Although the word planet is used often in astronomy, the term “planet” was not officially defined until 2006. Before the 1990s, only nine planets were known anywhere in the universe. They all reside in our little solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

The inner planets, Mercury, Venus, Earth, and Mars, were classified as terrestrial planets while, the outer planets, Jupiter, Saturn, Uranus, and Neptune were considered as gaseous planets.

Pluto, which at the time was the furthest planet from the Sun, was not included in either group due to its small size and isolated location within the Kuiper Belt—a ring of frozen, rocky objects between the orbit of Neptune and extending past the orbit of Pluto.

Pluto stayed a planet because there was really no need to change anything. However, the astronomers began questioning Pluto’s status near the end of the twentieth century when many smaller bodies—what are called trans-Neptunian objects—were discovered past the orbit of Neptune. In addition, hundreds of exoplanets (planets orbiting stars other than the Sun) were found to exist, too.

These discoveries inside and out of our solar system added a great diversity of sizes and characteristics to the term planet. Some bodies were as small as the Moon and other were as large as stars. In 2005, a body called 2003-UB-313 (now called Eres) was found outside Neptune’s orbit. It was presumed to be larger than Pluto—but no definite measurements had been made on it.

Astronomers decided it was time to define the term planet. Thus, on August 24, 2006, members of the International Astronomical Union (IAU) officially defined a planet as any “celestial body that: is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighborhood around its orbit.”

Because of this definition, Pluto was removed from its classification as being a planet. Instead, Pluto was called a dwarf planet—“a celestial body that: is in orbit around the Sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, has not cleared the neighborhood around its orbit, and is not a satellite.”

Recently, California Institute of Technology astronomers Michael Brown (who discovered Eris) and Emily Schaller measured Eres with respect to its moon Dysnomia with the use of NASA’s Hubble Space Telescope and the Keck Observatory in Hawaii. Based on various physical characteristics, the CalTech team found that Eris is about 27% more massive than Pluto—about 16.6 billion trillion kilograms for Eris and 13.1 billion trillion kilograms for Pluto.

In addition, Eris was found to have a longer diameter than Pluto: 2,400 kilometers for Eris versus 2,320 kilometers for Pluto.

Ceres has a diameter of only about 975 kilometers and has a mass of about 95 thousand trillion kilograms.

Therefore, based on these results, Pluto is now the second largest and second most massive dwarf planet in the solar system—bigger and more massive than Ceres, but smaller and less massive than Eris.

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