The definition of a planet and whether Pluto is one or not is a controversial subject. The parameters under consideration include:
Since 1991, Gibor Basri of the University of California, Berkeley, has been working on a formal definition of planets to propose to the IAU.
Basri would like to accommodate Pluto and those who can't fathom its demotion. He proposes that the lower limit for a planet gets set at a diameter of about 435 miles (700 kilometers). That's roughly the mass needed to allow gravity to shape an object into a sphere, depending on density. Smaller objects - both asteroids and comets -- tend to look like potatoes or bell peppers. If the size cutoff were accepted, 3 Kuiper Belt Objects - Sedna, Varuna and Quaoar - would immediately become planets and more would almost certainly be discovered in the future. Worse, the asteroid Ceres would also have to be reclassified as a planet under Basri's plan. Ceres is 930 kilometers (580 miles) wide.
Imke de Pater and Eugene Chiang of Berkley University would prefer to remove Pluto completely. "I would say a planet is a body in orbit about a star, but not forming part of a larger swarm, like the asteroids in the asteroid belt or the Kuiper Belt Objects," de Pater said. "A planet also would have to be in a stable orbit for a few billion years - it shouldn't be a KBO in transit to becoming a comet."
NASA say "We define a planet to be any body in the solar system that is more massive than the total mass of all of the other bodies in a similar orbit. For example, many asteroids cross the orbit of the Earth. Yet the Earth is more massive than all of those put together. Thus, the Earth is a planet. Ceres, the largest asteroid, is not greater in mass than the sum of the masses of the remaining asteroids. Hence, not a planet. Pluto sits squarely in the Kuiper belt, yet is not more massive than the total of the other Kuiper belt objects. Thus -- like Ceres -- Pluto is no planet, just the largest object in its class. "
A alternative definition promoted by astronomers is that anything in the solar system that is made round by its own gravity should be considered a planet. The definition takes the solar system from 9 planets to hundreds of planets, when you include all of the asteroids, satellites (the moon!), and Kuiper belt objects that are round.
Objects beyond Neptune (which lies at 30AU) are known as Trans-Neptunian Objects and were predicted to exist by both Thomas Edgeworth in 1949 and Gerard Kuiper in 1951. The vast majority are icy bodies analagous to comets (dirty iceballs). Although the most conform exactly to the prediction by Edgeworth and Kuiper lying beyond 40 AU from the Sun, a third of TNOs lie in resonance with Neptune - their orbital period is an integer fraction of the planet. They pass quite close to the orbit line of Neptune but never at a time when Neptune is actually near. The most common orbit observed lies at a mean distance of 39AU from the sun with a small eccentricity, a large inclination and an orbital resonance of 3:2. a resonant period of two orbits for every three of Neptune.
Pluto is a TNO that lies at 39AU with an orbit exactly as described in the previous sentence i.e. Pluto's orbit is controlled by Neptune's gravitational presence by being pulled twice around the sun for every three trips made by Neptune. It is therefore more accurately described as a the largest of the TNO population, rather than a planet and is the reason that this class of objects are often termed 'Plutinos'. Pluto is 1430 miles in diameter, only 2/3 the diameter of our Moon.
Sedna, the second largest TNO, was discovered in March 2004. It is 3 times further away from the Sun than Pluto, travels in an elliptical orbit and has a diameter of 800-1100 miles across.
In 1999 the International Astronomical Union, the body responsible for the naming of celestial objects, issued a press release stating its regret that "incomplete or misleading" press reports on the status of Pluto " appear to have caused widespread public concern." The statement went on to assure that "no proposal to change the status of Pluto as the ninth planet" had been made within the IAU. that Pluto should remain a planet even though it shares a number of characteristics with the newly discovered category of solar system bodies being called Trans-Neptunian Objects".
This was the official proclamation of the compromise argument which currently holds sway and which is populist rather than accurate. It says that the public would be confused and it would be unpopular to change Pluto's status from a planet so it should be left as the ninth planet but it should scientifically be kept in mind that it's really a KBO and we shouldn't increase the count of planets in our solar system beyond 9 even if further KBOs are found that compromise this position.
An IAU working group has come to some agreements. A planet is something that orbits a star but does not, like a moon, orbit another planet. That definition has proved inadequate in the face of new discoveries. In recent years, astronomers have found dozens of gas giant planets that are much more massive than Jupiter. Though they are not quite heavy enough to jumpstart the thermonuclear fusion that powers a real star, these gargantuan objects strain the definition of planets as we once knew them. Confusion results in part because astronomers had, back in the mid-1990s, agreed on a loose definition for middleweight objects called brown dwarfs. These failed stars are not massive enough to ignite hydrogen, but they do radiate more than Jupiter. No mass cutoff was set to distinguish between large planets and small brown dwarfs.
Basri suggests that the size limit of a planet be set at 13 times the mass of Jupiter, or roughly 4,000 Earth masses. Anything bigger can cause deuterium to fuse in the object's core, generating the sort of heat and low-level light typically associated with brown dwarfs. Geoffrey Marcy, University of California, Berkeley, co-discover of a 17-Jupiter-mass object, disagrees with Basri. He says an additional factor must be considered: How did the object form? If a large object condenses into being at the same time a companion star forms, then the object might be called a brown dwarf, Marcy suggests. But if the object forms later, out of the detritus of star formation (as did all the planets in our solar system) then it deserves consideration as a planet, regardless of mass. Marcy thinks it's too early to commit to any firm taxonomy. "It's way too early to define a planet," Marcy said. "No one would have predicted 10 years ago that we'd have any extrasolar planets. Even though we have now found more than 100 of them, these are still the early days in planet hunting."