Pluto
One of the debates that has stirred some interest in recent years is concerned with describing Pluto. Is it really a planet? (Notice I refrained from saying “defining.”) Since the orbit of Uranus was discovered to be irregular back in the 18th century, astronomers have been looking for additional solar objects that could be big enough and near enough to make it wobble. This search led first to the Italian Piazzi finding Ceres between Mars and Jupiter in 1801. It turned out to be both in the wrong place for Uranus and not big enough; it was the first of what we know now as asteroids. Next, in 1846 the Frenchman LeVerrier and the Englishman Adams discovered Neptune. Neptune was big, but it too had a wobble, so the questions weren’t answered completely. Then in 1930 Clyde Tombaugh, working in Flagstaff, Arizona, found a faint, slow-moving object farther away than Neptune that seemed to satisfy the need. It was named Pluto, and after it was more completely studied it turned out to be smaller even than our Moon. Therefore Pluto couldn’t be the missing large mass. But then what is Pluto? Pluto orbits around the Sun in a tilted ellipse at an average of about 40 AU (one Astronomical Unit is the distance from the Earth to the Sun). It is a rocky object like Earth, and like Earth it has a moon. It is spherical, not lopsided like most of the asteroids. And it has a thin atmosphere. It is a “wanderer”—the original definition of “planet.” Besides these qualities, it’s been called a planet for over you years. So do these qualities make it a planet? But Pluto’s small size (about 750 miles radius), and the fact that its orbit differs considerably from the other “accepted” planets, made astronomers question what it really is and look again for what else might be in its region. As far back as 1953 a British amateur astronomer, Kenneth Edgeworth, predicted that a very large number of small bodies might exist in the outer solar region. With more study, others suggested that if there were enough of them they might account for the wobbles. These bodies could also be stuff that was left over from the formation of the Sun and the planets we know already. One study concentrated on comets with the questions of where they come from and what they are. A striking number of comets originate in a region between 30 and 100 AU, the region where Pluto is orbiting. Apparently the region is shaped like a disk circling the Solar System; it has become known as the Kuiper Belt. The composition of comets is what would be expected in this cold outer region, that is, dirty snowballs, or frozen gasses loosely packed around a rocky core. With the help of a new generation of telescopes, the first real objects in this belt were photographed in 1992 by astronomers Jewett and Luu. Since then several other Kuiper Belt objects have been discovered, the largest of which is 15 miles in diameter. The current theory is that occasionally these objects are pulled out of their usual orbits and sent toward the Sun. When they approach the Sun they not only throw off a cloud of their gasses but also because of that cloud they become visible as comets. The study of the Kuiper Belt objects has still left unanswered the question of what Pluto is. It has a rocky core surrounded by gas; its orbit is elliptical like a comet; and its average distance places it well within the Kuiper Belt. Perhaps it’s a tiny planet, or perhaps it’s a large future comet. British scientist Alan Fitzsimmons may have come the closest: He remarked that Pluto is merely schizophrenic, a bit of both. References: Beyond Pluto, John Davis, Cambridge University Press, 2001 “The Kuiper Belt and The Oort Cloud,” http://seds.lpl.arizona.edu/nineplanets/nineplanets/kboc/html “The Kuiper Belt,” http://www.solarviews.com/eng/kuiper.htm