John Zarnecki follows arthur dent's unlikely adventures

The film's explanation of the Earth's origin is unorthodox: it was made by the planet designer Slartibartfast

The Hitchhiker's Guide to the Galaxy

Released April 28

We teach our students that planet Earth came about something like this: about 4.5 billion years ago, a large, rather anonymous cloud of galactic gas and dust suffered a random perturbation that caused its very tenuous material to start collapsing in on itself - or, as we prefer to phrase it, "the cloud exceeded its Jeans mass" (after astrophysicist Sir James Jeans 1877-1946).

Slowly, but inexorably, this started a process that would result in most of the material coalescing at the centre into a G2 star (better known as the Sun). A bunch of leftover material formed a large number of small bodies called planetesimals, which further coalesced into a small number of larger bodies that now go by the more familiar names of Mars, Jupiter, Mercury, Earth and so on. You get the picture.

But there are other theories. And in the film of Douglas Adams's now classic book, The Hitchhiker's Guide to the Galaxy , we are introduced to one of the more radical suggestions. More of that later.

Actually, many theses propounded here deviate rather radically from the scientific mainstream. One concerns the fate of Earth, namely the planet being suddenly and without warning vaporised to make way for a hyperspace bypass. Only two Earthlings survive - Arthur Dent and Trillian. They go through a series of unlikely adventures and meet some of the most bizarre beings imaginable, including the Vogons, who write the third worst poetry in the galaxy.

Incidentally, the fact that Adams's Milky Way seems to be teeming with life is no longer at odds with what we teach our students today. After all, we have found more than 130 stars with planets orbiting around them. Couple this with the discovery of biological life in the most extreme niches on Earth - including nuclear reactors - and there has been a shift in our preconceptions.

We even have the Drake equation, one of my favourites, which predicts the number of intelligent civilisations (N) we might find:

N = R * Pp * NE * PL * Pi * PC * T

The only trouble with this equation is that the values of all the terms on the right-hand side are so uncertain that you can get any value you want for N.

Perhaps, then, the variety of life forms encountered by our heroes is not so far-fetched.

But as I have noted, the film's explanation of the origin of the Earth is, um, a little unorthodox: it was designed and made by the award-winning planet designer Slartibartfast. Dent and Trillian manage to persuade this individual to build another one to replace the original so carelessly lost.

And that is exactly what he does, even though custom-built planets are described as "a bit of a luxury".

So our present abode is no more than the product of a galactic factory production line, carefully crafted by alien beings (though, of course, that still does not answer the question as to how the first one came about).

As for the film, it was a lot of fun - despite the Americanisation of two key characters. How Dent could have suspected Ford Prefect came from Guildford with an accent straight out of New York defeats me. But then again, conventional logic does not play a big part in The Hitchhiker's Guide to the Galaxy.

John Zarnecki is professor of space science at the Open University and a principal investigator on the Huygens probe that landed on Titan, one of the moons of Saturn, in January.