Round and Orbity? Must Be a Planet

Times Staff Writer

There goes the solar system.

The elite society of nine lordly bodies of rock, ice and gas would grow to at least 12 and as many as 53 members under a new definition of “planet” proposed Tuesday by the International Astronomical Union.

The core of the definition? Planets are round. And they orbit a star.

The proposal was hammered out after two years of intense astronomical debate among leading experts of the Astronomical Union, the international authority for naming celestial objects.


“We now have a new way to put the solar system together,” said MIT astronomer Richard Binzel, a member of the committee that drafted the proposal. “We think this definition is reasonable.”

It will be voted on next week by the group’s general assembly, which is now meeting in Prague, Czech Republic. Binzel said he was optimistic the definition would be approved.

The new list of planets would include UB313, which was recently discovered beyond Pluto’s orbit, as well as two bodies that have previously been rejected for planetary status: Pluto’s moon Charon and Ceres, the largest object in the asteroid belt between Mars and Jupiter.

Charon and Pluto would become the solar system’s first double planet, meaning they twirl around each other but neither dominates.


They would become part of a new subclass of planets called “plutons,” defined as having orbits around the sun that take at least 200 years.

There could be dozens more plutons added after the objects are further reviewed by the Astronomical Union. There are 12 awaiting evaluation.

Ceres would also become the sole member of a subclass called “dwarf planets.”

Astronomy professor Gibor Basri of UC Berkeley, praised the Astronomical Union for coming up with a definition that could help quell arguments over what makes a planet, a debate provoked by critics who questioned tiny Pluto’s status.


“I feel that they have made the most rational and scientific choices,” he said. “It does mean some adjustment for the public.”

But the definition has riled some astronomers.

Perhaps surprisingly, one of the strongest critics of the new solar system lineup is Caltech astronomer Michael Brown, who discovered one of the proposed new planets. He called the decision an “odd solution.”

“In my book, the word ‘planet’ was special. I liked it back when planets meant something other than it’s round,” said Brown, who has discovered 15 objects that appear to qualify as planets under the new definition, which might make him history’s most prolific planet hunter.


Brown said he had counted 53 objects that appeared to count as planets under the proposal.

It’s also an open question how the new definition would be received by the public, who grew up with mobiles of the nine-planet solar system in their bedrooms and learning mnemonic devices to memorize the planets, such as “My Very Excellent Mother Just Served Us Nine Pizzas.”

Some educators were delighted to learn the solar system might be about to change, considering it a “teachable moment” for students.

“Wow, that’s huge,” said Hilda Ramirez, a spokeswoman for the Los Angeles Unified School District.


The debate over what is and is not a planet was forced on the Astronomical Union by the recent discoveries of a new roster of Kuiper Belt objects orbiting far beyond Neptune.

Among the numerous objects, Brown identified one in 2005 -- UB313 (he nicknamed it “Xena”) -- that appeared larger than Pluto, which has a diameter of 1,400 miles.

The discoveries by Brown and astronomers at the Lowell Observatory in Arizona made it clear that the small, icy world of Pluto, discovered in 1930 by Clyde Tombaugh was hardly unique.

Something had to be done.


Many scientists advocated demoting Pluto, which would have left eight planets -- the four inner rocky worlds of Mercury, Venus, Earth and Mars, and the four outer gas giants, Jupiter, Saturn, Uranus and Neptune.

It was a simple approach, and one that Brown favored. “A gutsy move would be to bite the bullet and say that Pluto should never have been called a planet,” he said.

There was a precedent for this. When Ceres was discovered in 1801, it was classified as a planet. But as more objects were discovered in the asteroid belt, Ceres was demoted.

Reminded that dropping Pluto would dim the shine of his own discoveries, Brown said, “That’s true, but it’s still the right thing to do.”


In the end, the Astronomical Union took a slightly more nuanced approach.

Its proposal reads: “A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.”

The Astronomical Union estimates that a planet would have to be at least 480 miles in diameter to have enough mass to form itself into a round shape. For supporters of Pluto’s planet designation, who had been defending it for years, it was an important victory.

“Yes, Virginia, Pluto is a planet,” Binzel said.


The astronomical group said its decision was not a political compromise to satisfy traditionalists, who could not accept a solar system that snubbed Pluto.

Spokesman Lars Lindberg Christensen defended the decisions as “scientifically sound.”

Binzel said: “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.”

The new definition, if accepted, would also be used for systems orbiting other stars, some of which contain planets far larger than Jupiter.


One question left unsettled by the committee was the maximum size for a planet, some of which are now so large that they are considered brown dwarf stars.



Planets, redefined


The new “planet” definition proposed by the International Astronomical Union:

* Must be round

* Larger than 480 miles in diameter

* Must orbit the sun or another star



A new solar system


A new definition of “planet” could increase the size of our solar system from nine to 12 or more planets.


Under the International Astronomical Union’s proposal, the list of planets would include the asteroid Ceres, and three distant bodies -- including Pluto -- would fall under a new subclass of planets called “plutons”.



*--* Orbital period Avg distance Date of Diameter (Length of (from the sun Discovery (In miles) a year) in miles) Mercury 3,000 88 days 36 million Known by ancients Venus 7,500 225 days 67 million Known by ancients Earth 7,900 365 days 93 million Known by ancients Mars 4,200 687 days 142 million Known by ancients Ceres 592 1,680 days 258 million 1801, Giuseppe Piazzi Jupiter 88,800 11.9 years 484 million Known by ancients Saturn 74,900 29.4 years 886 million Known by ancients Uranus 31,800 84 years 1.8 billion 1781,William Herschel Neptune 30,800 164.8 years 2.8 billion 1846,Johann Galle




*--* Pluto 1,400 248 years 3.7 billion 1930,Clyde Tombaugh Charon 737 248 years 3.7 billion 1978,James Christy UB313 1,400-1,600 560 years 3.5-9 billion 2005, Michael Brown, David Rabinowitz and Chad Trujillo




Sources: Michael Brown, Caltech, International Astronomical Union, NASA