When - and if - planets collide

Scientists used to scoff at the idea that planets could wander free from a fixed orbit. Now they have learnt more, and nobody is laughing.

It's one of the basic rules of science: if you want more insight, get more data. But it's not working out so easily for astronomers trying to make sense of the hundreds of new planets they've discovered around other stars. Once upon a time, scientists trying to understand the origin of our galaxy were stuck with a motley collection of just five other worlds visible to the naked eye. But since the mid-1990s, ground-based and orbiting observatories have found over 470 new planets around stars beyond the solar system. They range from cosmic rubble barely the size of the moon to vast "gas giants" 10 times more massive than Jupiter, which orbit their stars on anything from long, cigar-shaped paths to perfect circles.

Yet despite having almost a hundred times more examples to study than their forebears, today's astronomers don't feel they have a hundred times more insight. On the contrary, the more they find out, the more they realise they don't know. One thing is for sure, though: the age-old view of planetary systems as some kind of beautiful celestial clockwork is light-years from reality. The picture now emerging is more akin to a cosmic pinball machine, with planets suddenly careening about and creating havoc.

Last week, a team of astronomers from the US and Germany announced their latest enigmatic discovery about alien worlds. It was made using an ingenious technique that allows planets to be detected around stars without seeing them directly. One normally thinks of planets as orbiting around the exact centre of their parent stars. In reality, both the planets and their stars revolve around their common centre of gravity. And if the planets are massive enough, this can be some distance away from the centre of the star. As a result, the whole star appears to wobble about in space, and by analysing this motion astronomers can extract a host of information about the planets - despite not being able to see them.

Last week, the team revealed details of two pairs of planets they had found using this technique around two different stars, both over 200 light-years away. What makes these pairs of planets so unusual is that they are following orbits that are both relatively close to one another, and in precise mathematical relationships. For example, the two planets orbiting the star code-named 24 Sextanis are just 113 million kilometres apart; by comparison, the two largest planets in our solar system always stay at least five times further apart. No less strange is the fact that they have orbital periods of 455 days and 910 days respectively, so that one of them completes an orbit in exactly half the time of the other.

The other pair of planets, orbiting the star HD 200964 are even closer together at just 53 million kilometres, and they too have oddly related orbital periods of 630 days and 830 days, ensuring that one completes four orbits in the time it takes the other to complete three. The astronomers admit to being baffled as to how these planets came to occupy such bizarre orbits. It's unlikely they just happened to form there. Instead, the suspicion is that the planets were involved in some primordial game of pinball which only ceased once they found refuge in the orbits they now occupy.

Until recently, the very idea of planets wandering around would have been greeted with derision. But the more extra-solar planets astronomers study, the more it appears that this is the only way to explain what they're finding. Many such planets are much larger than Jupiter, and yet complete their orbits in a matter of days rather than decades. That, in turn, means they are so close to their parent stars that they reach temperatures of over 1,000°C - raising the question of how such planets ever formed in the first place. While the idea that they wandered there from much further away might seem outlandish, it is a lot less problematic than the alternative.

If planets do wander around from time to time, presumably they can also collide. This would explain another perplexing discovery made over recent months by astronomers in the US and Europe. Using the orbiting European observatory named Corot, they have been able to compare the tilts of the orbits of many planets relative to the axis of their host stars. According to current theories, planets are formed from the dust and gas that accumulates in a disc surrounding the star. That, in turn, suggests that the orbits of planets should all be roughly aligned with the equator of their parent stars. That's certainly the case with our solar system, where all the major planets have orbits tilted only a few degrees relative to the sun's equator.

Yet observations of planets beyond the solar system have shown that many are following orbits tilted at over 30 degrees. One possible explanation is that they have been knocked out of their original orbits by past "fly-bys" or even direct collisions. If many planets beyond our solar system have a violent past, what about our own? Although the orbits of the major planets are relatively stable, some of them seem to bear the scars of past upheaval. For example, Uranus spins on its side, suggesting it was knocked over by some primordial collision, while Venus is actually orbiting upside down. Closer to home, the rocks brought back by the Apollo missions 40 years ago suggest that our moon was smashed out of the Earth around 4,500 million years ago by the impact of an object around the size of Mars.

In the 1950s, a Russian psychiatrist named Immanuel Velikovsky published a best-selling book entitled Worlds in Collision. Much of it was pretty nonsensical, but the scientific world was particularly incensed by the claim that Venus and Mars had once come perilously close to the Earth. At the time, a leading astronomer at Harvard University declared: "If Dr Velikovsky is right, the rest of us are crazy."

It was a put-down that got a few laughs at the time, but no one is laughing now. Robert Matthews is a visiting reader in Science at Aston University, Birmingham, England