Nine's a charm

The International Astronomical Union announces that Pluto is still a Planet

Feb. 4, 1999: Pluto, the tiniest planet in the solar system, will remain a planet the International Astronomical Union announced yesterday. In recent weeks rumors had it that astronomers were about to vote to declassify Pluto as the ninth planet and re-categorize it as a "Trans-Neptunian Object" (TNO). In a press release dated February 3rd, the IAU denied that Pluto was about to be demoted. Instead, "a working group under the IAU Division of Planetary Systems is conducting a technical debate on a possible numbering system for TNO's," the press release said. Pluto might be included in that numbering scheme but it would still remain a planet.

Right: Pluto and its tiny satellite Charon photographed by the Hubble Space Telescope's Faint Object Camera. Click the picture for a larger image.

Despite the IAU announcement that Pluto's status is not in jeopardy, astronomers have realized for some time that Pluto is different from the other planets. These differences include the following:

All the other planets in the outer solar system are gaseous, giant planets whereas Pluto is a small solid object.
Pluto is more than a factor 2 smaller than any other planet. It's even smaller than Earth's moon.
Pluto's orbit is by far the most inclined with respect to the plane of the solar system and also the most eccentric.
Pluto's orbit is the only planetary orbit which crosses that of another planet. At the moment Pluto is closer to the sun than Neptune, but it will cross Neptune's orbit later this year thus regaining its status as the most distant planet.
Pluto's satellite, Charon, is larger in proportion to its planet than any other satellite in the solar system.

D. Morrison and T. Owen, in their textbook The Planetary System (first edition, 1988, Addison-Wesley), suggested (p371) "Perhaps Pluto should be considered the largest asteroid rather than the smallest planet." (Pluto has more than twice the diameter of Ceres, the largest asteroid.) They then summarized the outer planets, saying (p375) "Pluto is an anomalous object, resembling an icy satellite more closely than the other planets."

The debate about Pluto's status began soon after the 1992 discovery of 1992 QB1 by D. Jewitt and J. Luu. QB1 is a small icy object, similar in size to an asteroid, orbiting 1.5 times further from the sun than Neptune. There are now almost 100 known objects like QB1. Many orbit in 3:2 resonance with Neptune, as does Pluto.

Right: Kuiper Belt object 1995 WY2 observed UT 1995 Nov 18 using the Keck 10 meter telescope by D. Jewitt and J. Luu. These are the discovery images. Each is a 900 second integration. The images are separated by about 18 minutes, during which time the object moves 1 arcsecond. Stationary background galaxies are also evident in this short movie. The apparent magnitude is V = 24.2. For more information see David Jewitt's Kuiper Belt Home Page.

This swarm of Pluto-like objects beyond Neptune is known as the Kuiper Belt, after Gerard Kuiper, who first proposed that such a belt existed and served as a source of short period comets. Astronomers estimate that there are at least 35,000 Kuiper Belt objects greater than 100 km in diameter, which is several hundred times the number (and mass) of similar sized objects in the main asteroid belt.

For now Pluto's status as a planet seems secure. It's a sentimental favorite of school children and astronomers alike. However, if more Trans-Neptunian Objects are discovered that are even larger than Pluto, the debate could begin anew.

Web Links

IAU Press Release-- on the status of Pluto, Feb 3, 1999

The status of Pluto-- from the IAU

Facts about Pluto - from SEDS

The Kuiper Belt and the Oort Cloud

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Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack