Facts of the Matter
Pluto and its three moons, Charon, Nix and Hydra
Pluto upsets historic definition of planets
EVERYBODY KNOWS what a planet is. We learned in school that there are nine planets in our solar system.
In order of increasing distance from the sun, there are the rocky or terrestrial planets: Mercury, Venus, Earth and Mars. They are small, with diameters smaller than Earth's 8,000 miles. The terrestrial planets are composed primarily of rock and metal, and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.
The asteroid belt, which lies between the orbits of Mars and Jupiter and marks a division between the inner and outer solar system, is a collection of orbiting rocks ranging in size from Ceres (575 miles in diameter) down to the size of pebbles.
The gas planets (also called "gas giants") Jupiter, Saturn, Uranus and Neptune are composed primarily of hydrogen and helium, and generally have low densities, rapid rotation and deep atmospheres. They range in size from Neptune's 30,000-mile diameter to Jupiter's 87,000 miles.
And then there is Pluto, a renegade outer planet 3 billion miles distant, a mere 1,400 miles in diameter, that appears only as a point of light from Earth-based telescopes.
Pluto is also unusual in that it has three moons. Charon is half the size of Pluto, so the two actually revolve around one another, making them a double planetary system. Nix and Hydra, discovered in 2004, are much smaller, somewhere between 30 and 100 miles in diameter.
OUTSIDE THE ORBIT of Neptune, extending roughly 2.75 to 4.5 billion miles from the sun, is the Kuiper Belt, a disk-shaped region containing many small, icy bodies presumably left over from the origin of the solar system that did not coalesce to form planets.
Classifying solar system objects was straightforward until Pluto's status as a planet recently came under question with the discovery of several Kuiper Belt objects similar to Pluto, with one reportedly as large as Pluto.
Occasionally the gravitational attraction of the giant outer planets will perturb the orbit of a Kuiper Belt object (KBO) and cause it to cross inside the orbit of Neptune.
Astronomers estimate that there are around 35,000 KBOs, all composed of mixtures of rock and ice, and having a total mass greater than that of the asteroid belt.
KBOs are not planets. They are more like asteroids, except that asteroids are made of metal and rock rather than the ice and rock thought to comprise KBOs.
Currently, nine known KBO-like objects are orbiting between Jupiter and Neptune. The International Astronomical Union (IAU) has designated this class of objects as Centaurs.
Their orbits are not stable. Centaurs are almost certainly "refugees" from the Kuiper Belt. Some show cometary activity (i.e., their images are a little fuzzy, indicating the presence of a diffuse coma). The largest of these is Chiron, which is about 65 miles in diameter, 20 times larger than comet Halley, but which shares orbital characteristics with Pluto. Both are locked into a 3-2 ratio with Neptune, which means that Neptune makes three revolutions around the sun in the same time that Pluto and Chiron make two revolutions.
With the discovery of these new objects, scientists have become engaged in a controversy about Pluto's status.
Is Pluto a planet, a KBO or a Centaur? There are supportable arguments on all sides.
Chiron, which is classified as a Centaur, has an orbit virtually identical to Pluto's in all respects except orientation.
Their highly elliptical orbits extend well into the Kuiper belt but also cross inside the orbit of Neptune. Although they are closer to the sun than Neptune for about 20 years of their 248-year orbit, the orbits are inclined so steeply that there is no chance of collision.
But Pluto is nearly 20 times more massive than Chiron, which strengthens its candidacy for planetary status. On the other hand, Pluto is entirely unlike the gas giants, and not like the terrestrial planets, either.
BECAUSE OF HISTORICAL precedent, no one really wants to demote Pluto from planetary status. But what if we find something larger than Pluto? Do we create a new planet? What if we find five things larger than Pluto? Are they all planets?
One reason we know so little about Pluto is because it is the only "planet" never to have been visited by NASA spacecraft.
Two Voyager probes that explored the outer solar system in the 1970s could not get close to Pluto. The two probes were launched at a time when they could sweep past Jupiter, Saturn, Neptune and Uranus, but Pluto was not in a position that allowed it to be included in the trajectory of either probe.
On Jan. 19, after five years of preparation, a New Horizons probe was launched, with an ETA at nearest approach to Pluto in 2015.
New Horizons will use Jupiter's intense gravitational field to slingshot the probe and accelerate it, saving two years of travel time.
There was much concern that bad weather would delay the launch until after Jupiter had moved out of a position where the probe could take advantage of the slingshot effect, and missing that window would have added three years to the 15-year project.
Many of the scientists who began work on the probe will be retired by the time it arrives, and many of those who will be on the project at arrival are now in elementary school.
New Horizons instruments are the most sophisticated yet most energy-efficient ever sent on a first flyby of any planet. They are:
» Ralph: Visible and infrared imager/spectrometer to provide color, composition and thermal maps.
» Alice: Ultraviolet imaging spectrometer to analyze composition and structure of Pluto's atmosphere and look for atmospheres around Charon and KBOs.
» REX: Passive radiometer to measure atmospheric composition and temperature.
» LORRI: Telescopic camera to obtain visual data at long distances, map Pluto's far side and provide high-resolution geologic data.
SWAP: Solar wind and plasma spectrometer to measure atmospheric "escape rate" and observe Pluto's interaction with solar wind.
» PEPSSI: Energetic particle spectrometer to measure the composition and density of plasma (ions) escaping from Pluto's atmosphere.
» SDC: Built and operated by students to measure the space dust peppering New Horizons during its voyage across the solar system.
Regardless of whether it is called a planet, a KBO or a Centaur, Pluto is expected to provide many new clues to the origin of the solar system, because it lies within the transition between the region where planets formed from primordial material and the region where they did not.
Pluto, being potentially the largest example of primordial matter in the solar system, might contain the basic material that formed all the planets, each of which has evolved in a unique way from the time of its formation.
picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at firstname.lastname@example.org