TIME travel is normally fixed in both direction and speed: forward only, and no more or less than twenty four hours a day. Part of the thrill of a new year is the sense of this stately progress leaping, just a little, as a year ticks over in a second. When two digits tick by at once the thrill expands further as the grain gets coarser. Dividing history into ten-year chunks on the basis of the last-but-one digit is arbitrary, but knowing that it will be thus divided, despite the senselessness of so doing, gives the 09 to 10 transition and its ilk added relish. Something new, or at least different, has begun.

Given that time travel moves only forward, it might seem that this would be the only way new periods can be added to history. Not so, though, if you are a geologist or Earth scientist. For them the past offers billions of years of room for redefinition and subdivision, with the beginnings and endings of various geological periods and subperiods tussled over continuously. A recent paper goes further, offering the possibility of adding periods where there were previously none: of extending the Earth's history, not further into the future, but further into the past.

SPL

To appreciate this, a quick recap of the Earth's origins. About 4.6 billion years ago a large cloud of gas, probably shocked by a nearby supernova, fell in on itself, forming grains of dust and, rather more spectacularly, a star. In the disk of gas and dust around this nascent sun smaller objects coalesced into larger ones, encountering each other with ever greater violence as they became ever larger. Near the end of this process a Mars-sized object crashed into a Venus-sized object, in the process creating the Earth (which is larger than Venus) and the Moon (which is smaller than Mars). This knockabout phase lasted only a few tens of millions of years, though the orbits of the planets, especially the big ones further from the sun, did not settle down until a few hundreds of millions of years later.

At present, the Earth's geological history does not deal with this period well, for the simple reason that geologists deal mostly with rocks, and rocks older than about 3.8 billion years are rare indeed. Rocks from 3.8 billion years ago to 2.5 billion years ago are assigned to the Earth's earliest geological eon, the Archaean. Anything earlier—a few lumps of Greenland and Canada, and rock-residues preserved now only as inclusions in larger, later, rocks—are referred to as belonging to the Hadean, an informal and ill-defined but useful and evocative term.

The new proposal suggests not just that the Hadean should be formalised, but also that a new aeon, the Chaotian, should be recognised as extending extend further back in time than the Earth itself. The Chaotian would begin with the beginning of the cloud's collapse, be punctuated in the middle with the ignition of the sun and come to an end with the collision that created the Earth-moon doublet in its sort of modern form. In a fit of further distinctions, the authors—Colin Goldblatt, a young researcher at NASA's Ames Research Centre, and three older colleagues with considerable previous form in the framing of provocative hypotheses—suggest the Nephelean and Erebrean for the periods before the sun's ignition, the Hyperitian and Titanomachean for those after.

Other than demonstrating that there are scientists abroad with classical educations, lively imaginations and a little spare time on their hands, what would such a seemingly silly extension of geological history to the pre-rock, pre-planet era actually achieve? Two things. One, which the authors themselves put forward, is that given the increasing amount of serious and informative science being done with respect to the early history of the Earth and the solar system—not least in the context of observations of other systems round other stars—an agreed vocabulary with which to discuss the timing of various events and transitions would be helpful. A second is that if the solar system is to be considered a system, in the sense of something more than the sum of its parts, it needs a system-wide timescale that stretches back to its beginning, and into which the timescales of the various planets can be pieced as they become better understood. To use the Earth's geology, which is the best understood of that of any planet, as the basis for this is as good a solution as any.

Following on from that, and of less specialist interest, is the degree to which such thinking expands people's conceptions of their planet and its place in the scheme of things. The Earth is not an isolated lump of rock, a system unto itself; it is the result of events that created it, and of processes that extend far beyond its physical limits. It is sprinkled with dust older than the sun, constantly pulled this way and that by tides while occasionally smacked about by comets and asteroids. Most crucially, it is shot through with the energy of the sun, energy which drives the planet's winds and rivers, stirs its oceans and brings life to its plants and all who eat them—and thus accounts for almost all of what is available in the renewable energy business. To see the Earth and its properties in the cosmic contex of the processes that have formed and sustained them, rather than just as lumps of stuff, is salutory—whether looking billions of years back or a decade or so into the future.