Definition of 'Planet' Expected in September

Historians and educators have joined astronomers in an effort to break a deadlock on contentious discussions over a definition for the word planet.

A decision is expected in September, but history suggests rewriting the textbooks could be more challenging than finding tiny new worlds at the edge of the solar system.

The International Astronomical Union (IAU) is expected to propose wording to delineate planets from other small, round objects at its 12-day General Assembly meeting in Prague this August. The proposal will be based on recommendations from a newly formed committee that includes experts outside the realm of astronomy tasked to break a deadlock in earlier committee discussions.

Depending on the outcome of a separate controversial procedural issue-whether IAU members should be allowed to vote on such things-astronomers might then have the chance to weigh in on the definition later in the same meeting, has learned.

If approved, the definition would then be announced in September.

Rewrite the textbooks ...

Some might think it ironic that the world's governing body for astronomy does not have a definition for planet.

The problem stretches back to the late 1990s, when astronomers began discovering Pluto-like objects in the distant reaches of our solar system.

All the newfound worlds-there are several known now-were until recently smaller than Pluto, but they are round and orbit the Sun, two characteristics that had for centuries been sufficient for the implicit definition of planet. The hitch: These small objects are typically on wild, elongated orbits that stretch well above and below the main plane of the solar system where eight of the traditional planets travel (Pluto has a wild orbit, too, which is one reason many astronomers do not consider it a planet anymore).

So what to call them? Astronomers have been arguing about it in earnest since 1999.

The controversy came to a head with the July 2005 announcement of 2003 UB313, an object roughly the size of Pluto that orbits the Sun beyond Neptune. The object's discoverer, Mike Brown of Caltech, has argued it should be called a planet.

But other astronomers say that if planethood is bestowed upon 2003 UB313, then several similar way-out bodies should gain the same status, and the number of planets in our solar system could ultimately climb into the thousands as search technology improves.

Still waiting ...

The IAU had deferred judgment until it could come up with a definition.

That process was debated in an IAU committee for more than a year. But the dozen or so astronomers on the committee could not agree whether to define planet strictly by mass, or to consider orbital characteristics as well as how and where a planet formed, among other things. Last fall they argued over possibly putting adjectives in front of planet, such as gas giant, terrestrial, asteroidal and perhaps even traditional or historic to grandfather Pluto into the family of "regular" worlds. Those talks broke down, however.

Recently, the issue was handed off to a new committee that includes historians and educators, said Alan Boss, a planet-formation theorist at the Carnegie Institution of Washington who was on the first committee.

"They wanted a different perspective from that of planetary scientists," said Edward Bowell, an astronomer at Lowell Observatory who is also vice president of the IAU's Division III-Planetary Systems Sciences group.

Uncertain future

Neither Bowell nor Boss knows what exactly might happen next, however. Nor does Brian Marsden, leader of the Minor Planet Center where newfound objects are catalogued. Marsden was also on the first definition committee.

"The new committee is supposed to recommend what 'should be done' about Pluto, 2003 UB313 and other 'largish' small bodies, but it is not clear that what they decide will depend on mass," Marsden told

Marsden said it's also unclear how the IAU will reach an ultimate resolution. 

"The IAU made the interesting policy decision in 2003 to disenfranchise its members, and they were therefore not allowed to vote on 'scientific matters' (such as what a planet is) at the last plenary General Assembly session at the Sydney meeting [in 2003]," Marsden said. "There are rumors that there may be an administrative decision to re-enfranchise us at the first of the upcoming plenary sessions this August in Prague-so that suggested vote might be possible at the second."

That is indeed the plan, IAU General Secretary Oddbjorn Engvold explained yesterday. The advisory Committee is scheduled to meet June 30, Engvold said by email.

"Their proposal and advice will be forwarded to the IAU Executive Committee, who will present the matter for decision at the IAU General Assembly in Prague," Engvold said. "Assuming that the proposed change in voting rules will be accepted at the first session of the General Assembly, all IAU members will be allowed to vote on all scientific issues at this General Assembly."

IAU officials appear to have some confidence this will all work out. A statement on the IAU web site states: "The IAU will publish beginning of September 2006 the definition of a 'Planet.'"

Broad definition possible

Astronomers, meanwhile, are eager to know if the definition will include just mass, which would likely mean 2003 UB313 and eventually hundreds of other worlds will be added to the list of original nine planets, or if it will exclude those worlds by defining planets as being also in somewhat circular orbits or some other qualifier.

Might the definition go beyond mass, to include orbit characteristics and formation scenarios?

"Yes," IAU President Ronald Ekers told "The scope of the definition may include all these aspects."

  • All About Planets
  • Large Planets Undefined, Too
  • Gallery: Casting Our Solar System In a New Light
  • Gallery: The Solar System - Sun and Nine Planets
  • Video: Interplanetary Traffic Jam
  • Video: Planet Hunter

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at:

Robert Roy Britt
Chief Content Officer, Purch

Rob has been producing internet content since the mid-1990s. He was a writer, editor and Director of Site Operations at starting in 1999. He served as Managing Editor of LiveScience since its launch in 2004. He then oversaw news operations for the's then-parent company TechMediaNetwork's growing suite of technology, science and business news sites. Prior to joining the company, Rob was an editor at The Star-Ledger in New Jersey. He has a journalism degree from Humboldt State University in California, is an author and also writes for Medium.