What Makes a Planet? New Classification for Exoplanets and Planets

First Posted: Nov 12, 2015 08:11 AM EST

What is a planet? What causes a world to be a planet, a dwarf planet or an exoplanet? Since the late 1980s, researchers have discovered nearly 5,000 planetary bodies orbiting stars other than the sun. Now, scientists have defined what should be called a planet and what shouldn't be.

The current official definition of a planet applies only to bodies in our solar system. However, this has created a "definition limbo" for the newly discovered bodies outside our solar system. Now, researchers propose to extend the planet definition to all planetary systems.

The new approach would actually only require estimates of the star's mass and the planet's mass and orbital period. All of these things can easily be obtained with Earth- or space-based telescopes. According to the criteria, all eight planets in our solar system and all classifiable exoplanets would be confirmed as planets.

The current definition is based primarily on the ability of a planet to "clear its orbit." This means whether it can evacuate, accumulate or dominate small bodies in its orbital neighborhood. This newly created test can be used to determine whether a body can clear a specific region around its orbit within a specific time frame, such as the lifetime of its host star. The test is easy to implement and it could immediately classify 99 percent of all known exoplanets.

"The disparity between planets and non-planets is striking," said Jean-Luc Margot, the scientists who came up with the new system, in a news release. "The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature."

The findings are published in The Astrophysical Journal.

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