Two planets collected most of the rocky material that was left in the inner nebula after the gas and volatile elements were swept away by early solar activity. They finished up with nearly an equal share. Here I investigate why these twins that have almost the same mass and density have turned into such different worlds.

Venus

Venus, rising in the morning or setting in the evening skies, is the most brilliant object in the sky, after the Sun and the Moon, and has been admired since antiquity. Because it is an apparent twin of the Earth, it has always been of interest as the only similar planet in the solar system. When Venus was found to have an atmosphere, it did not take much imagination to make it a hotter version of the Earth. In popular literature, it was clothed with thick tropical forests and swamps, that were populated with various monsters. Dinosaur-like creatures were favorites of science-fiction writers [1].

Early observers thought that the planet was either spinning rapidly, similar to the 24-hour period of the Earth, or perhaps on a monthly period. But Venus was discovered from radar observations to be unique among the planets. Although the atmosphere of Venus rotates in about four days, the planet itself has a retrograde rotation that takes 243 days [2] (Table 12.1). Venus orbits around the Sun in 225 days and so the day on Venus is longer than the year.

A principal task in writing in the first edition of this book was to examine the series of events that led to the formation of the solar system. The conclusion, so greatly illuminated by the previous three decades of planetary exploration by spacecraft, was that random events had predominated in the construction of the great variety of planets and satellites. Thus it was unlikely that duplicates might be found elsewhere. This was in contrast to earlier views that the solar system was as orderly as a clock and that, given sufficient computer power, one might simulate the construction of such a clockwork system from first principles according to the laws of physics and chemistry.

In the 1992 edition, I commented that “the … common occurrence of disks around young … stars strengthens the case for the existence of other planetary systems. If so, would they resemble our own? Would we see something like the Galilean satellite system of a few equal-sized planets, systems with one giant planet, or a single brown dwarf companion?” After contemplating the satellite systems around our giant planets, I concluded that “no simple sequence of reproducible events has occurred in our solar system. Other planetary systems … will be different in detail to our own. What their satellites might look like is only for bold spirits to predict” (p. 251).

The pre-Copernican view

Pre-Copernican theories in which the Earth was the center of the universe have long lost the attention of scientists. This is not only because such theories have been superseded since the Copernican Revolution, but also because in such hypotheses the origin of the Earth, Sun and planets is inextricably bound up with the origin of the universe. The Earth could hardly be younger than the rest of the universe if it occupied the central position. We are now aware that the solar system is less than one-third of the age of the observable universe. This makes it no longer necessary, as was the case with the authors of the Book of Genesis, to seek a common origin for Earth, Moon, Sun and stars. Most of this progress has been made by the discovery of new facts, not by theories. Galileo's observations, like those of Darwin, have done more to give us a correct view of the world than most of the theorising about it over the centuries.

The Greeks

The Babylonian and Greek astronomers observed the strange motion of the planets against the fixed positions of the stars [1]. In this manner, they became aware that there were two classes of heavenly objects in addition to the Sun and the Moon. It is curious that although the ancient astronomers devoted much study to the movements of the planets, they did not spend much time considering the origin of the solar system.

“After the Sun, the Moon of all the heavenly bodies is that which interests us the most; its phases afford us a measure of time so remarkable that it has been primitively in use among all people” [1]. The Moon and Mercury represent special cases even by the standards of the solar system. Mercury is unique due to its high density, with an iron/silicate ratio about twice that of the other inner planets (Section 11.1). In contrast, the Moon is of interest because of its low density and low metal/silicate ratio [2]. Explanations for the peculiar nature of both bodies have a long history and much effort has been expended in attempts to fit one or both into overall schemes of planetary formation, but without conspicuous success.

The Moon has played a central role in the recent development of theories of the origin and evolution of the solar system. This is not without irony, as it has proven one of the most difficult objects to explain. It is in plain sight, accessible even to naked-eye observation, as Harold Urey (1893–1981), who persuaded NASA to go to the Moon, was accustomed to remind us. The Moon was often thought to be a kind of Rosetta Stone, so that the general belief in pre-Apollo times was that we could discover much about the origin of the solar system by going to the Moon [3].

In this chapter I discuss ideas on the construction of planets. These notions are naturally heavily biased toward observations within our solar system, although I include some comments based on the discovery of extrasolar planets. Although limited, these are already shedding light on some of the problems and importantly on the distinction between planets and brown dwarfs. The conclusions from these new bodies appear to be broadly consistent with present ideas. The extrasolar planets that are found in close orbits probably originated in a similar manner to our own gas giants, although subject to later migration, something that may have also happened to our own giants.

The problem of building planets is fundamental to the entire question of the origin of the solar system. Historically, this latter question has frequently been considered to have been solved, but the wide variety of explanations and solutions that have been offered, from the creation myths of primitive societies, to the more recent, but numerous, scientific attempts, have generally collapsed when faced with new information about the system [1].

There are two principal difficulties. The first dilemma is that until very recently planetary scientists, like historians, had only one example, the present scene, together with whatever relics have survived from previous epochs, to tell the tale of former events [2].

A reluctant conversion

A quotation from Ursula Marvin headed this chapter in the 1992 edition: “A hypervelocity meteorite impact is an extraordinary event – wreaking change instantaneously. Such a process violates every tenet of uniformitarianism. Largely for this reason, hypotheses of impact origin for craters on the Earth and the Moon were vigorously opposed for the better part of the last [19th] century … research has now established, beyond doubt, the authenticity of impact as a geological process, but … a wide chasm still persists between the views of impact specialists and those of terrestrial geologists” [1]. By 1999 a major philosophical advance had occurred so that it was possible for her to write that “Today we realise that collisions in space are the most fundamental process that has operated throughout the history of the solar system. This is a truly revolutionary insight that requires a fresh vision of the basic tenets of geology” [2].

It might be considered surprising to include a separate chapter on this topic in a book that is principally concerned with examining the origin and evolution of the solar system from a cosmochemical perspective. However, it has become clear that collisions between bodies have played a significant role in the evolution of the solar system. These effects have occurred at all times and stages, beginning with the sticking together of grains in low-velocity collisions in the dusty midplane of the nebula, and continuing with the growth of planetesimals from meter- to kilometer- and eventually to planet-sized bodies.

The giant planets, their satellites, the asteroids and the comets formed very early. In the inner portions of the nebula, within a couple of AU of the Sun, a collection of dry rocky planetesimals that had survived the early solar winds, began slowly to assemble into larger bodies. Moonsized objects appeared. Finally several the size of Mercury and Mars dominated the scene but most were swept up by the two bodies that became Earthand Venus. A couple escaped into stable orbits and survive as Mercury and Mars.

Mercury

Although there is a common perception that Mercury is so close to the Sun that it is difficult to observe, it is often clearly visible away from city lights as a morning or evening star. Mercury is unique on account of its high density that tells us that it has a high content of metallic iron relative to rock. In contrast, the Moon has the reverse; a low content of metal relative to rock. Explanations for the peculiar nature of both bodies have a long history and much effort has been expended in attempts to fit one or both into overall schemes of planetary formation.

Like the Moon, the real significance of the anomalous nature of Mercury has only recently been appreciated; both these objects have played the role of red herrings in our attempts to understand the solar system [1].

The Sun and the solar system (and ourselves) are latecomers in the universe. The universe was in existence for around ten billion years before the formation of the solar system. Another four-and-a-half billion years passed before Homo sapiens arose to survey the surroundings. When the solar system formed, the universe would have long settled down into its present familiar appearance, complete with galaxies and stars, and would have looked much the same as today. However, due to the slow relative movements of the stars, our familiar constellations, such as Orion the Hunter and his companion, the Great Dog (Canis Major), will be rearranged and replaced by other groupings in the future. Edmond Halley (1656–1742) seems to have been one of the first to have realized this, when he observed that the positions of many stars in the early 18th century differed from those recorded in the catalogue of Hipparchus in the second century BC.

Our nearest star (Proxima Centauri, an 11th magnitude M5 red dwarf and the faintest of a triple-stellar system of which Alpha Centauri is the brightest) is about 4.3 light-years or about 1.3 pc distant from Earth. Although Proxima Centauri is the nearest star at present, the dwarf star Ross 248 will succeed to the title in about 33,000 Earth years [1].

The plurality of worlds?

The discovery of planets orbiting other stars and the widespread occurrence of dusty circumstellar disks, some with gaps in which planets may be lurking, has raised once again and in a dramatic fashion, the ancient question posed amongst others by Albertus Magnus in the 13th century:

“Since one of the most wondrous and noble questions in Nature is whether there is one world or many, a question that the human mind desires to understand, it seems desirable for us to enquire about it” [1].

These questions have been discussed under many headings for the past 24 centuries since Democritus and Epicurus in Greece favored a multitude of worlds in contrast to Plato and Aristotle who considered the Earth to be unique [2]. The concept of a multitude of habitable worlds has appeared historically under several headings such as “the plurality of worlds” [3], “the principle of plenitude (abundance)” [4] and “the principle of mediocrity” which states that our neighbourhood is more or less typical of the rest of the universe [5]. The whole question of the existence of Earth-like planets is inextricably intertwined with the debate over the existence of extraterrestrial life (which is usually assumed overtly or covertly to be intelligent). On this topic the biologists, familiar with the random course of evolution, have mostly been sceptical [6] while the physicists have been less so [7].

Jupiter and Saturn are splendid planets. With their associated rings and satellites, they excite the admiration of all observers. One perceptive writer compared the giant planets to French impressionist paintings. “Jupiter's orange and yellow bands are so roiled up that its disk might have been painted by van Gogh at Arles. Saturn, with more delicate bands of ochre, resembles a Monet haystack in a sunlit mist. Uranus' disk, though, is so featureless and limpid as to suggest the still pond around a Monet water lily” [1].

Saturn, recorded from the seventh century BC by the Babylonian astronomers, was the outermost planet known to the ancients. Uranus was the first planet to be discovered since antiquity. Its discovery by William Herschel (1738–1822) in 1781, although often called accidental, resulted from his careful checking of star positions and so was inevitable. He called it George's Star (Georgium Sidus) after King George III, who presented Herschel with a lifetime pension. Such nationalistic fervor did not receive universal acclaim and the planet was soon given the classical name of Uranus. Next it was discovered that the orbit of Uranus was being affected by another large body further away from the Sun. Study of these variations in the orbit of Uranus led eventually to the discovery, in 1846, of another large planet, Neptune.

The true dimensions of the terrestrial globe were revealed mostly in the fifteenth and sixteenth centuries, principally through the technical development of truly ocean-going vessels and the magnetic compass, which enabled the exploration of the oceans. Even with these advantages, the real extent of the Great Southern Continent or terra australis incognita had to await the voyages of Cook in the eighteenth century. This flood of new geographic knowledge replaced the medieval view of the world (although Flat-Earth societies still persist). Such understanding was gained only very slowly. Adetailed knowledge of the topography of the ocean floors and our understanding of their composition and origin has been obtained only a little ahead of our radar pictures of the surface of Venus.

Our exploration of the solar system is at a similarly heroic stage. The distant points of light, barely resolvable in telescopes, have been revealed through the use of space vehicles, the latter-day equivalent of the Portuguese caravels, as separate worlds, with an astonishing amount of diversity.

The information has been rapidly and widely disseminated, electronic media having superseded the printing press of the Renaissance. Everyone is informed of the striking new discoveries. Although no Eldorados have emerged, the pictures reveal a plurality of worlds unimagined by the Elizabethans. Every satellite has turned out to differ in some significant feature from its neighbor: “… the sense of novelty would probably not have been greater if we had explored a different solar system” [1].

The most-ancient samples

Meteorites present us with tantalizing evidence concerning the origin of the solar system and they provide us with the best evidence of the time when the solar system formed. However, as the citizens of Ensisheim, Alsace, noted about the meteorite that fell there in 1492, “many know much about this stone, everyone knows something, but no one knows quite enough” [1].

Some of the grains in primitive chondrites retain a memory of presolar conditions. The refractory inclusions in chondrites tell us of the earliest recorded events in the solar system. Even though the CI carbonaceous chondrites were somewhat altered in an aqueous environment, the elemental abundances of these meteorites resemble those of solar photospheric spectra for the nongaseous elements, providing a basis for estimating the composition of the solar nebula [2]. Much chemical processing occurred very early, which resulted in (i) the formation of refractory inclusions, chondrules; (ii) the separation of metal, sulfide, and silicate phases; and (iii) the depletion of volatile elements in the inner parts of the solar nebula. Differentiated meteorites such as the eucrites and the iron meteorites tell us of the existence of both planetesimals and of heat sources that induced melting. This differentiation also occurred very early, with the production of true igneous rocks close to T0.

Even after the cessation of the turbulent conditions that accompanied meteoritic and planetary formation, meteorites that reach the Earth have had an interesting dynamical history.

My purpose in writing this book is to enquire into the solar system and how it came to be. So much progress has been made in the past decade that this book has been completely rewritten from the first edition. The seven large chapters in that edition have been restructured into fifteen smaller ones that deal more readily with the increased flood of information.

As in the first edition, I have tried to place the solar system in the broader context of the universe. My excuse for venturing into fields such as cosmology is to reinforce the point that the solar system is a relative newcomer in the universe and came about through a fortunate sequence of chance events. A secondary purpose is to try to overcome the narrow and potentially hazardous specialization that is endemic in science and that I talk about in the Prologue. I have attempted to educate myself in fields remote from my own through discussions with many colleagues, listed in the Acknowledgments.

The book does not follow the usual descriptive arrangement of starting with Mercury and marching stolidly out through the giant planets to Pluto. Instead, the unconventional arrangement I have adopted here has arisen naturally as I have tried to explain how the system arose from the solar nebula and why the various bodies happen to be where they are. The result is that there are many associations that may at first sight seem surprising.

The initial concept

The fundamental and perhaps the most obvious fact about the solar system is that the planets and satellites mostly lie close to the plane of the ecliptic (the Sun–Earth plane) and, with minor and informative exceptions, rotate in the same sense, both around the Sun and about their axes of rotation. Their orbits, although elliptical, have very low eccentricities. In the 18th century, scientists were unaware of the retrograde rotation of Venus and of other irregularities: what they saw appeared to be as well ordered as a clock. This fortunate lack of too much information enabled the French astronomer and mathematician, Pierre-Simon, marquis de Laplace, to propose in 1796 that the solar system originated from a rotating disk of dust and gas [1]. He called this disk the solar nebula. In his model, the planets condensed successively from rings as the nebula contracted. This elegant concept survived in its original form until late in the 19th century. The crucial and ultimately fatal flaw of the original laplacian theory was the failure to account for the concentration of (i) angular momentum in the planets and of (ii) mass in the Sun [2].

The view that the Sun and the planets formed from a rotating disk of gas and dust, the solar nebula, provides such an obvious explanation that it has become axiomatic. Nevertheless, it is wise to question established viewpoints.

The scale and structure of the universe

In order to obtain some perspective on the solar system, it is useful to contemplate the scale of the universe as we perceive it at present, as “on the galactic scale of things, the solar system is a rather small place” [1]. The mean distance between the Sun and the Earth is 149.6×106 km or one astronomical unit (AU). The diameter of the Sun is 1.392×106 km, close to 0.01 AU. Mercury is close to the Sun at 0.4 AU, but many of the extrasolar planets are within 0.05 AU of their parent star. Our planetary system extends out to the orbit of Neptune at about 30 AU. Beyond that, from about 35 to over 1000 AU, lies the Edgeworth–Kuiper Cloud of icy comets. Further out is the inner Oort Cloud extending from 5000 to over 50,000 AU. The classical Oort Cloud of comets marks the outer bound of the solar system extending from 50,000 to about 105 AU. All these distances are very small on a galactic scale.

Titius–Bode Rule

The distances of the planets from the Sun can be expressed as a series 0.4, 0.7, 1.0, 1.6, 2.8, 5.2 etc., that is close to their separation in AU. This sequence of numbers can be arrived at by adding a constant 0.4 to the doubling sequence of 0, 0.3, 0.6, 1.2, 2.4, 4.8 etc. There are many other more sophisticated formulations.