PLUTO ... & BEYONDKey Concepts
(1) Pluto and its moon Charon are icy worlds that resemble Triton, the giant moon of Neptune.Uranus was discovered in the 18th century, serendipitously.Neptune was discovered in the 19th century, using Newton's laws. Pluto was discovered in the 20th century, by sheer dumb luck. Computations of the orbits of Uranus and Neptune, during the early 20th century, indicated that there might be a ninth planet outside the orbit of Neptune, perturbing the orbits of Uranus and Neptune by its gravitational influence. Astronomers at Lowell Observatory, in Flagstaff, Arizona, thought that the ninth planet was in the constellation Gemini at the time. A 24-year-old Kansan, by the name of Clyde Tombaugh, was hired for the tedious task of searching for the planet by comparing pairs of photographs of the sky, taken several nights apart. Any planet captured in the photographs would have moved from one night to the other. In February 1930, Tombaugh discovered a planet, which was given the name Pluto, after the Roman god of the underworld. The ironic part of the story (where the ``dumb luck'' comes in) is that Pluto is far too small to affect the orbits of Uranus and Neptune. The perturbations that astronomers thought they saw were the result of measurement errors. It was pure luck that Pluto happened to be in Gemini at the time. The orbital period of Pluto is 248 years; Kepler's 3rd Law tells us that the semimajor axis of Pluto's orbit is a=40 A.U. (Compare to 30 A.U. for the orbit of Neptune.) The rotation period of Pluto is 6.4 days, longer than that of the Jovian planets. The orbit of Pluto is highly eccentric; from January 1979 to March 1999, Pluto was actually closer to the Sun than Neptune was. The orbit of Pluto is inclined, or tilted, relative to the orbits of the other planets; this tilt prevents Pluto's orbit from intersecting that of Neptune. Pluto and Neptune aren't going to collide. In 1978, it was discovered that Pluto has a moon, which was given the name Charon, after the ferryman who ferries dead souls to the underworld, where they enter the realm of Pluto. A picture of Pluto and Charon, taken by the Hubble Space Telescope, is given below. Pluto is the larger blob, and Charon is the smaller one. At maximum separation, they are less than an arcsecond apart as seen from Earth.
Pluto has never been visited by spacecraft from Earth, unlike all the other planets in the solar system. The best map we have of Pluto, compiled from information gathered by the Hubble Space Telescope, is tantalizingly vague:
The white region near the north pole of Pluto is probably an ice cap of frozen methane and nitrogen, like that of Neptune's moon Triton. In fact, Pluto and Charon are similar to Triton in all their properties, as far as we can tell from Earth. At the moment, Pluto, Charon, and Triton are all at about the same distance from the Sun (30 A.U.). They all have about the same surface temperature. They all have a density of about 2000 kg/m3, indicating they are made of thick icy mantles over a rocky core. Because Pluto is smaller than Triton, it has a lower escape velocity and an even more tenuous atmosphere. Because Charon is smaller than Pluto, it has a still lower escape velocity, and no atmosphere at all. Pluto is the only planet that has not been visited by a spacecraft from Earth. A reconnaissance mission called New Horizons is currently scheduled for launch in January 2006. If all goes well, it will reach Pluto and Charon in the year 2015. (2) There exist many small, Pluto-like objects in the outer solar system.What lies beyond Pluto? Recent studies have revealed that icy objects, similar to Pluto and Charon, exist within the Kuiper belt. The Kuiper belt is the region beyond Neptune in the plane of the solar system. It stretches from 30 A.U. to roughly 500 A.U. from the Sun, and is shaped like a stepped-on bagel. In 1951, the astronomer Gerard Kuiper reasoned that if there are no Jovian planets beyond Neptune, then the icy planetesimals that formed in the solar nebula should still be running around loose in the Kuiper belt.In 1951, the available telescopes were not powerful enough to detect icy planetesimals more than 30 A.U. from the Sun. In the past decade, with large ground-based telescopes, searches have been under way for icy objects within the Kuiper belt. These icy objects are generally referred to in the astronomical community by the generic name of ``Kuiper belt objects''. So far, 628 Kuiper belt objects have been found (as of November 29, 2002), and the list is growing steadily. It is estimated that the Kuiper belt contains 35,000 icy objects with diameters of 100 kilometers or more. Kuiper belt objects are hunted for in the same way Tombaugh hunted for Pluto almost seven decades ago. Take two photographs of the same region of the sky several hours or days apart; anything that moves between one exposure and the next is a candidate Kuiper belt object. The images directly below show the largest Kuiper belt object found so far; it has been given the name `Quaoar' (pronounced `kwah-o-whar').
(Quaoar is the name of the creator god in the religion of the Tongva -- Native American inhabitants of the Los Angeles basin.) Quaoar, discovered in June 2002, is about 1200 kilometers in diameter; the same as Charon, and more than half the diameter of Pluto. Known objects in the Kuiper Belt:
(3) If planets exist beyond Pluto, they must be very small or very distant.Do planets exist beyond Pluto within our Solar System? To prevent argument, let's define a planet as something at least as massive as Pluto, orbiting the Sun. (This means that the Kuiper belt objects discovered so far are too small to qualify as bona fide planets.) If any planets exist beyond Pluto, they would be dim and slowly moving; they might be detectable, however, via their gravitational effect on other bodies (just as Neptune was first detected by its gravitational effect on Uranus).The search undertaken by Clyde Tombaugh, and other similar searches, tell us that there are no more Jovian planets within 200 A.U. of the Sun. This is discouraging news for those who want to find new planets. There are, however, two loopholes that we can wiggle through. Loophole number one: Small, Pluto-like planets may exist within the Kuiper belt. An object similar in size to Pluto, but with a larger orbit, would be too dim to be easily detected, and would not have a significant gravitational effect on the inner planets. So far, surveys of the Kuiper belt have been woefully incomplete; only 628 objects (out of an estimated 35,000 objects bigger than 100 km across) have been detected. There may well be objects orbiting within the Kuiper belt that are similar in size to Pluto. Loophole number two: Massive planets may exist more than 200 A.U. from the Sun. At such a large distance, even a Jovian planet would be hard to detect. updated: |
