Encyclopedia of the Solar System

Lucy-Ann McFadden, Torrence Johnson, Paul Weissman

• Publisher: Academic Press
• Year: 2006
• ISBN:
  9780080474984
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Astronomy & Astrophysics

Long before Galileo published his discoveries about Jupiter, lunar craters, and the Milky Way in the Starry Messenger in 1610, people were fascinated with the planets and stars around them. That interest continues today, and scientists are making new discoveries at an astounding rate. Ancient lake beds on Mars, robotic spacecraft missions, and new definitions of planets now dominate the news. How can you take it all in? Start with the new Encyclopedia of the Solar System, Second Edition.This self-contained reference follows the trail blazed by the bestselling first edition. It provides a framework for understanding the origin and evolution of the solar system, historical discoveries, and details about planetary bodies and how they interact—and has jumped light years ahead in terms of new information and visual impact. Offering more than 50% new material, the Encyclopedia includes the latest explorations and observations, hundreds of new color digital images and illustrations, and more than 1, 000 pages. It stands alone as the definitive work in this field, and will serve as a modern messenger of scientific discovery and provide a look into the future of our solar system.· Forty-seven chapters from 75+ eminent authors review fundamental topics as well as new models, theories, and discussions
· Each entry is detailed and scientifically rigorous, yet accessible to undergraduate students and amateur astronomers
· More than 700 full-color digital images and diagrams from current space missions and observatories amplify the chapters
· Thematic chapters provide up-to-date coverage, including a discussion on the new International Astronomical Union (IAU) vote on the definition of a planet
· Information is easily accessible with numerous cross-references and a full glossary and index

CHAPTER 1 The Solar System and Its Place in the Galaxy

Paul R. Weissman

Jet Propulsion Laboratory California Institute of Technology Pasadena, California

1. Introduction

2. The Definition of a Planet

3. The Architecture of the Solar System

4. The Origin of the Solar System

5. The Solar System’s Place in the Galaxy

6. The Fate of the Solar System

7. Concluding Remarks Bibliography

Bibliography

1. Introduction

The origins of modern astronomy lie with the study of our solar system. When ancient humans first gazed at the skies, they recognized the same patterns of fixed stars rotating over their heads each night. They identified these fixed patterns, now called constellations, with familiar objects or animals, or stories from their mythologies and their culture. But along with the fixed stars, there were a few bright points of light that moved each night, slowly following similar paths through a belt of constellations around the sky (the Sun and Moon also appeared to move through the same belt of constellations). These wandering objects were the planets of our solar system. Indeed, the name “planet” derives from the Latin planeta, meaning wanderer.

The ancients recognized five planets that they could see with their naked eyes. We now know that the solar system consists of eight planets, at least three dwarf planets, plus a myriad of smaller objects: satellites, asteroids, comets, rings, and dust. Discoveries of new objects and new classes of objects are continuing even today. Thus, our view of the solar system is constantly changing and evolving as new data and new theories to explain (or anticipate) the data become available.

The solar system we see today is the result of the complex interaction of physical, chemical, and dynamical processes that have shaped the planets and other bodies. By studying each of the planets and other bodies individually as well as collectively, we seek to gain an understanding of those processes and the steps that led to the current solar system. Many of those processes operated most intensely early in the solar system’s history, as the Sun and planets formed from an interstellar cloud of dust and gas, 4.56 billion years ago. The first billion years of the solar system’s history was a violent period as the planets cleared their orbital zones of much of the leftover debris from the process of planet formation, flinging small bodies into planet-crossing, and often planet-impacting, orbits or out to interstellar space. In comparison, the present-day solar system is a much quieter place, though many of these processes continue today on a lesser scale.

Our knowledge of the solar system has exploded in the past four decades as interplanetary exploration spacecraft have provided close-up views of all of the planets, as well as of a diverse collection of satellites, asteroids, and comets. Earth-orbiting telescopes have provided an unprecedented view of the solar system, often at wavelengths not accessible from the Earth’s surface. Ground-based observations have also continued to produce exciting new discoveries through the application of a variety of new technologies such as charge-coupled device (CCD) cameras, infrared detector arrays, adaptive optics, and powerful planetary radars. Theoretical studies have also contributed significantly to our understanding of the solar system, largely through the use of advanced computer codes and high-speed, dedicated computers. Serendipity has also played an important role in many new discoveries.

Along with this increased knowledge have come numerous additional questions as we attempt to explain the complexity and diversity that we observe on each newly encountered world. The increased spatial and spectral resolution of the observations, along with in situ measurements of atmospheres, surface materials, and magnetospheres, have revealed that each body is unique, the result of a different combination of physical, chemical, and dynamical processes that formed and shaped it, as well as its different initial composition. Yet, at the same time, there are broad systematic trends and similarities that are clues to the collective history that the solar system has undergone.

We have now begun an exciting new age of discovery with the detection of numerous planet-sized bodies around nearby stars. Although the properties and placement of these extra-solar planets appear to be very different from those in our solar system, they are likely the prelude to the discovery of other planetary systems that may more closely resemble our own.

We may also be on the brink of discovering evidence for life on other planets, in particular, Mars. There is an ongoing debate as to whether biogenic materials have been discovered in meteorites that were blasted off the surface of Mars and have found their way to Earth. Although still very controversial, this finding, if confirmed, would have profound implications for the existence of life elsewhere in the solar system and the galaxy.

The goal of this chapter is to provide the reader with an introduction to the solar system. It seeks to provide a broad overview of the solar system and its constituent parts, to note the location of the solar system in the galaxy, and to describe the local galactic environment. Detailed discussions of each of the bodies that make up the solar system, as well as the processes that have shaped those bodies and the techniques for observing the planetary system are provided in the following chapters of this Encyclopedia. The reader is referred to those chapters for more detailed discussions of each of the topics introduced.

Some brief notes about planetary nomenclature will likely be useful. The names of the planets are all taken from Greek and Roman mythology (with the exception of Earth), as are the names of their satellites, with the exception of the Moon and the Uranian satellites, the latter being named after Shakespearean characters. The Earth is occasionally referred to as Terra, and the Moon as Luna, each the Latin version of their names. The naming system for planetary rings is different at each planet and includes descriptive names of the structures (at Jupiter), letters of the Roman alphabet (at Saturn), Greek letters and Arabic numerals (at Uranus), and the names of scientists associated with the discovery of Neptune (at Neptune).

Asteroids were initially named after Greek and Roman goddesses. As their numbers have increased, asteroids have been named after the family members of the discoverers, after observatories, universities, cities, provinces, historical figures, scientists, writers, artists, literary figures, and, in at least one case, the astronomer’s cat. Initial discoveries of asteroids are designated by the year of their discovery and a letter/number code. Once the orbits of the asteroids are firmly established, they are given official numbers in the asteroid catalog: over 136,500 asteroids have been numbered (as of September 2006). The discoverer(s) of an asteroid are given the privilege of suggesting its name, if done so within 10 years from when it was officially numbered.

Comets are generally named for their discoverers, though in a few well-known cases such as comets Halley and Encke, they are named for the individuals who first computed their orbits and linked several apparitions. Because some astronomers have discovered more than one short-period comet, a number is added at the end of the name in order to differentiate them, though this system is not applied to long-period comets. Comets are also designated by the year of their discovery and a letter code (a recently abandoned system used lowercase Roman letters and Roman numerals in place of the letter codes). The naming of newly discovered comets, asteroids, and satellites, as well as surface features on solar system bodies, is overseen by several working groups of the International Astronomical Union (IAU).

2. The Definition of A Planet

No formal definition of a planet existed until very recently. Originally, the ancients recognized five planets that could be seen with the naked eye, plus the Earth. Two more jovian planets, Uranus and Neptune, were discovered telescopically in 1781 and 1846, respectively.

The largest asteroid, Ceres, was discovered in 1801 in an orbit between Mars and Jupiter and was hailed as a new planet because it fit into Bode’s law (see discussion later in this chapter). However, it was soon recognized that Ceres was much smaller than any of the known planets. As more and more asteroids were discovered in similar orbits between Mars and Jupiter, it became evident that Ceres was simply the largest body of a huge swarm of bodies between Mars and Jupiter that we now call the Asteroid Belt. A new term was coined, “minor planet,” to describe these bodies.

Searches for planets beyond Neptune continued and culminated in the discovery of Pluto in 1930. As with Ceres, it was soon recognized that Pluto was much smaller than any of the neighboring jovian planets. Later, measurements of Pluto’s diameter by stellar occultations showed that it was also smaller than any of the terrestrial planets, in fact, smaller even than the Earth’s Moon. As a result, Pluto’s status as a planet was called into question.

In the 1980s, dynamical calculations suggested the existence of a belt of many small objects in orbits beyond Neptune. In the early 1990s the first of these objects, 1992 QB1 was discovered at a distance of 40.9 astronomical units (AU). More discoveries followed and over 1000 bodies have now been found in the trans-Neptunian zone. They are collectively known as the Kuiper belt. All of these bodies were estimated to be smaller than Pluto, though a few were found that were about half the diameter of Pluto.

The existence of the Kuiper belt suggested that Pluto, like Ceres, was simply the largest body among a huge swarm of bodies beyond Neptune, again calling Pluto’s status into question. Then came the discovery of Eris (2003 UB₃₁₃), a Kuiper belt object in a distant orbit, which turned out to be slightly larger than Pluto.

In response, the IAU, the governing body for astronomers worldwide, formed a committee to create a formal definition of a planet. The definition was presented at the IAU’s triennial gathering in Prague in 2006, where it was revised several times by the astronomers at the meeting. Eventually the IAU voted and passed a resolution that defined a planet.

That resolution states that a planet must have three qualities: (1) it must be round, indicating its interior is in hydrostatic equilibrium; (2) it must orbit the Sun; and (3) it must have gravitationally cleared its zone of other debris. The last requirement means that a planet must be massive enough to be gravitationally dominant in its zone in the solar system. Any round body orbiting the Sun that fails condition (3) is labeled a “dwarf planet” by the IAU.

The outcome left the solar system with the eight major planets discovered through 1846, and reclassified Ceres, Pluto, and Eris as dwarf planets. Other large objects in the asteroid and Kuiper belts may be added to the list of dwarf planets if observations show that they too are round.

Although most astronomers have accepted the new IAU definition, there are some who have not, and who are actively campaigning to change it. There are weaknesses in the definition, particularly in condition (3), which are likely to be modified by an IAU committee tasked with improving the definition. However, the likelihood of the definition being changed sufficiently to again classify Pluto as a planet is small.

In this chapter we will use the new IAU definition of a planet. For a...

Table of contents

• Cover image
• Title page
• Table of Contents
• Inside Front Cover
• Copyright
• Contributors
• About the Editors
• Foreword
• Preface to the Second Edition
• Preface to the First Edition
• Chapter 1: The Solar System and Its Place in the Galaxy
• Chapter 2: The Origin of the Solar System
• Chapter 3: A History of Solar System Studies
• Chapter 4: The Sun
• Chapter 5: The Solar Wind
• Chapter 6: Mercury
• Chapter 7: Venus: Atmosphere
• Chapter 8: Venus: Surface and Interior
• Chapter 9: Earth as a Planet: Atmosphere and Oceans
• Chapter 10: Earth as a Planet: Surface and Interior
• Chapter 11: The Sun-Earth Connection
• Chapter 12: The Moon
• Chapter 13: Meteorites
• Chapter 14: Near-Earth Objects
• Chapter 15: Mars Atmosphere: History and Surface Interactions
• Chapter 16: Mars: Surface and Interior
• Chapter 17: Mars: Landing Site Geology, Mineralogy and Geochemistry
• Chapter 18: Main-Belt Asteroids
• Chapter 19: Planetary Satellites
• Chapter 20: Atmospheres of the Giant Planets
• Chapter 21: Interiors of the Giant Planets
• Chapter 22: Io: The Volcanic Moon
• Chapter 23: Europa
• Chapter 24: Ganymede and Callisto
• Chapter 25: Titan
• Chapter 26: Triton
• Chapter 27: Planetary Rings
• Chapter 28: Planetary Magnetospheres
• Chapter 29: Pluto
• Chapter 30: Physics and Chemistry of Comets
• Chapter 31: Comet Populations and Cometary Dynamics
• Chapter 32: Kuiper Belt: Dynamics
• Chapter 33: Kuiper Belt Objects: Physical Studies
• Chapter 34: Solar System Dust
• Chapter 35: X-Rays in the Solar System
• Chapter 36: The Solar System at Ultraviolet Wavelengths
• Chapter 37: Infrared Views of the Solar System from Space
• Chapter 38: The Solar System at Radio Wavelengths
• Chapter 39: New Generation Ground-Based Optical/Infrared Telescopes
• Chapter 40: Planetary Radar
• Chapter 41: Remote Chemical Sensing Using Nuclear Spectroscopy
• Chapter 42: Solar System Dynamics: Regular and Chaotic Motion
• Chapter 43: Planetary Impacts
• Chapter 44: Planetary Volcanism
• Chapter 45: Astrobiology
• Chapter 46: Planetary Exploration Missions
• Chapter 47: Extrasolar Planets
• Appendix
• Glossary
• Index