23 October 2019


Paul Murdin. The Secret Lives of Planets. Hodder and Stoughton, 2019

Paul Murdin is a policymaker for the UK government, a fellow of the Royal Astronomical Society and also a commentator for the BBC. He draws on a lifetime of astronomy, which manifests in this clearly written description of the wonders of the universe.
The author takes you on a journey through space in a splendidly detailed and informative book that gathers together all the latest research to tell the history of our solar system. He describes how all eight major planets have been born from violence and how they grew up together to become living, breathing worlds - and how they're set to fade away as they age. He also talks about moons, stars, comets, and asteroids.

The book begins by describing the outdated reasoning, from only a few centuries ago, that the solar system is perfectly regular, running predictably like clockwork. The wondrous disposition of the sun, planets, and comets could only be the work of an all-powerful and intelligent Being. According to Newton, God orchestrates the movements of the planets and other bodies within the solar system and controls them through the Laws of Gravity. This has become known as the 'Divine Watchmaker' argument.

Many believe that the quantum-mechanics revolution of the 1920s is settled science I am not of that mind, and there may be other alternatives. The standard quantum model only allows us to know either the position or trajectory of a subatomic particle — not both at the same time. Calculating the orbits of planets' behaviour is in the short term predictable but as the author relates "in the long term, depends so much on the initial state that you cannot calculate in the long term". He continues: "The displacements in position that arise as a result of slight initial displacement grow uncontrollably".

He then compares this theory to the methods of meteorologists who usually can predict the weather, more or less accurately one day or even a week ahead. However, in the well-known adage, they cannot predict how air disturbances from the flapping wings of every butterfly in Brazil can affect where a hurricane will strike Florida next year. This tiny unknowable 'Butterfly Effect' has completely changed the nature of long-range weather forecasts. There are simply too many unknown variables that come into play and increase over time. These are highly complex matters, but I found to my pleasure that the author is adept at explaining them in uncomplicated terms.

He informs us that there are 3,800 planets known in orbit around stars other than the Sun, known as exoplanets and there are more than 2500 other stars with planets orbiting them in our galaxy discovered so far. There are likely to be many more planetary systems out there waiting to be discovered!

The eternal question of the possibility of extraterrestrial life is put in perspective by Murdin as he states "The bottom line is that our solar system has no parallel among known planetary systems. Astronomy has no fully accepted explanation for this yet."

Planets are the main result of a process in which large bodies that had solidifies from a disc of material that originally surrounded this Sun as it was forming, the so-called "solar nebula". The asteroids, comets and other orbiting bodies were detritus leftover from this process and fragments created since that time by collisions of asteroids. This helps put Pluto in a new light as being more detritus than planet.

A new definition was adopted in 2006 which ruled out Pluto as a planet, as to be a planet an object has to have enough of a size that it has cleared out of its orbit of other bodies, and has to dominate the orbital zone that it inhabits. Pluto does not  qualify: it crosses Neptune’s orbit and orbits among other TNO's (Trans-Neptunian Objects), so is regarded as a "dwarf planet". The asteroid Certes is also regarded as a dwarf planet.

Murdin continues in a light-hearted manner describing Mercury as "bashed, bashful and eccentric”. This airless planet and its cratered surface speaks of a cosmic bashing known as the Late Heavy Bombardment. About 3.9 billion years ago, and is also the shyest planet, hard to inspect closely even now with our technology of the Space Age. It hides in the skirts of the Sun and is difficult to see so near to the bright sunlight, peeping out briefly from time to time.

Mercury was the messenger from the gods, fleet of foot: it is the fastest planet in orbit, it revolves around the Sun in only 88 days, as against Earth's 365. Greek astronomers had two names for Mercury: Apollo and Hermes, as the period of visibility of Mercury is different from its orbit and they assumed there were two planets. It was Pythagoras who pointed out in about 500BC that the two were identical.

Space-probes that even just orbit Mercury have difficulty coping with the Sun's heat, so surface landers or rovers cannot operate there, as most materials would melt or decompose at Mercury’s equator and so far nothing feasible has been found that would be viable. As an aside, reading this book made me think is there mercury on Mercury? I discovered “At Mercury's temperature, mercury the element would likely be vaporised, but cinnabar, the mercury bearing ore, could potentially be found in its crust.”


I was interested to learn that three hundred pieces of the Moon have been found that fell to Earth after being knocked off the Moon’s surface by the impact of asteroids. The oldest lunar rocks are those collected from lunar highlands, lighter areas of the Moon. Individual rocks from the lowlands, the dark maria, have ages that seem to cluster between 3.85 to 4 billion years old, which was when they last solidified. It appears therefore that the crust of the Moon was strongly heated 3.9 billion years ago.

Sheffield University astronomers suggested that after the Moon had first solidified about 4.5 billion years ago, asteroids heavily bombarded its surface for 200 million years and re-melted it. This event was known as the 'Lunar Cataclysm' (an early name for the Late Heavy Bombardment); but the reason the bombardment happened is as yet unresolved.

Murdin then explains how simulations - calculations about a large number of possible scenarios of various degrees of invention - are important to astronomers to understand their own theories about the birth of our universe. The most interesting scenario for the Late Heavy Bombardment has been put forward as a result of what is known by astronomers as the “Nice Simulation” describing an event at happened in the first billion years or so of the history of the solar system. At that period it was like a gigantic game of interplanetary billiards or pool played by hyperactive children let loose around a pool-table. This Nice simulation is one of several calculations of how the planets might have interacted, at the stage when they had just formed in the solar system.

Murdin goes rather poetic: “There was at that time a counter-factual future life for Earth, in which the Earth became an interstellar planet roving around the Galaxy like a lone coyote on the icy, vacant prairie” explaining that this did not happen to our planet but it may well have happened to one of Earths former neighbors. Although the Earth was not ejected from the Solar System in this chaotic period in its development, the Earth shifted its orbit back and forth towards and away from the Sun. Our planet ended up in the so-called 'Goldilocks Zone' (not too hot, not too cold) of the Solar System, which made the evolution of life possible, purely as a matter of luck.

Finally, we (Earth) are battered and punched into shape! In Murden's lively description: “Asteroids were pulled and swung out of their orbits, heaved about by massive effects of Jupiter and Saturn. Some asteroids, jaywalking across the more orderly circular paths of planets, fell on planets, especially those inwards towards the Sun, like Mercury”. They pounded their surfaces, making craters – perhaps this was the event that we know as the Late Heavy Bombardment."

Throughout the book, the author presents thought-provoking questions on what distinguishes an asteroid from a meteorite, which is the only planet named after a Greek, rather than a Roman God and which boasts of a Volcano 100 times the size of Earths largest and much more.

He takes us on a venture through all our planets and the space in between, this mysterious journey is very eventful and exciting and rewarding. I am impressed by the publisher's production, the book has a very cool cover and the paper feels luxurious, there are many colour images, and the price makes it most  accessible. An excellent book both for the amateur stargazer, as well as the more academic reader.  –  Gerrard Russell.

1 comment:

astrodreamer said...

The erasure of astrology is notable.