At the 26th General Assembly for the International Astronomical Union, held in 2006, a new definition for a “planet” was introduced. It became defined “as 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 assume a hydrostatic equilibrium (nearly round) shape and has cleared the neighborhood around its orbit.”
This same meeting was also responsible for Pluto being stripped of its planetary status. The former ninth planet is now known as a dwarf planet.
Univ. of California, Los Angeles (UCLA) professor Jean-Luc Margot recently presented a simple test that will separate planets from other bodies, such as dwarf and minor planets.
He presented his new method at this week’s American Astronomical Society meeting.
According to Margot, the current system has created a “definitional limbo” for newly discovered bodies outside the solar system, as one of the prerequisites for a planet is that it orbits the sun. Exoplanets are left out of the definition entirely. According to NASA, close to 5,000 planetary bodies orbiting other stars have been discovered.
“One should not need a teleportation device to decide whether a newly discovered object is a planet,” said Margot, who teaches planetary astronomy.
Under the new approach, classification criteria would include the star’s mass, the planet’s mass and orbital period, all of which are attainable from Earth- or space-based telescopes, according to UCLA.
“When a body has sufficient mass to clear its orbital neighborhood, it also has sufficient mass to overcome material strength and pull itself into a nearly round shape,” Margot said.
According to UCLA, the test is easy to implement and could help classify 99% of the known exoplanets.
“The disparity between planets and non-planets is striking,” said Margot. “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.”
When applied to the solar system, the test places the eight planets into a distinct category, and the dwarf planets—Pluto, Ceres and Eris—into another.
A paper regarding Margot’s proposal is forthcoming from the Astronomical Journal.
According to Discover News, Alan Stern, who heads the New Horizons science team, deemed the new test an oversimplification. “I think that there’s pretty clear appreciation among planetary scientists that Pluto is much more similar to the planets of our solar system than anything else,” he said to Discover News. “Astronomers don’t seem to understand that attributes matter.”
The next International Astronomical Union general assembly is scheduled for 2018.