What is a planet?


Senior Member
Oct 1, 2003
What Is a Planet? Debate Forces New Definition
By Robert Roy Britt
Senior Science Writer
posted: 07:00 am ET
02 November 2000

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.

What do you think?

If you want to know my opinion, they truly believe pluto is not a planet
I dont know to believe!


Junior Member
Dec 13, 2003
So what else do ya'll think of in Vermont? Isn't that where the Bob Newhart show was supposed to be filmed at? I was trying to think of how to say the location was supposed to be up north. My favorite was "HI I'm Larry and this is my brother Daryl and my other brother Daryl".