UCLA professor of planetary astronomy Jean-Luc Margot describes a simple “planet test” that can be readily applied to bodies orbiting the Sun and other stars. According to this new test, all eight solar system planets and all classifiable exoplanets are confirmed as planets. Margot presented his classification tool at the American Astronomical Society’s 47th Annual Meeting of the Division for Planetary Sciences.
The official definition of a “planet” applies only to bodies in our solar system, which has created a “definitional limbo” for thousands of planetary bodies orbiting stars other than the Sun, Margot said. In a paper accepted for publication in the Astronomical Journal, he proposes to extend the planet definition to all planetary systems.
Nine years ago, the International Astronomical Union (IAU) defined what it takes to be a planet, but left the classification of exoplanets for future consideration. The recent flood of exoplanet discoveries, now numbering close to 5,000, requires an extension of the current limited definition, argues Margot, who encourages the IAU to refine and extend its current definition.
The IAU definition of a planet is based primarily on the ability of a planet to “clear its orbit” — to evacuate, accumulate, or dominate small bodies in its orbital neighbourhood. Margot derived a simple test that can be used to determine whether a body can clear a specific region around its orbit within a specific time scale, such as the lifetime of its host star. The test is easy to implement and allows immediate classification of 99 percent of all known exoplanets.
His proposed criterion requires only estimates of the star mass, planet mass, and orbital period, all of which can be easily obtained with Earth-based or space-based telescopes. Ease of implementation was an essential driver for the new classification tool. “One should not need a teleportation device to decide whether a newly discovered object is a planet,” Margot said.
When applied to the solar system, the test divides bodies into two dramatically different groups.
“The disparity between planets and non-planets is striking,” Margot said. “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.”
Margot also found that bodies that can clear their orbit are expected to be round. This finding is important because the IAU definition currently requires that a planet be nearly round, which is difficult or impossible to verify for exoplanets.
“When a body has sufficient mass to clear its orbital neighbourhood, it also has sufficient mass to overcome material strength and pull itself into a nearly round shape,” Margot said. This means that the IAU planet definition could be simplified by eliminating the superfluous roundness requirement that is difficult to verify observationally.
It is not known whether the new classification tool will be considered by the IAU, whose resolutions are typically crafted and reviewed by committees before possible adoption by IAU members during a general assembly. The next IAU general assembly will be held in Vienna in 2018.