Secrets of the Solar System

Planets: A Very Short Introduction

OXFORD UNIV. PRESS: 2010. 135 pp. £7.99

Half a century of space exploration has yielded strange and exciting insights into the Solar System, including hints at a molten middle in one of Jupiter's moons, and the possibility of a ninth planet.

In Planets: A Very Short Introduction, David Rothery takes the reader on a whirlwind tour of the planets of the Solar System, together with their moons and various other bodies. Included in this latter category are asteroids, comets and trans-Neptunian objects — rocky objects exceeding 100 km in size that whizz round the Sun beyond the orbit of Neptune.

Rothery's pared-down description of star formation is one of the most memorable parts of the book. A great cloud of hydrogen contracts under its own weight, generating vast quantities of heat. This, in turn, triggers the fusion of hydrogen nuclei, the formation of helium, and the birth of a star. Rothery describes planets as a mere by-product of this process: left-over fluffy dust grains collide and stick together. Over a period of thousands of years, these masses grow into centimetre-scale globules. Eventually they become rocky bodies hundreds of kilometres in size and “massive enough for their own gravity to pull them into spherical shapes”. When these large rocky bodies crash into one another, they melt and meld; following a sufficient number of collisions, a planet emerges.

“The surface of Venus — hidden beneath a permanent layer of cloud — remained a mystery until radar measurements in the nineties revealed extensive trails of lava.”

The geology of the inner, terrestrial planets of the Solar System — Mercury, Venus, Earth and Mars — and their satellites is explored in some depth. Particularly striking is Rothery's portrayal of the prevalence and magnitude of volcanism. The surface of Venus — hidden beneath a permanent layer of cloud — remained a mystery until radar measurements in the nineties revealed extensive trails of lava. These trails are thought by some to have emanated from a massive burst of volcanism that began around 500–700 million years ago. Over on the Moon, flood basalts are attributed to the expulsion of “fountains of incandescent molten lava” billions of years ago, through fissures in the satellite's surface.

But it's not just internal processes that shape the surface of the terrestrial planets and their moons. Rothery elaborates on the surface-sculpting effects of the solar wind, ultraviolet light and meteorites, in a process termed space weathering. Particularly vulnerable are Mercury and the Moon, which have too low a mass to capture an atmosphere — or, in Rothery's words, “too little gravity to hang on to a gas blanket”.

The discussion of the massive outer planets — Jupiter, Saturn, Uranus and Neptune — is intriguing, too. Sometimes referred to as the gas giants, these planets contain copious quantities of hydrogen and helium, and their lower atmospheres are thought to merge “seamlessly into a fluid interior at temperatures and pressures so high that there is no distinction between gas and liquid”. As such, it is impossible to gauge the exact size of these planets or their internal dynamics. But a general understanding of the Solar System has led to the notion that “a sea of hydrogen” surrounds the inner cores of Jupiter and Saturn. The atmospheres of the gas giants are more accessible, and observations have revealed ammonium-bearing clouds and disfiguring giant rotating storms.


No less interesting are the moons of the giant planets, which Rothery describes as “substantial worlds” in their own right, which have much in common with the terrestrial planets. Europa, Rothery's favourite and one of Jupiter's moons, is thought to harbour a salty ocean, which heralds hopes for finding life beneath its surface of ice. And Saturn's moon, Titan, hosts lakes of ethane-tinted liquid methane.

Rothery's playful and evocative language — be it about the “cold wind howling” in the valleys of Mars or his description of Jupiter's moon, Io, as “a fantastic place for a volcanologist to visit”, although “thoroughly inimical to human exploration” — bring these alien landscapes to life, and mean that each turn of the page yields palpable insights into the Solar System.

The final chapter delves into the galaxy beyond our Solar System. The first exoplanet was detected in 1995 — since then more than 300 planets have been found orbiting Sun-like stars (including, of course, the six latest additions to the list; Nature 470, 53–58; 2011). There are numerous methods for detecting these far-away planets, including one that relies on the gravitational pull of the planet on the star. As Rothery explains, “an unseen orbiting companion will tug the star from side to side”, causing it to dance out of position.

Given Rothery's lively imagination and his enthusiasm for all things space, it's not surprising that his hopes run high when it comes to the likelihood of life on other planets: “with countless trillions of appropriate organic molecules in an exoplanet's ocean, and with million of years to play with, life will inevitably start.” But, as he notes at the end, in what seems like a desperate final plea, “if life is so abundant, and if intelligence is a common outcome of life, then where is everybody?”


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Armstrong, A. Secrets of the Solar System. Nature Geosci 4, 133 (2011).

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