Moons Aplenty, Moons Galore
by Dr. Laurence Trafton
Most of the planets in our solar system have moons — planet-like bodies that orbit a bigger body. Earth has only one moon, but some planets have many: Jupiter, for example, has 63 known moons -- more than any other planet -- while Saturn has 33, and Uranus has at least 27.
A planet and its moons actually revolve about each other, around a common center of gravity that orbits the Sun. For such a system, the planet is always the biggest body and all the other bodies are called moons or satellites. "Satellite" is a more general term because it includes any body revolving about another. Thus, both the Moon and man-made objects orbiting Earth are called satellites.
In our solar system, all the planets revolve in roughly the same plane and in the same direction around the Sun. This direction is clockwise when viewed from "above" the solar system, and is called prograde. In most cases, the planets also rotate in this same prograde direction. Uranus and Pluto are exceptions because they are tilted almost on their sides. This unusual orientation probably resulted from a severe collision with another large body early in the life of the solar system when the planets and moons were still forming.
During that time, many bodies of various sizes orbited the Sun. Many of them coalesced to form planets, moons, comets, and other bodies. Evidence of this massive debris during the later stages of planet formation is recorded by the many impact craters now visible not only on our Moon but also on other planets and moons with rocky surfaces.
Moons with icy surfaces show few ancient craters because ice flows, filling in and hiding the older craters. Planetary atmospheres also erode all but the biggest craters. So even though the planets were subject to this early bombardment, there is less evidence of it than on the surfaces of many moons. Of course, the giant planets (Jupiter, Saturn, Uranus, and Neptune), which do not have solid surfaces, don't have any craters at all.
The asteroids may be fragments of a once planet-sized body, or debris that failed to form a planet, between Mars and Jupiter. Such a body might have been broken up during the bombardment period. Astronomers are trying to piece together the history of the asteroid belt by studying the evolution of asteroid orbits and the composition of asteroids and certain meteorites, which probably are pieces of asteroids.
During the early days of the solar system, planets formed from the cloud of gas and solid particles surrounding the newly-formed Sun. As these "proto-planets" swept up more and more of the material in the solar nebula, they formed their own small clouds of debris. It is from these smaller nebulae that many of the major moons — in particular, the largest moons of Jupiter — are thought to have formed. Like the planets, particles in these smaller nebulae clumped together to form bigger bodies: the moons.
Many other moons may have been captured by the gravity of a planet as they passed by. This process may explain the moons that are too small to have formed from a nebula. An example is the two moons of Mars, Phobos and Deimos, which appear to be captured asteroids. Neptune's moon, Triton, is large but orbits Neptune in a retrograde direction — opposite the direction of most bodies in the solar system. This is strong evidence that it was captured, since the process of capture could result in either a prograde or retrograde orbit. Triton probably is the largest captured object in the solar system.
Earth's moon probably formed after our planet collided with another large body early in the life of the solar system. So did Pluto's moon, Charon. This system comes closest to forming a "double planet," because Charon is more than half as big as Pluto. This is the only case in the solar system where the center of gravity of a planet-moon system is above the planet's surface. In the case of the Earth-Moon system, the center of gravity is about 1,000 miles below Earth's surface. The Moon causes tides in Earth's oceans. The process that causes the tides also plays a major role in determining the length of a moon's day.
The mutual tidal force between a moon and its planet distorts and heats both bodies. As the heat radiates into space, the energy is removed from the system. Over time, the orbits will become circular and the relative rotation of the bodies will cease; in other words, the same side of the moon will always face the same side of the planet. Although the solar system isn't old enough for this to have occurred with any planet-moon system except Pluto-Charon, many moons have already reached circular orbits and keep the same hemisphere facing their planets. This is the case with our own moon, the largest moons of Jupiter, and Triton.
Mercury and Venus are the only planets without moons. They are also the closest planets to the Sun, where solar tides are greatest. Solar tides also affect oceans on Earth and are much stronger for bodies closer to the Sun. These planets may not have moons because this tidal force pulled the moon and planet apart, making it impossible for fledgling moons to maintain stable orbits.
Dr. Laurence Trafton is a McDonald Observatory research scientist.
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