The atmosphere is the gaseous outer portion of a planet. Atmospheres have been detected around all planets and several satellites, and each is unique. Some atmospheres are very dense, and gradually blend into fluid envelopes which contain most of the planet's mass. Others are extremely tenuous, so tenuous that even the best vacuum on Earth seems dense in comparison. The composition of planetary atmospheres varies from the solar-like hydrogen/helium envelopes of the giant planets to atmospheres dominated by nitrogen, carbon dioxide, or esoteric gases such as sulfur dioxide or sodium for terrestrial planets and satellites of giant planets. However, even though all atmospheres are intrinsically different, they are governed by the same physical and chemical processes. For example, clouds form in many atmospheres, but with vastly different compositions since the gases available to condense differ. The upper layers of an atmosphere are modified by photochemistry, with the particulars depending on atmospheric composition. Variations in temperature and pressure lead to winds, which can be steady or turbulent, strong or weak. The various processes operating in planetary atmospheres are discussed in this chapter, and the characteristics of the atmospheres of bodies within our Solar System are summarized.

In this Appendix we briefly summarize aspects of observational planetary science. Techniques specific to extrasolar planets are discussed in §12.2. References to more extensive treatments are provided in the FurtherReading section at the end.

Photometry

Photometry is a photon-counting technique, wherein the brightness of an object is measured. Time series of brightness measurements can be combined into a photometric lightcurve that shows, e.g., the variations in a body's brightness as the object rotates around its axis (Fig. 9.4).

Graphs of an asteroid's brightness in reflected sunlight as a function of phase angle, φ, usually show an abrupt increase in intensity at φ ≲ 2°, referred to as the opposition effect. The opposition effect for the Moon, shown in Figure E.1, is very large; the intensity increases by ∼20%; from φ ∼ 2° down to φ = 0°. This is why a ‘full Moon’ can appear to be much brighter than a nearly full gibbous Moon. Part of the opposition effect can be attributed to the hiding of shadows when the Sun and observer are located in the same direction as seen from the object. Laboratory simulations of this phase angle effect, however, show that this is not the complete story. In any particulate material, multiple reflections diffusely scatter the incoming waves in all directions. At zero phase angle the waves interfere constructively, and the reflected intensity can be amplified considerably. This process is known as the coherent backscatter effect.

Each of the four giant planets in our Solar System is surrounded by flat, annular features known as planetary rings. Planetary rings are composed of vast numbers of small satellites, which are unable to accrete into large moons because of their proximity to the planet.

When Galileo Galilei first observed Saturn's rings in 1610, he believed them to be two giant moons in orbit about the planet. However, these ‘moons’ appeared fixed in position, unlike the four satellites of Jupiter which he had previously observed. Moreover, Saturn's ‘moons’ had disappeared completely by the time Galileo resumed his observations of the planet in 1612. Many explanations were put forth to explain Saturn's ‘strange appendages’, which grew, shrank and disappeared every 15 years (Fig. 11.1a). In 1656, Christiaan Huygens finally deduced the correct explanation, that Saturn's strange appendages are a flattened disk of material in Saturn's equatorial plane, which appear to vanish when the Earth passes through the plane of the disk (Fig. 11.1b).

For more than three centuries, Saturn was the only planet known to possess rings. Although Saturn's rings are quite broad, little structure within the ring system was detected from Earth (Fig. 11.2). Observational and theoretical progress towards understanding the physics of planetary rings was slow.

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.

Temperature is one of the most fundamental properties of planetary matter, as is evident from everyday experience such as the weather and cooking a meal, as well as from the most basic concepts of chemistry and thermodynamics. For example, H2O is a liquid between 273 K and 373 K (at standard pressure), a gas at higher temperatures, and a solid when it is colder; silicates undergo similar transitions at substantially higher temperatures and methane condenses and freezes at lower temperatures. Most substances expand when heated, with gases increasing in volume the most; the thermal expansion of liquid mercury allowed it to be the ‘active ingredient’ in most thermometers from the seventeenth century through the twentieth century. The equilibrium molecular composition of a given mixture of atoms often depends on temperature (as well as on pressure), and the time required for a mixture to reach chemical equilibrium generally decreases rapidly as temperature increases. Gradients in temperature and pressure are responsible for atmospheric winds (and, on Earth, ocean currents) as well as convective motions that can mix fluid material within planetary atmospheres and interiors. Earth's solid crust is dragged along by convective currents in the mantle, leading to continental drift. Temperature can even affect the orbital trajectory and rotation state of a body, as we have seen in our discussions of the Yarkovsky and YORP effects (§2.7.3 and §2.7.4, respectively).

The four largest planets in our Solar System are gas giants, with very deep atmospheres and no detectable solid ‘surface’. All of the smaller bodies, the terrestrial planets, asteroids, moons and comets, have solid surfaces. These bodies display geological features that yield clues about their formation, as well as past and current geological activity. The surface reflectivity varies dramatically from one body to another; some surfaces have very low albedos (such as the maria on the Moon, carbonaceous asteroids, comet nuclei), while others are highly reflective (Europa, Enceladus). Large albedo variations may even be seen on a single object (Iapetus). Some bodies are almost completely covered by impact craters (Moon, Mercury, Mimas), while others showlittle or no sign of impacts (Io, Europa, Earth). The terrestrial planets and many of the larger moons show clear evidence of past volcanic activity, and some (Earth, Io, Enceladus, Triton) are active even today. Past volcanic activity may be seen in the form of volcanoes of different shapes and size (Earth, Mars, Venus) or large solidified lava lakes (Moon).

In addition to the eight known planets, countless smaller bodies orbit the Sun. These objects range from dust grains and small coherent rocks with insignificant gravity to dwarf planets that have sufficient gravity to make them quite spherical in shape. Most are very faint, but some, the comets, release gas and dust when they approach the Sun and can be quite spectacular in appearance (Fig. 10.1); comets are discussed in Chapter 10. In this chapter, we describe the orbital and physical properties of the great variety of non-cometary small bodies ranging in radius from a few meters to over 1000 km that orbit the Sun. We refer to these bodies collectively as minor planets.

Minor planets occupy a wide variety of orbital niches (see Fig. 1.2). Most travel in the relatively stable regions between the orbits of Mars and Jupiter (known as the asteroid belt), exterior to Neptune's orbit (the Kuiper belt), or near the triangular Lagrangian points of Jupiter (the Trojan asteroids).

The origin of the Solar System is one of the most fundamental problems of science. Together with the origin of the Universe, galaxy formation and the origin and evolution of life, it is a crucial piece in understanding where we come from. Because planets are difficult to detect and study at interstellar distances, we have detailed knowledge of only one planetary system, the Solar System. Data from other planetary systems around both main sequence stars and pulsars are now beginning to provide further constraints (Chapter 12). But even though 98%; of known planets orbit stars other than the Sun, the bulk of the data available to guide modelers of planet formation is from objects within our Solar System. Models of planetary formation are developed using the detailed information we have of our own Solar System, supplemented by astrophysical observations of extrasolar planets, circumstellar disks and star-forming regions. These models are used together with observations to estimate the abundance and diversity of planetary systems in our galaxy, including those planets which may harbor conditions conducive to the formation and evolution of life (§12.5).

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 farther away than the Moon.

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 πλαντηζ, 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.

Most planets are surrounded by huge magnetic structures, known as magnetospheres. These are often more than 10–100 times larger than the planet itself, and therefore form the largest structures in our Solar System, other than the heliosphere. The solar wind flows around and interacts with these magnetic ‘bubbles’. A planet's magnetic field can either be generated in the interior of the planet via a dynamo process (Earth, giant planets, Mercury), or induced by the interaction of the solar wind with the body's ionosphere (Venus, comets). Large-scale remnant magnetism is important on Mars, the Moon and some asteroids.

The shape of a planet's magnetosphere is determined by the strength of its magnetic field, the solar wind flow past the field and the motion of charged particles within the magnetosphere. Charged particles are present in all magnetospheres, though the density and composition varies from planet to planet. The particles may originate in the solar wind, the planet's ionosphere or on satellites or ring particles whose orbits are partly or entirely within the planet's magnetic field. The motion of these charged particles gives rise to currents and large-scale electric fields, which in turn influence the magnetic field and the particles' motion through the field.

Although most of our information is derived from in situ spacecraft measurements, atoms and ions in some magnetospheres have been observed from Earth through the emission of photons at ultraviolet and visible wavelengths.