‘Planet’ May Get New Definition, but Pluto Is Still Out

For many people, the definition of a planet was only made clear in 2006, when the spunky world Pluto was demoted from a planet to a dwarf planet. Now, a team of astronomers is pushing for “planet” itself to be redefined to encompass bodies that orbit stars besides our Sun.

The team’s paper is currently hosted on the preprint server arXiv and comes in advance of the International Astronomical Union’s General Assembly next month. The current definition of planet was established at the union’s General Assembly in 2006, back when Pluto failed to meet those standards.

Why are we still talking about Pluto?

To be clear, Pluto is not the main focus of the team’s petition. Rather, the team’s focus is on the thousands of exoplanets—worlds orbiting stars beyond our solar system—that are excluded from the current definition of planet.

NASA has confirmed nearly 6,000 exoplanets in the observable universe, but the agency expects that billions are out there. Exoplanets are intriguing venues for all kinds of research, including questions about planetary evolution, the growth of star systems, and even astrobiology—the search for life beyond our planet.

“All the planets in our solar system are dynamically dominant, but other objects—including dwarf planets like Pluto and asteroids—are not,” said Jean-Luc Margot, a planetary scientist at UCLA and the study’s lead author, in a university release. “So this property can be included in the definition of planet.”

How does the proposed definition of ‘planet’ differ from the existing definition?

In 2006, the IAU famously dropped Pluto from the pantheon of planets because it did not meet the union’s criteria. As stated on the union’s website, a planet in the solar system:

• orbits its host star, just as the Earth and Jupiter orbit the Sun
• is large enough to be mostly round
• must have an important influence on the orbital stability of the other objects in its neighbourhood

The recent team’s definition is broader, and does not specifically apply to bodies in our solar system, a pretty small subdivision of the galactic backyard, much less the universe. Their definition of a planet is one that:

• orbits one or more stars, brown dwarfs, or stellar remnants
• is more massive than (2.2*10²³) lbs (10²³kg) and
• is less massive than 13 Jupiter masses

When some otherwise-planets are too large, like some gas giants, they undergo nuclear fusion, thereby becoming a brown dwarf rather than a planet. That’s why the team put an upper bound on mass in their definition of a planet. And in case you’re wondering, Pluto weighs approximately 2.89*10²² (1.31*10²² kg). The team noted that rogue planets—bodies floating through space, unbound by the gravitational field of a star or similar body—should satisfy the second two criteria that they outlined.

Related article: Astronomers Spot Upwards of 170 Rogue Exoplanets, the Largest Trove Yet

“Having definitions anchored to the most easily measurable quantity—mass—removes arguments about whether or not a specific object meets the criterion,” said Brett Gladman, a planetary scientist at the University of British Columbia and co-author of the paper, in the same release. “This is a weakness of the current definition.”

The definition of planet won’t change immediately

The team is only presenting its revised definition of planet in the meeting next month, so don’t expect a change to occur overnight. But if the team is successful, the thousands of worlds we call exoplanets may simply be called planets.

In a certain light, you could say the move would take us away from an isolationist classification of our own existence; instead of planets just being our cosmic neighbors, planets could refer to any one of the countless worlds in our universe.