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Traditional views of what makes a celestial body a planet, in place for centuries and defined almost entirely by the nine with which we're most familiar, have become thoroughly antiquated in five short years as a host of new objects have been discovered.
And so the word "planet" will be redefined by the world organization authorized to do so, SPACE.com has learned. The change could come as early as mid November.
In fact, the International Astronomical Union (IAU), charged with classifying heavenly objects, has never had a definition on record for planets. Never needed one. Everyone instinctively knew what a planet was.
But starting in 1995, discoveries of huge planets around other stars, plus new objects that are neither planet nor star, have forced the IAU to draw some distinctions. The movement gained momentum in recent weeks with the announcement of free-floating objects in space that look like planets.
Exoplanets and brown dwarfs
Since the first planet was discovered orbiting another star in 1995, more than 50 extrasolar planets, or exoplanets, have been found. These planets are nothing like what we're used to. They are huge -- often many times the mass of Jupiter -- and some are altogether more like another class of object, the brown dwarf.
Suspected since 1963 and confirmed to exist in 1995, brown dwarfs are failed stars. Though huge, they never grew massive enough to initiate the thermonuclear fusion that drives a real star. Yet most brown dwarfs are enormous compared to the planets in our solar system. They can be up to 75 times as massive as Jupiter.
While a planet emits no visible light, a brown dwarf emits just enough to be detected. Yet some appear very planet-like. They can have a diameter closer to Jupiter's and, frequently, orbit a star just as a planet would.
Adding to this confusion are the objects that freely drift though space. A batch of 18, revealed in October, have been called planets. But this designation is totally counter to the notion that a planet is something that orbits a star.
Though the free-floaters might be brown dwarfs, they have the light signatures of planets, and they are just five to 15 times as massive as Jupiter, a size range typically thought to be planetary.
"We are beginning to see this whole series of objects that we were not able to detect before, and it's completely changing our ideas of planetary formation and the mass of the objects we find," said Morris Aizenman, a senior science associate in the Mathematical and Physical Science Directorate of the National Science Foundation.
Aizenman and others say planetary and stellar sciences are undergoing a revolution. The definition of a planet, meanwhile, is crumbling under the weight of discovery.
Problems big and small
At the other end of the size spectrum, tiny Pluto should never have been called a planet in the first place, most mainstream scientists agree.
Pluto is less than half the size of any other planet, and its orbit is at a distinct angle from the plane in which the other planets travel around the Sun. Most significant, Pluto roams so far beyond the orbit of Neptune that researchers say it is part of the Kuiper Belt, a region of distant, frozen rocks only confirmed to exist in 1992.
"If we'd known about the Kuiper Belt when Pluto was discovered [in 1930], it would have been a giant Kuiper Belt Object," said Michael A'Hearn, a University of Maryland astronomer and past president of the IAU's Planetary Systems Sciences division.
In early 1999, the IAU wrangled over giving Pluto dual status -- both as a planet and as a Trans-Neptunian Object, reflecting its distant location. The plan was dropped after a public outcry led to hundreds of e-mails to the IAU.
Collectively, the smaller bodies in the solar system, including comets and asteroids, have come to be called minor planets (a term that is also being debated). A'Hearn said the all the definitions are fluid for good reason: Scientists are still figuring out what the objects are.
Even in our own backyard. A'Hearn and others say it's reasonably likely that another object as big or larger than Pluto will be found orbiting the Sun even farther out. What happens then?
"Start the fight over again," A'Hearn said.
Textbook definition? Good luck
Finding a textbook definition for the word "planet" is tricky. Amazingly, many science and astronomy books -- just like the IAU -- don't define the word. You won't find a basic definition in the New York Public Library's Science Desk Reference, for example. And in the 1999 edition of Universe, a comprehensive tome used widely in college courses, "planet" is not even an entry in the 21-page glossary.
How could this be?
FUTURE SPACE
Alan Shephard became the first American in space 40 years ago. Check out SPACE.com's Special Report on Friday on the historic flight. More
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