Saturday science subject: What makes a planet?

The solar system has been known to have nine full-fledged planets for a while—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto—but this list could soon be extended to include three other bodies. The International Astronomical Union (IAU) will vote next week on a new definition of the term "planet" that could turn Pluto's moon Charon, the asteroid Ceres, and "trans-neptunian" object 2003 UB313 into our solar system's fifth, eleventh, and twelfth planets, respectively. According to the proposed definition, a planet would be "any body in orbit around a star that is not a star itself nor in orbit around a much larger planet, and that is massive enough for gravity to have squished it into an approximately spherical shape."

Object 2003 UB313 in particular prompted the new definition, because it is currently not considered a planet despite orbiting the Sun and being larger than Pluto. However, some astronomers believe the definition proposed by the IAU is flawed. California Institute of Technology Astronomy Professor Michael Brown in particular contends that objects inside the Kuiper belt with radii as little as 200-400 km can be round because of the large amount of ice they contain (for reference, Pluto has a radius of around 1,150 km, and the Earth's moon has a radius of 1,737 km.) Brown believes that if the IAU's proposed definition is accepted, the addition of objects from the Kuiper belt could bump the solar system's planet count to 53.

To avoid such a situation, an IAU committee has proposed dubbing trans-neptunian planets that take more than 200 years to orbit the Sun "plutons." Those planets would include Pluto, Charon and 2003 UB313.

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