The Ex-Planet Debate
From the beginning, the planet classification of Pluto has been heatedly debated. Many scientists believed Pluto was not technically a planet because it shared more similarities with the icy objects in the Kuiper Belt than with the eight other planets in our Solar System. The International Astronomical Union (IAU), which is the authority on classifying objects in our skies, finally defined the criteria for what constitutes a planet. (You can also view the IAU's press release at this link.)
IAU's Resolution 5A
- A "Planet" is a celestial body that:
- Is in orbit around the Sun
- Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- Has cleared the neighborhood around its orbit
- A "Dwarf Planet" is a celestial body that:
- Is in orbit around the Sun
- Has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
- Has not cleared the neighborhood around its orbit
- Is not a satellite
- All other objects except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies.”
Unfortunately, Pluto has not cleared other objects out of the way in its orbital neighborhood because icy “wrecks” from the Kuiper Belt orbit in Pluto’s neighborhood. Therefore Pluto is defined as a Dwarf Planet.
Pluto's Orbit
Pluto orbits in an ellipse, which is titled upwards at 17 degrees and also rotates on its side. (Uranus also rotates on its side). It takes 248 years for Pluto to orbit the Sun. During this long orbital track, Pluto comes closer to the Sun than Neptune, jokingly referring to Pluto as the Eighth Planet in our Solar System. This period of time is known as perihelion and it recently occurred in 1989. (And it will be around another 230 years until the perihelion happens again for Pluto.) Furthermore, for about 20 years in its orbital rotation, Pluto nestles inside Neptune’s orbit. The last time Pluto was in Neptune’s orbit was from January 23, 1979 to February 11, 1999. Still unlike Earth, which spins on its axis once every 24 hours, it takes Pluto about six Earth days to spin around once on its axis.
The Kuiper Belt
The Kuiper Belt is a vast region beyond Neptune’s orbit, which holds thousands of objects (some thought to be potential planets, some icy wrecks, and even some comets). Gerald Kuiper, who was a Dutch-American astronomer, proposed the existence of “small bodies” in this region sometime in the 1950s. However, there has been a dispute over the naming of the Kuiper Belt. Some scientists call this region the Edgeworth/Kuiper Belt for both Gerald Kuiper and Kenneth Edgeworth, who was an Irish scientist who published the idea of objects existing outside Neptune ten years earlier. These objects within the Kuiper Belt are known as KBOs or Kuiper Belts Objects. (However, to make this more complicated, these objects are also just called the Trans-Neptunian Objects or TNOs). The first discovered KBO (or TNO) was in 1992 by Dave Jewitt and Jane Luu. Today there are one thousand known KBOs, most notably are Sedna, Quaoar, and Eris.
So far there are seven KBOs with diameters between 600-900 miles (1,000 - 1,500 kilometers), which in comparison to Pluto’s diameter of 1400 miles (2300 km) they are really, really small objects. Quaoar belongs to this category with a diameter of 800 miles. Sedna is slightly bigger at 1100 miles and Eris, which started the process of defining planetary status, is bigger than Pluto. Even so, Pluto is unique because it shares characteristics of both KBOs and planets. For instance, Pluto is the only KBO, whose orbit takes it out of the Kuiper Belt and into Neptune’s orbit. However, while Pluto and Charon share a planet/moon relationship, Pluto is not much larger than Charon. And, Pluto and Charon are a binary system, which is common with other KBOs. In the end, Pluto is the brightest KBO, which accounts for why it was the first discovered object and named as a planet while the next KBO was not discovered until 1992 (not counting Charon in 1978).
More Resources
I.) The National Space Science Data Center (NSSDC), "Pluto." NASA Lunar and Planetary Science. http://nssdc.gsfc.nasa.gov/planetary/planets/plutopage.html (accessed 27. November, 2006).
The NSSDC features general information on Pluto as well as current press releases from NASA about this dwarf planet. There is also of interest a link to the 1999 press release from the IAU about Pluto’s planetary status.
II.) Buie, Marc W. "Buie: Pluto IS a Planet." Lowell Observatory. http://www.lowell.edu/users/buie/pluto/planet.html (accessed 1. December, 2006).
Marc Buie's website responds to the IAU's recent planetary decision regarding Pluto and voices his opinions on why Pluto is still a full fledged planet.
III.) Alexander, Amir. "And Then There were Eight: IAU Vote Shakes Up the Solar System ." The Planetary Society. http://planetary.org/explore/topics/our_solar_system/pluto/new_solarsystem.html (accessed 3. December, 2006).
Amir Alexander's article from The Planetary Society, discusses Pluto's demotion to a dwarf planet and the harsh reality that eight planets is enough even though more would be planets (both larger and smaller than Pluto) are being discovered.
IV.) NASA, "Hotly-Debated Solar System Object Gets a Name." http://www.nasa.gov/vision/universe/solarsystem/erisf-20060914.html (accessed 4. December, 2006).
This website looks at the controversy surrounding the discovery and final naming of 2003 UB313 to Eris.
The orbits of the planets and the Kuiper Belt pictures are the property of the New Horizon's Website.