At present there is a popular consensus in favor of intelligent life elsewhere in the universe. These notions are at least 2500 years old and rest on several assumptions, that have run as a common thread throughout history. Many lie outside the realm of scientific enquiry coming under the heading of “must be” arguments. The first is that the universe is infinite and so must contain planets identical to the Earth. This is often phrased as the “Big Numbers” argument. There are so many stars with planets that somewhere out there must be a replica of us. The second is that because life exists here, it must be common elsewhere. The third is that the development of intelligence is inevitable and happens elsewhere concurrently with, or more commonly in advance of, the evolution of life on Earth. These themes are addressed below under several headings.

THE PLURALITY OF WORLDS [2]

The discovery of many planets orbiting other stars, free-floating objects and the widespread occurrence of dusty circum-stellar disks, some with gaps in which planets are lurking, has raised once again in a dramatic fashion, the ancient question posed amongst others by Albertus Magnus in the thirteenth 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” [3]. One of the favorite current quotations of astrobiologists comes from Metrodorus of Chios (350 BCE). It is usually stated as “it is unnatural in a large field to have only one ear of corn and in the infinite universe, only one living world”. These questions have been discussed under many headings for the past 25 centuries since Democritus, Epicurus and Metrodorus favored a multitude of worlds.

AN EXPANDING VIEW

In order to obtain some perspective on the place of planets in the scheme of things, it is useful to contemplate the scale of the universe, as we perceive it at present. Everything in the universe is very isolated. The nearest star to us is Proxima Centauri, a red dwarf and the faintest member of a triple star system of which Alpha Centauri is the brightest. This star is familiar to dwellers in the southern hemisphere, as it forms one of the Pointers to the Southern Cross. Light from this nearest star takes 4.2 years to reach us. 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. Because of the slow relative movements of the stars, our familiar constellations, such as Orion the Hunter and his companion, the Great Dog, will be rearranged and replaced by other groupings in the future. Edmund Halley (1656–1742), of comet fame, seems to have been one of the first to realize this, by observing that the positions of many stars in the early eighteenth century differed from those recorded in the catalogue of Hipparchus in the second century BCE.

We live in remarkable times replete with technical advances, a consequence of the great intellectual advances of the seventeenth and eighteenth centuries in Europe. Destiny or Chance in 1998 looked at the solar system to examine the question whether our planets were likely to be reproduced elsewhere. From the evidence then available, this was judged to be very unlikely, while the possibility of intelligent life resembling Homo sapiens [1] elsewhere was assessed to be zero. In the succeeding dozen years, major improvements in technology have resulted in the discovery of thousands of exoplanets. Has the situation changed? Yes, in the sense that it has gotten worse. Not only are the exoplanets “Strange New Worlds” as a popular book title has it, but our familiar solar system itself, with its tidy circular orbits, appears to be a rarity. The very architecture of the solar system, familiar to every schoolchild, appears to have arisen through chance collisions and migrations half a millennium after it formed.

Destiny or Chance was written following a close look at our solar system. The numerous planets, satellites, TNOs, asteroids, centaurs and other assorted debris that surround our Sun provided no evidence of design. The resulting array, strange enough when looked at objectively, was clearly the result of a series of chance events. Halfway through writing Destiny or Chance, the first exoplanets were discovered. These “Hot Jupiters” were totally unexpected by astronomers, although less surprising to students of the solar system. Lurking in the background is the expectation that something like the Earth, complete with its set of interesting inhabitants, might be discovered.

Our planet is the very model of a habitable planet. Accompanied by its silvery Moon, the Earth forms our only current example of habitability. For this reason, these currently unique objects require a separate section, to consider why this might be so and how such a welcome situation came about. But the random processes involved during formation and the level of complexity exposed by subsequent geological and biological evolution give one pause in seeking clones among the exoplanets.

THE EARTH

John Updike remarked that “this planet is exceptional; clearly Venus or Jupiter wouldn’t be agreeable to us” [2]. How did it manage to achieve this status? Can one deduce any general principles from a unique planet accompanied by a unique satellite? A good example has been the difficulty in recognizing on the Earth that the formation of craters by the impact of asteroids, comets and meteorites is an important planetary process. Erosion has removed most of the evidence. It is only within living memory that professional geologists have come to accept that the craters were not caused by internal eruptions. Our experience with the geology and geochemistry of the Moon, with its subtle but crucial distinctions from our hard-won experience on the Earth, should also remind us of the hazards of trying to extrapolate from unique terrestrial conditions.

WHAT IS A PLANET?

This question is less important than one might suppose, given the uproar about the status of Pluto. Although labels are useful, trying to define a planet runs into the philosophical difficulty of attempting to classify any set of randomly assembled products. A bewildering array of objects form in the nebular disks around stars. These items include in our system, dust, asteroids, Trojans, Centaurs, comets, TNOs, our eight planets from tiny Mercury to mighty Jupiter and their 160 satellites. All differ from one another in some salient manner. A rational view would merely define our planetary system as having four planets (the gas and ice giants) with some assorted rocky rubble sunwards and icy rubble beyond. The significant question is how did they form and evolve, not what pigeonholes this variety of objects can be forced into. The strange varieties of exoplanets and brown dwarfs have added much extra complexity [2].

Trying to write a current account about exoplanets reminds one of the definition of Post-Modernism: “it's so new that it's out of date already”. Looking for exoplanets around stars used to be an unpopular topic, shunned by professional astronomers; but times change. One is reminded of the comment by Talleyrand that “treason is a matter of dates”. Now a barely concealed excitement suffuses much of the serious scientific literature. Among many quotations the following give a flavor: “the search for extra-terrestrial planets – rocky worlds in orbit around stars other than the Sun – is one of humanity's most exciting science goals” [2], while the real goal seems to be “the detection and characterization of habitable worlds” [3].

The previous section showed what we have attempted to learn about the formation of planets, based strongly on observations of our own system. The discovery of the exoplanets has revealed many surprises and demonstrates once again the triumph of observations over theory. Although “one of the most surprising aspects of the hundreds of known exoplanets is their broad diversity” [2], this was less surprising to students of our own solar system, in which all the planets and satellites display an astonishing range of sizes and compositions. This results in the notorious difficulties of trying to classify or even define planets, exacerbated now that we have such a variety of exoplanets. The long established upper limit for planets of 12 or 13 Jupiter-masses, that marks the lower limit of deuterium burning, has now been extended to at least 25 Jupiter-masses into the brown dwarf desert, and will need to go to higher masses. Nevertheless and consistent with its name, the desert remains thinly populated. Below the 12 Jupiter-mass limit, problems also occur because of the discovery of free-floating objects down to at least 3 Jupiter-masses. Are these bodies that are not linked to any star, “free-floating planets” ejected from the nest, or “sub-brown dwarfs”? We seem to be approaching a classification for planets, brown dwarfs and the like based on where they formed. Truly a nightmare for classifiers.

SOURCES

This book is derived from many discussions and sources, the first six books being particularly useful:

1. Apai, D. and Lauretta, D. Protoplanetary Dust, Cambridge University Press, 396 pp., 2010.

2. de Pater, I. and Lissauer, J. J. Planetary Sciences, second edition, Cambridge University Press, 647 pp., 2009.

3. Lodders, K. and Fegley, B. Chemistry of the Solar System, RSC Publishing, Cambridge, UK, 476 pp., 2011.

4. McFadden, L. et al. (Editors). Encyclopedia of the Solar System, second edition, Academic Press, 935 pp., 2006.

5. Perryman, M. The Exoplanet Handbook, Cambridge University Press, 410 pp., 2011, which contains over 3200 references.

6. Seager, S. (Editor) Exoplanets, Arizona University Press, 526 pp., 2010.

7. Taylor, S. R., Solar System Evolution: A New Perspective, Cambridge University Press, 460 pp., 2001.

8. Taylor, S. R. On the difficulties of making Earth-like planets (Leonard Medal address). Meteoritics and Planetary Science, 34, 317–329, 1999.

9. Taylor, S. R. and McLennan, S. M. Planetary Crusts: Their Composition, Origin and Evolution, Cambridge University Press, 378 pp., 2009.

The series of books on planetary and space science published by the University of Arizona Press are especially relevant to the theme of this book, in particular the following volumes:

1. Asteroids III (Editors: W. Bottke et al.) 2002.

2. Jupiter (Editor: Fran Bagenal) 2007.

3. Meteorites and the Early Solar System II (Editors: D. Lauretta and H. Y. MacSween) 2006.

4. Protostars and Planets V (Editors: B. Reipurth et al.) 2007.

How does our solar system fit among the great host of exoplanetary systems? It is not a typical sample of what is out there so it is not a good example of the “Principle of Mediocrity”. Its obvious advantage is that it is the only one that we can examine in detail and so forms the basis for current models of planetary origin. What is revealed are not only our eight planets, interesting though they are as possible analogs for exoplanets, but also hosts of other bodies in orbit either around the Sun or planets. We have over 160 satellites, four ring systems, dust, swarms of many different species of asteroids, KBOs, TNOs and wanderers such as comets, centaurs, NEAs and NEOs. The system resembles a half-finished long-abandoned building site, littered with leftover rubble and assorted debris. Although little of this detail is yet evident in exoplanetary systems, it is useful to examine our solar system in depth to see what actually happened in one disk around a star. It is not only a matter of just forming planets.

The solar system has long been a benchmark, becoming from familiarity, what was commonly expected to be a norm. But reality was different, as the first exoplanet discovery, Pegasi 51b, demonstrated so dramatically. Now even our system of planets is seen to be uncommon, the end result of chaotic and chance events.

Planets were curiosities in most ancient models of the universe. But the discovery of Uranus and later Neptune led to speculation about their origin. Clearly they were distinct from stars and so might form in a different manner. But such notions came very late once it was established that they orbited the Sun and that the stars were remote.

The problem of building planets is fundamental to the entire question of the origin of planets. 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. The latest problems stem from the bewildering variety of exoplanets. There are several difficulties.