Since the discovery of Pluto by American astronomer Clyde Tombaugh in 1930, scientists and textbooks have told us that there are nine planets in the solar system. A debate has been going on for the past decade over the planetary status of Pluto, fueled by the discovery of other Pluto-sized objects in the outer solar system. Many of these objects, called Kuiper belt objects, have been discovered in recent years. One of these, 2003 UB313, is believed to be even larger in diameter than Pluto.
Some astronomers have argued that there should be only eight planets in the solar system: the four terrestrial planets (Mercury, Venus, Earth, and Mars) and four gas giant planets (Jupiter, Saturn, Uranus, and Neptune), and that all other objects that orbit the sun ought to be classified as something else. Their arguments for demoting Pluto are as follows:
- Pluto is very small, with only 0.2% of the mass of Earth, and only 3.8% of the mass of the next smallest planet, Mercury.
- Pluto is smaller than seven moons that orbit other planets (Earth’s moon, Io, Europa, Ganymede, Callisto, Titon, Triton).
- Pluto’s orbit is considerably more eccentric than the orbits of the eight other planets.
Perhaps the best argument for the planetary status of Pluto, according to these scientists, is that “Miss Parker told be so in second grade.”
If Pluto were demoted, it would not be the first time that a “planet” had been discovered only to be later demoted. In 1801, Giuseppe Piazza discovered an object between Mars and Jupiter, and named it Ceres. Ceres remained on planet lists for fifty years, until astromomers with better telescopes discovered that it was only the largest of a large number of objects in what we now know as the asteroid belt.
The International Union of Astronomers (IAU) is the organization that decides what is a planet, and what is not, and for the past several years they have been saying that they were not going to demote Pluto. The group is presently meeting in Prague, and are expected to come out with a definition for “planet.” Their proposed definition, which is likely to pass, has two parts:
- In order to be a planet, an object must be have enough mass to pull itself into a spherical shape. Smaller bodies, such as most asteroids, tend to be irregular, potato-shaped objects.
- The object must orbit the sun. This would exclude bodies such as the Earth’s moon.
Debate at the IAU assembly continues, with a vote expected on August 24th. If the proposal passes, the solar system will have twelve planets:
- 2003 UB313
Pluto and Charon orbit each other, with a center of gravity in between them, so both will be considered planets. The Earth’s moon is not a planet, because the center of gravity of the Earth-Moon system is deep inside the Earth.
2003 UB313 needs a name. The discoverer nicknamed the object “Xena,” as in “princess warrior” from TV. We ran out of Greek and Roman gods and goddesses a long time ago. Other names have been submitted to the IAU; perhaps they will give it a name when they announce their decision on the 24th.
The IAU proposal apparently divides the planets into two groups: the “classical” planets, and “plutons,” or Pluto-like planets. Ceres would belong to neither group. I don’t like the name “pluton,” as it has a geological definition as well: an intrusive (formed underground, as opposed to volcanic, which is formed on the surface) igneous body, such as a batholith, stock, dike, or sill.
Also, if the new definition prevails, the list of planets will grow as more objects are discovered in the far reaches of the solar system.
Grace and Peace