Pluto is a planet, but one of 12?

Astronomers consider embracing Pluto's moon, the largest asteroid, and an icy body nicknamed Xena as planets in their own right.
By | Published: August 16, 2006 | Last updated on May 18, 2023
Pluto and Charon
In this artist’s concept, Pluto and its moon Charon are seen from the surface of one of Pluto’s newly discovered candidate satellites.
David A. Aguilar (Harvard-Smithsonian Center for Astrophysics
The new solar system?
A new definition of “planet” would let Pluto keep its status — and add the asteroid Ceres, Pluto’s moon, and 2003 UB313 to the list.
IAU/Martin Kornmesser
August 16, 2006
If the world’s astronomers agree to a plan announced this morning, our solar system would have 12 planets, not 9. The proposal stems from a panel’s 2-year effort to define what, exactly, the word “planet” means.

“Our goal was to find a scientific basis for a new definition of ‘planet’ and we chose gravity as the determining factor. Nature decides whether or not an object is a planet,” says the Massachusetts Institute of Technology’s Richard Binzel, a member of the group proposing the definition.

Under the plan, a planet would be any object that orbits a star, is neither a star itself nor the moon of another planet, and contains enough matter that gravity forces it into a nearly round shape. Pluto, whose planetary status some astronomers have questioned in recent years, meets these requirements. But if astronomers approve the proposal, three objects not now classed as planets would also join the club: Ceres, the largest asteroid; Pluto’s moon Charon; and the Kuiper Belt object nicknamed Xena.

“I feel that they have made the most rational and scientific choices — namely ones which are physically based and can be most readily verified by observations,” says Gibor Basri, an astronomer at the University of California at Berkeley.

“I’m pretty happy with it,” says Alan Stern, principal investigator on the recently launched New Horizons mission to Pluto.

The proposal comes from a seven-member panel the International Astronomical Union (IAU) charged with defining where to draw the line between planets and the solar system’s numerous smaller bodies. Some 2,500 astronomers now gathered in Prague for the IAU’s General Assembly meeting will vote August 24 on whether the organization should accept the new definition. The IAU is the scientific body empowered with classifying and naming space objects.

“I’m impressed. This group really did a very creative job,” Alan Boss, an astronomer at the Carnegie Institution of Washington, tells Astronomy. “It’s very controversial.”

Name game
Everyone agrees that the largest worlds circling the Sun — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune — make the cut in any planet definition. But lowly Pluto, less than 1/400th Earth’s mass, has been a source of contention for years. When Clyde Tombaugh discovered Pluto in 1930, astronomers estimated it might be as massive as Earth. But with ever-improving observations, Pluto looked less and less like it belonged in the club.

Since the 1990s, astronomers have discovered hundreds of icy objects that, like Pluto, course through the fringes of the solar system in a zone astronomers call the Kuiper Belt. Many began regarding Pluto as little more than the region’s largest object.

Then came Xena.

A team led by Caltech astronomer Michael Brown discovered the object — officially dubbed 2003 UB313 but better known by its TV-inspired nickname — in January 2005. Xena is slightly larger than Pluto. It seemed reasonable that If Pluto is a planet, and Xena is bigger than Pluto, then Xena must be a planet, too.

But the IAU decided not to approve a formal name for Xena until the object’s planetary status could be determined. This meant waiting for the panel’s proposed definition and its acceptance by the astronomical community.

“I credit the IAU with trying, but I think they sort of blew it on this one,” says Brown. He explains that potentially 50 objects in the Kuiper Belt could be large enough to meet the panel’s roundness criterion.

“What’s the value of [a definition] when you add on another 50 bodies? I think it sort of cheapens the word ‘planet,'” says Boss.

Planet Charon?
Under the new plan, Charon, regarded as Pluto’s moon since its 1978 discovery, becomes a planet itself. Charon is so large relative to Pluto that the pair’s center of mass lies between the two worlds. For this reason, astronomers frequently refer to Pluto and its outsize satellite as a “double planet.” If approved, the new definition would formalize this idea.

“For me, Charon as a planet doesn’t really pass the smell test, but I think that is simply a gut reaction,” Brown tells Astronomy. He would have preferred a definition that takes away Pluto’s planetary status and recognizes only eight planets. “Scientific decisions should be based on science, not sentiment,” he says.

Plutons and dwarf planets
“[O]ur committee felt that the time was ripe to recognize Pluto as the prototype of a different sort of planet,” writes Owen Gingerich, who chairs the IAU panel. “Consequently, we propose to distinguish between the eight classical planets discovered before 1900, and a new class of trans-neptunian objects, for which we recommend the name ‘plutons.'”

So, while Pluto is a planet under the proposed definition, Gingerich notes, “[I]t will generally be preferable to call it a pluton to emphasize its role as the prototype for a physically distinct category of planetary bodies.”

The panel’s proposed definition also crowns Ceres, the largest asteroid, as a planet and warns that others — notably Pallas, Vesta, and Hygeia — may follow if further study proves them nearly round. The committee suggests astronomers apply the informal term “dwarf planet” to Ceres and other asteroids that meet the definition’s requirements for planethood.

“It does mean some adjustment for the public — particularly the inclusion of Ceres as a planet again,” Basri notes. Ceres, the first asteroid discovered, was originally thought to be a new planet, but astronomers quickly found other bodies in similar orbits between Mars and Jupiter.

“This is going to be the talk of Prague this week and the next,” says Boss. “There’ll be a lot to discuss.”