University of Toronto Magazine University of Toronto Magazine

What Is a Planet?

Recent discoveries force astronomers to rethink long-held definition

The flood of planetary discoveries over the last 10 years has given astronomers more than just a treasure of exotic worlds to study. It’s forcing them to rethink the definition of the word “planet.”

Some of the massive extra-solar planets are so large that astronomers wonder if they belong in the same category as other giant gas planets, such as Jupiter. They seem to have more in common with brown dwarfs, which are cool stars that emit only feeble amounts of light (mostly at infrared wavelengths).

At the same time, astronomers are questioning the status of Pluto, long considered the outermost planet in our solar system. Recent discoveries of objects of a similar size, in orbits just as far from the sun, suggest that the number of planets in our solar system may not be nine, but 10 or more. Most of these new objects lie within a ring of rocky and icy debris known as the Kuiper Belt, which extends from the orbit of Neptune – a distance of 30 Astronomical Units (AU) from the sun – out to at least 100 AU. (One AU is the average distance from the Earth to the sun, or about 150 million kilometres.) Some astronomers argue that the discovery of these small, distant planet-like objects simply makes Pluto less unique. Others contend that these objects – including Pluto – don’t deserve the moniker of planet at all. They would include Pluto in a relatively new class of body within our solar system called “trans-Neptunian objects” or, simply, “planetary bodies.”

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