.

The ancient Greeks counted the Moon and Sun as planets along with Mercury, Venus, Mars, Jupiter and Saturn. Earth was not considered a planet, but rather was thought to be the central object around which all the other celestial objects orbited.

By the 17th century, astronomers realized that the Sun was the celestial object around which all the planets - including Earth - orbit, and that the Moon is not a planet, but a satellite (moon) of Earth. Uranus was added as a planet in 1781 and Neptune was discovered in 1846.

The asteroid Ceres was discovered between Mars and Jupiter in 1801, and was at first classified as a planet. Many more objects were found in the same region and also classified as planets. Eventually Ceres and the rest of the bodies in the asteroid belt were reclassified as minor planets, but they still retain the asteroid label.

Color image of Pluto and its moons.
Pluto and its moons.

Pluto was discovered in 1930 and identified as the ninth planet. But Pluto is much smaller than Mercury and is even smaller than some of the planetary moons. It is unlike the terrestrial planets (Mercury, Venus, Earth, Mars), or the gas giants (Jupiter, Saturn), or the ice giants (Uranus, Neptune). Its huge satellite, Charon, is nearly half the size of Pluto and shares Pluto's orbit. As more became known about Pluto, some astronomers questioned whether it should really be called a planet. Pluto kept its planetary status through the 1980s, but in the 1990s, things began to change.

Technical advances in telescopes led to better observations and improved detection of very small, very distant objects. In the early 1990s, astronomers began finding numerous icy worlds orbiting the Sun in a doughnut-shaped region called the Kuiper Belt beyond the orbit of Neptune - out in Pluto's realm. With the discovery of the Kuiper Belt and its thousands of icy bodies (known as Kuiper Belt objects, or KBOs; also called transneptunians), it was proposed that it is more useful to think of Pluto as the biggest KBO instead of a planet. Then, in 2005, a team of astronomers announced that they had found the tenth planet - it was a KBO even larger than Pluto. People began to wonder what planethood really means. Just what is a planet, anyway?

Suddenly the answer to that question didn't seem so self-evident, and, as it turns out, there are plenty of disagreements about it. The International Astronomical Union (IAU), a worldwide organization of astronomers, took on the challenge of classifying the newly found KBO (later named Eris). In 2006, the IAU passed a resolution that defined planet and established a new category, dwarf planet. Eris, Ceres, Pluto, and two more recently discovered KBOs named Haumea and Makemake, are the dwarf planets recognized by the IAU as of September 2009. Pluto, Eris, Haumea, and Makemake are also classified as KBOs.

Astronomers and planetary scientists did not unanimously agree with these definitions. To some it appeared that the classification scheme was designed to limit the number of planets; to others it was incomplete and the terms unclear. Some astronomers argued that location (context) is important, especially in understanding the formation and evolution of the solar system.

One idea is to simply define a planet as a "natural object in space that is massive enough for gravity to make it approximately spherical." But some scientists objected that this simple definition does not take into account what degree of measurable roundness is needed for an object to be considered round. In fact, it is often difficult to accurately determine the shapes of some distant objects. Others argue that where an object is located
or what it is made of do matter and there should not be a concern with dynamics; that is, whether or not an object sweeps up or scatters away its immediate neighbors, or holds them in stable orbits.

The lively planethood debate continues. As our knowledge deepens and expands, the more complex and intriguing the universe appears. Our explorations have really just begun, and there are many questions still to answer.

Additional Resources:

  • What is a Planet? (PDF, 178 KB): Students learn about the characteristics of planets, comets, asteroids, and trans-Neptunian objects through a classification activity.

Last Updated: 24 January 2011

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Last Updated: 24 Jan 2011