Ocean Worlds:The Story of the Seas on Earth and Other Planetsby Jan Zalasiewicz and Mark WilliamsOxford University Press, 2015The fact that we live in a Golden Age of astronomy is the reason Ocean Worlds, the new book by University of Leicester geology professors Jan Zalasiewicz and Mark Williams, has a plural in its title; thanks to advances in equipment (such as the great Kepler space telescope and the European Space Agency's Herschel Infrared Telescope) and assiduous field work, we now know about the existence of a couple of thousand exoplanets – worlds circling distant stars in such profusion as to make Galileo weep with joy.Scientists detect the existence of those exoplanets in a variety of ways, including measuring the dimming of the light of their home stars as they pass between them and Earth's watching eyes. And scientists detect the characteristics of those exoplanets in a variety of ways as well, including scrutinizing the tiniest variations in the nature of those incredibly distant occultations. It's extremely abstruse science, and it yields staggering imaginative dividends. From what the science of astronomy best theorizes here in 2015, we know that there are at the very least hundreds of worlds in the universe that have oceans. Hence, ocean worlds.Zalasiewicz and Williams can't tell their readers much about those alien oceans yet (an update of this book in ten years' time will surely look nothing at all like this one), but they can tell those readers quite a bit about the only oceans we know well: Earth's own. Our authors keep those home oceans in carefully-balanced perspective, often using alien worlds as yardsticks – like, for example, the exoplanet 55 Cancri e, which circles a star 40 light years from Earth in the constellation of Cancer. That world circles star at such a frightfully close orbit that its year lasts only 18 hours, and yet it seems to have enormous oceans of some kind, and Zalasiewicz and Williams are quick to point out some of the key differences between those oceans and anything found on Earth:

55 Cancri e is a water world, of a sort. Earth is not a water world and never will be. Even if the world warmed sufficiently to melt all of the ice of Antarctica and Greenland, of the Himalayas and the Alps and the Cordillera, sea levels would rise by only 70 metres. This is enough to inundate many of the world's great cities, granted: London, Shanghai, and New York would all be beneath the waves. But it would by no means submerge the world completely.

Water accounts for only 0.05 per cent of Earth's total mass, whereas there are indications that alien worlds may be more than 10 percent water, with oceans so deep that the sheer pressure would turn the liquid at their bases to solid. And in addition to size and pressure, there's also the variable of time: our authors point out the latest strong indications that a near-Earth planet like Mars once had wide and sparkling oceans, before time and environmental forces withered them away – a process, they remind us, to which Earth itself is hardly immune:

But the universe, we now know, began 13.8 billion years ago in the Big Bang, while our solar system, and the Earth, are just 4.6 billion years old. On the timescale of modern science, what prospects do we have? Will oceans last until the end of the Earth? Or will the Earth's old age be parched and arid? If so, it may become a planet that we might not recognize, were we to take a trip here in that old time-travelling telephone box. Indeed, we might be well advised not to open the doors.

Naturally, in a book of this nature, the subject of oceans can hardly be raised without mentioning the appalling treatment humans now routinely doll out to the waters of their own home world, where enormous islands of plastic trash float around the world, where toxin levels are hundreds of times higher than they were only half a century ago, and where all strata of ocean life are being devastated by over-fishing and environmental degradation. “It would be ironic,” they conclude,

in that very human way, to discover a wealth of strange and bizarre oceans out in the cosmos just as we are dismantling the beautiful and unique oceans on our own doorstep. Those distant oceans are, for any foreseeable human prospects, entirely unreachable. Many will be intricate and fascinating as regards their physics and chemistry, but will be biologically dead. Of the living ones that now seem likely to be out there, most will be dominated by microbes - the condition of life in the Earth's oceans for more than three-quarters of their history, after all. Few will have the kind of biological riches of Earthly seas. None of them will suit us as well as our own oceans do.

The oceans of Earth are undergoing dramatic, almost yearly change, and Zalasiewicz and Williams do a very agile job of balancing their book between the wonders of the oceans we know and the potentials of the oceans we don't. The result is both a call to arms and an almanac of awesome splendor.