The generally unexpected and sometimes spectacular appearances of comets have triggered the interest of many people throughout history. A bright comet can easily be seen with the naked eye, and its tail can extend more than 45° on the sky (Fig. 10.1). The name comet is derived from the Greek word κωμητηζ which means ‘the hairy one’, describing a comet's most prominent feature: its long tail. The earliest records of comets date back to ∼6000 bce in China. In the time of Pythagoras (550 bce) comets were considered to be wandering planets, but Aristotle (330 bce) and subsequent natural philosophers thought comets were some kind of atmospheric phenomenon. Comets were therefore scary, and often considered bad omens. An apparition of Comet 1P/Halley is depicted on the Bayeux Tapestry (Fig. 10.2), which commemorates the Norman conquest of England in 1066.

The first detailed scientific observations of comets were made by Tycho Brahe in 1577. Brahe determined that the parallax of the bright Comet C/1577 VI was smaller than 15 arcminutes, and concluded that therefore the comet must be further away than the Moon. Edmond Halley used Newton's gravitational theory to compute parabolic orbits of 24 comets observed up to 1698. He noted that the comet apparitions in 1531, 1607, and 1682 were separated by 75–76 years, and that the orbits were described by roughly the same parameters. He hence predicted the next apparition to be in 1758. It was noticed much later that this Comet Halley, as it was named subsequently, has returned 30 times from 240 bce to 1986; records of all of these apparitions have been found with the exception of 164 bce.

Preface to the First Edition

The study of Solar System objects was the dominant branch of Astronomy from antiquity until the nineteenth century. Analysis of planetary motion by Isaac Newton and others helped reveal the workings of the Universe. While the first astronomical uses of the telescope were primarily to study planetary bodies, improvements in telescope and detector technology in the nineteenth and early twentieth centuries brought the greatest advances in stellar and galactic astrophysics. Our understanding of the Earth and its relationship to the other planets advanced greatly during this period. The advent of the Space Age, with lunar missions and interplanetary probes, has revolutionized our understanding of our Solar System over the past forty years. Dozens of planets in orbit about stars other than our Sun have been discovered since 1995; these massive extrasolar planets have orbits quite different from the giant planets in our Solar System, and their discovery is fueling research into the process of planetary formation.

Planetary Science is now a major interdisciplinary field, combining aspects of Astronomy/Astrophysics with Geology/Geophysics, Meteorology/Atmospheric Sciences, and Space Science/Plasma Physics. We are aware of more than ten thousand small bodies in orbit about the Sun and the giant planets. Many objects have been studied as individual worlds rather than merely as points of light. We now realize that the Solar System contains a more dynamic and rapidly evolving group of objects than previously imagined. The cratering record on dozens of imaged bodies shows that impacts have been quite important in the evolution of the Solar System, especially during the epoch of planetary formation. Other evidence, including the compositions of meteorites and asteroids and the high bulk density of the planet Mercury, suggests that even more energetic collisions have disrupted objects. More modest impacts, such as the collision of comet D/Shoemaker–Levy 9 with Jupiter in 1994, continue to occur in the current era. Dynamical investigations have destroyed the regular ‘clockwork’ image of the Solar System that had held prominence since the time of Newton.

The first eleven chapters of this book covered general aspects of planetary properties and processes, and described specific objects within our Solar System. We now turn our attention to far more distant planets. What are the characteristics of planetary systems around stars other than the Sun? How many planets are typical? What are their masses and compositions? What are the orbital parameters of individual planets, and how are the paths of planets orbiting the same star(s) related to one another? What are the relationships between stellar properties such as mass, composition, and multiplicity and the properties of the planetary systems that orbit them? These questions are hard to answer because extrasolar planets, often referred to as exoplanets, are far more difficult to observe than are planets within our Solar System.

Just as the discoveries of small bodies orbiting the Sun have forced astronomers to decide how small an object can be and still be worthy of being classified as a planet (Chapter 9), detections of substellar objects orbiting other stars have raised the question of an upper size limit to planethood. We adopt the following definitions, which are consistent with current International Astronomical Union (IAU) nomenclature:

1. • Star: self-sustaining fusion is sufficient for thermal pressure to balance gravity (≳0.075 M⊙ ≈ 80 M2 for solar composition; the minimum mass for an object to be a star is often referred to as the hydrogen burning limit).

2. • Stellar remnant: dead star–no more fusion (or so little that the object is no longer supported primarily by thermal pressure).

3. • Brown dwarf: substellar object with substantial deuterium fusion–more than half of the object's original inventory of deuterium is ultimately destroyed by fusion.

4. • Planet: negligible fusion (≲0.012 M⊙ ≈ 13 M2, with the precise value again depending upon composition), plus it orbits one or more stars and/or stellar remnants.

The wonders of the night sky, the Moon and the Sun have fascinated mankind for many millennia. Ancient civilizations were particularly intrigued by several brilliant ‘stars’ that move among the far more numerous ‘fixed’ (stationary) stars. The Greeks used the word πλανητηζ, meaning wandering star, to refer to these objects. Old drawings and manuscripts by people from all over the world, such as the Chinese, Greeks, and Anasazi, attest to their interest in comets, solar eclipses, and other celestial phenomena.

The Copernican–Keplerian–Galilean–Newtonian revolution in the sixteenth and seventeenth centuries completely changed humanity's view of the dimensions and dynamics of the Solar System, including the relative sizes and masses of the bodies and the forces that make them orbit about one another. Gradual progress was made over the next few centuries, but the next revolution had to await the space age.

In October of 1959, the Soviet spacecraft Luna 3 returned the first pictures of the farside of Earth's Moon (Appendix F). The age of planetary exploration had begun. Over the next three decades, spacecraft visited all eight known terrestrial and giant planets in the Solar System, including our own. These spacecraft have returned data concerning the planets, their rings and moons. Spacecraft images of many objects showed details which could never have been guessed from previous Earth-based pictures. Spectra from ultraviolet to infrared wavelengths revealed previously undetected gases and geological features on planets and moons, while radio detectors and magnetometers transected the giant magnetic fields surrounding many of the planets.