It’s official: IAU demotes Pluto

The world's astronomers have removed Pluto's planetary status.
By | Published: August 24, 2006 | Last updated on May 18, 2023
Mapping Pluto
This is our best look at Pluto to date. Scientists mapped the planet using the Hubble Space Telescope. This view is centered on the Pluto hemisphere that always faces Charon.
The new solar system?
A new definition of “planet” would let Pluto keep its status — and add the asteroid Ceres, Pluto’s largest moon, and 2003 UB313 to the list.
IAU/Martin Kornmesser
August 24, 2006; updated August 25
Astronomers meeting in Prague have decided by a wide margin that Pluto no longer merits consideration as a full-fledged planet. The historic decision follows a week of sometimes contentious debate over how to define a planet.

Xena discoverer Mike Brown; Alan Stern, lead scientist on the New Horizons mission to Pluto; and Patricia Tombaugh, widow of Pluto’s discoverer, react to the decision in Astronomy‘s podcast.

Jocelyn Bell Burnell, an expert on neutron stars at the University of Oxford in England, moderated the proceedings with the help of some playful visual aids. A blue toy balloon stood in for Neptune, while a plush toy of Disney’s cartoon dog Pluto played the ninth rock from the Sun. Applause followed this morning’s critical vote at the International Astronomical Union’s (IAU) General Assembly meeting.

“I was relieved,” says Caltech’s Michael Brown. Brown’s January 2005 discovery of Xena (officially, 2003 UB313), which is larger than Pluto, forced the IAU to address this long-simmering issue. “Scientifically, there is no question this is the right way to go,” he says. “Eight is enough.”

“I am delighted that rationality has prevailed,” says Richard Conn Henry of Baltimore’s Johns Hopkins University.

However, not everyone is happy with the decision. The American Astronomical Society ‘s 1,300-member Division of Planetary Sciences — the world’s largest group of planetary scientists — recommended acceptance of the original proposal announced last week. If passed, this would have retained Pluto’s status and declared three other bodies — Xena, the largest asteroid, and Pluto’s largest moon — planets as well.

“I just think the IAU has embarrassed itself,” says Alan Stern, principal investigator for NASA’s New Horizons mission to Pluto. “If you read the definition that they have adopted in that room today, it is scientifically indefensible.”

“I think there’s going to be a protest,” says Mark Sykes at the Planetary Science Institute in Tucson. “A minority of the astronomical community passed this.”

So, what is a planet?
Following fractious public and closed-door discussions, Pluto no longer makes the cut. Astronomers agreed to define a planet 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 assumes a … nearly round shape, and has cleared the neighborhood around its orbit.”

Objects that meet all but the last requirement — like puny Pluto and asteroid Ceres — are to be classed as “dwarf planets.” But a proposal that would give these objects and the “classical” planets — Mercury through Neptune — equal solar-system stature failed.

Everything else in the solar system — comets, moons, and the rest of the asteroids — will be known as “small solar system bodies.”

“Tell me where else in astronomy we classify objects by what else is around them?” objects Stern. “It’s ridiculous.”

“I think there will be a lot of people who just choose to ignore it,” Sykes adds.

Nature vs. nurture
A week ago, the International Astronomical Union (IAU) presented its first attempt to define the word planet. If an object orbited the Sun and had enough mass that its own gravity forced it into a nearly round shape, it would qualify. If passed, this meant Charon, Pluto’s largest moon; Ceres, the largest asteroid; and the Kuiper Belt object nicknamed Xena would immediately join the rank of planets.

But at least a dozen additional objects orbiting in the Kuiper Belt beyond Neptune also would qualify under this definition — and possibly as many as 53. “Many more Plutos wait to be discovered,” Richard Binzel of the Massachusetts Institute of Technology noted.

Many astronomers found this potential explosion of planets troubling. “Maybe planets shouldn’t be so special,” Sykes counters.

Others said that pushing Pluto’s moon Charon to planetary status went too far. “There was a little over-reaching with this thing about ‘plutons’ and making Charon a planet,” Stern says, “but the core … was all pretty good.” The term proposed for Pluto-like objects — “pluton” — was roundly rejected in early discussions, largely owing to its well-established use in geology.

But scientists who study how planets interact were the most perturbed. The original definition looked only at an object’s physical nature without regard to its orbital environment.

During Friday’s debate, Julio Fernández of the University of the Republic in Montevideo, Uruguay, suggested an alternative that defined a planet as “by far the largest body in its local population.” This would keep Pluto, Charon, Ceres, and the Kuiper Belt objects from graduating to planets.

Debate continued Tuesday when the IAU’s planet-definition committee put forward only a slightly modified version of the original definition — one that avoided any reference to a planet’s orbital environment. “They have presented practically the same resolution as before,” Fernández complained during the discussion. Astronomers hammered out a compromise in closed-door negotiations that afternoon.

“They’ve put together a hastily drafted definition that’s purely dynamical,” explains Sykes. “Perhaps there could have been a more dynamical definition that achieved their goal that would be better crafted, but unfortunately they just didn’t have the time.”

Sykes argues that, under the adopted definition, a Mars-mass planet discovered at 200 astronomical units would be classed as a dwarf. (An astronomical unit is the average distance between Earth and the Sun.) “There are some common-sense issues with this,” he adds.

“I think the demotion of Pluto into the realm of other minor objects outside the orbit of Neptune is the most consistent thing to do to straighten out the nomenclature of our solar system,” says William Blair at Johns Hopkins University. “However, I don’t find the wording of the official planet definition to be very clear, and hence it will continue to be open to interpretation.”

“Pluto should never have been called a planet,” Brown says. “The only reason Pluto exists in its current orbit is that Neptune keeps it there and protects it.” Neptune forces Pluto — and many other icy objects, called plutinos — to go around the Sun twice for every three Neptune orbits.

“Pluto and Xena never fit in,” Brown notes, although he admits he’s wistful about missing the opportunity of having discovered the tenth planet. “We have a chance now to actually educate people on how the solar system really works. I think that’s exciting.”