1. Evolving perspectives - a historical prologue

Satellites, rings and small distant objects

Discovery of satellites and rings

For nearly half a century, the only satellites known in the solar system were the Earth’s Moon and Jupiter's four largest satellites, discovered by Galileo in 1610 and now often called the “Galilean satellites” in his honor. They are named after four of Jupiter’s lovers. Io is the innermost of the four Galilean satellites, succeeded by Europa, Ganymede, and Callisto.

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In 1655 the Dutch astronomer-physicist, Christiaan Huygens (1629-95), discovered Titan, the largest satellite of Saturn, named after Saturn's older brother. Within a few decades Gian Domenico Cassini (1625-1712), at the Paris Observatory, had discovered four more moons circling Saturn; they are named Iapetus, Rhea, Tethys and Dione.

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Huygens turned his telescope toward Saturn itself, and explained its mysterious handle-like appendages. Galileo had noticed that the planet was not round, but had blurry objects on each side. When these objects disappeared two years later, Galileo wondered if Saturn “had devoured his own children”. In 1656 Huygens, then only 27 years old, realized that the planet was surrounded by “a thin flat ring, nowhere touching it, and inclined to the ecliptic”. Because the ring is tipped with respect to the plane of the Earth’s orbit around the Sun, it changes its shape when viewed from Earth, slowly opening up and then turning edge-on as Saturn makes her slow 29.5-year orbit around the Sun. When the ring is opened up, it resembles handle-like appendages, but when it is viewed edge-on the ring virtually disappears.

In the meantime, more planetary satellites had been found, and for the next three centuries their discovery progressed more or less in tandem with the development of increasingly powerful telescopes.

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Jupiter has eight small outer moons in eccentric tilted orbits that are so far from the planet that the Sun competes for their gravitational control. These eight outer moons fall into two widely separated groups. The innermost four move in the same direction as the planet rotates, but in highly inclined orbits that are not in the plane of the planet's equator. The outermost four circle Jupiter in the backward, or retrograde, direction. The Sun’s gravitational perturbations would have dislodged the outermost satellites if they had direct orbits.

Pluto - a small world with an oversized companion

The discovery of Neptune in 1846 resulted from a mathematical study of the differences between the predicted and observed positions of Uranus, attributed to the gravitational pull of the then unknown planet. Astronomers hoped that similar irregularities in Neptune's motion would lead to the discovery of another remote planet; but because of Neptune's long 165-year orbit there were insufficient observations. Prediction of another unknown planet therefore had to be based upon perturbations in Uranus' motion, after corrections for the gravitational effects of Neptune.

The most ambitious search for the trans-Neptunian planet was directed by Percival Lowell, at his observatory in Flagstaff, Arizona, but no new planet was found at a variety of predicted locations between 1905 and 1919. The search from the Lowell Observatory continued a decade later using a new 0.33-meter (13-inch) photographic telescope. Once three photographs had been taken at intervals of several days, they were set in pairs in a blink microscope that would show the apparent motion of a planet, asteroid or comet against a background of nearly half a million stars on each photograph.

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After months of painstaking work, Clyde William Tombaugh (1906-1997) discovered, on 18 February 1930, the sharp, faint, moving image of the elusive quarry. The new object was named Pluto, for the Roman god of the underworld. It is a small frozen world at the outer fringe of the planetary system, with a highly-elongated orbit that carries it between 29.7 and 49.3 AU from the Sun.

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Pluto is now known to be a double object, with a companion that is half as big as Pluto is. This discovery was an accidental by-product of observations made for another purpose. In 1978, astronomers at the United States Naval Observatory were obtaining a series of photographs to improve the accuracy of Pluto's orbit, when several of the images appeared slightly distorted, from a round to oblong shape. The elongation seemed to disappear every few days, and careful examination showed that it is caused by another small world that orbits Pluto. The two objects are so close together and so far away that they remain blurred together when viewed with even the best telescopes on the ground, but they can be clearly resolved with the Hubble Space Telescope that orbits the Earth above its obscuring atmosphere. Pluto’s companion is named Charon, after the boatman who ferried new arrivals across the river Styx at the entrance to Pluto's underworld, Hades.

The announcement of this remarkable doubling was a happy surprise, for it permitted determining the mass of Pluto. Charon's slow revolution about Pluto is a result of Pluto's small mass - only 0.2 percent (0.002) of the Earth’s mass and only about one-sixth the mass of our Moon. This means that Pluto was not found because it was correctly predicted. Its mass is far too small to have noticeably influenced the past motions of Uranus.

Small cold worlds in the outer precincts of the planetary system

As it turned out, Pluto shares a similar orbit and composition with numerous small balls of ice and rock, forming a distant, flat ring just outside of Neptune's orbit. It is now known as the Kuiper belt in recognition of Gerard P. Kuiper's 1951 prediction of its existence. He argued that the dark outer edge of the planetary realm is not empty, but is instead full of small, unseen bodies created from the left-over debris of the formation of the giant planets. The low-density material in these distant regions would have been spread out into such a large volume, and moving in such slow, ponderous orbits around the Sun, that it could not gather or coalesce into a body any larger than Pluto.

Comet reservoirs

There are probably many more small objects than large ones in the Kuiper belt. Most of them have been hibernating in the deep freeze of outer space since the formation of the solar system. Yet, Neptune’s gravity slowly erodes the inner edge of the belt, within about 40 AU from the Sun, and gradually pulls some of its members closer to the Sun. The Kuiper belt is therefore a likely reservoir for a certain class of comets, called the short-period comets, that become visible when they emerge from cold storage and move toward the Sun. When a Kuiper-belt object has been launched into the inner solar system, within a few AU of the Sun, the increased solar heat vaporizes the object’s icy surface, forming an Earth-sized cloud of gas and dust that reflects enough sunlight to be seen as a short-period comet. The short-period comets have periods of less than 200 Earth years, typically 5 to 20 years, and conform somewhat to the pattern of planetary motion. They all move in the same prograde direction as the planets and their orbits are tilted only slightly from the orbital plane of the Earth, known as the ecliptic. This is consistent with an origin in the outer parts of the disk of the solar system, or in the Kuiper belt. The other class of comets, those with long periods greater than 200 Earth years, come into the planetary realm at every possible angle – their orbits are inclined at all angles to the ecliptic. Roughly half of them orbit the Sun in the retrograde direction, opposite to the motion of the planets. These long-period comets approach the Sun on very elongated orbits, coming from enormous distances of 50 thousand AU or more. The orientation and size of the orbits of long-period comets indicate an origin in a vast, remote spherical shell of icy objects, It surrounds the solar system and extends to interstellar distances of about 100 thousand AU, or up to one quarter the distance to the nearest star, Alpha Centauri at 250 thousand AU. This comet reservoir is named the Oort cloud, after Jan Hendrik Oort (1900-1992) who first postulated its existence in 1951.

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The random jostling of stars or giant molecular clouds passing near the Oort cloud can knock some of these icy objects from their stable orbits, sending them into the heart of the solar system where they can be seen as a long-period comet. When tossed near the Sun, these dirty balls of ice become vaporized and light up with long tails and changing shapes that have inspired awe for centuries.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University