Meet your nemesis

In July 1994, a spectacular celestial fireworks show had astronomers across the world spellbound for weeks. Comet Shoemaker-Levy, broken into many pieces, barrelled into Jupiter, setting off a series of gargantuan explosions and visibly changing the face of that giant planet. From the safety of 630 million km away, we on Earth marvelled at this display of cataclysmic power. All our bombs and missiles seemed puny, irrelevant, in comparison. I'm sure even hardened professional astronomers felt a small chill running down their spines as they watched the pyrotechnics. How fortunate that Shoemaker-Levy had chosen to plough into Jupiter, and not our own Earth! Can you imagine the destruction it would cause here?

In fact, we don't need to imagine the destruction. We know quite a bit about it already, and we knew well before the spectacle of Shoemaker-Levy. For such events have happened on Earth. Not just some small pebbles from space that fall on our heads, either. Our planet has been in collisions on just the scale of what happened on Jupiter in 1994.

Paleontology, the science that studies ancient fossils and tells us about life on Earth millions of years ago, has long puzzled over a singular event in our past. The fossil record shows that about 65 million years ago, dinosaurs suddenly went extinct. Till that time, they had roamed the Earth freely for millions of years, the undisputed rulers of our planet.

But almost overnight, 65 million years ago, their reign came to an abrupt end. Dinosaurs vanished, never to be seen again on Earth. What had caused this stunning disappearance?

In 1979, a physicist called Luis Alvarez came up with an answer. His son Walter, a geologist, found a layer of clay deposited in our oceans that dated to exactly the time the dinosaurs had vanished. From certain peculiarities in the clay, the Alvarezes and two nuclear chemists showed that it had to have been caused by an extraterrestrial object -- perhaps a comet or an asteroid, rocks floating through space -- crashing into the Earth. The enormous impact, they said, had caused the extinction of the dinosaurs.

Astronomers had known for a while that disastrous impacts like this one were more than likely to have happened. They had observed comets and asteroids whose paths in space cross that of the Earth. Now, in that layer of clay, there was proof of just such an impact. What's more, there was now an explanation for what had happened to the dinosaurs.

Actually, Alvarez had found evidence for two such impacts: one 65 million years ago, and another about 39 million years ago. It turns out that the second also coincides with an extinction. Many land mammals died out suddenly then, the fossil record tells us. So we've had at least two huge collisions with something in our past. Both had catastrophic effects on life on Earth at the time.

But the Alvarez theory was only the beginning. Things really started getting interesting in about 1983.

By then, paleontologists David Raup and John Sepkoski had carefully gathered a lot of data on extinctions from fossils. Analysing it, they found not just two, but several enormous extinctions in our planet's history. That there had been many is not particularly surprising. But there was a startling thing about these events: they had happened every 26 million years. Like clockwork. Even the two extinctions Alvarez had blamed on asteroid impacts fit the pattern.

Here was a puzzle indeed. Why these regular disasters? There wasn't any evidence that they were due to collisions with asteroids -- except, of course, for Alvarez's findings. But if they were, they should happen randomly. Rocks, even the Earth, that move through the huge emptiness of space certainly must collide from time to time. But regularly? How was that possible? There was nothing wrong with Raup and Sepkoski's results, except that they seemed ridiculous. What could possibly explain them?

Luis Alvarez himself initially dismissed the Raup-Sepkoski theory. The two collisions he had found, he insisted, were random events. Being so, they did not belong in any pattern. It was just a coincidence that they had happened 26 million years apart. In fact, if you removed them from Raup and Sepkoski's data, the evidence for regularity became tenuous indeed.

But what if there was a way that celestial objects could hit the Earth every 26 million years? Richard Muller, an ex-student of Alvarez, confronted with the puzzle of Raup and Sepkoski's data, asked himself just this question. With two colleagues, he then suggested a way that our planet might, in fact, experience enormous collisions like clockwork.

Muller proposed that the Sun has a companion star orbiting around it. This is not a novel idea. We know of and have observed many such binary star systems. Of course, nobody had thought the Sun might be part of one. Muller's hypothetical star takes 26 million years to complete its orbit. The diameter of such an orbit is easily calculated: about 2.8 light years. That puts this mysterious star closer than Alpha Centauri -- at 4 light years away, the nearest known star to us.

Every 26 million years, Muller's star approaches the Earth and triggers storms of comets. Some of these hit the Earth and cause the extinctions that Raup and Sepkoski had discovered. For obvious reasons, Muller called his star "Nemesis".

It was a nearly perfect theory. Muller and others later found evidence that impacts on the Earth occured at the same times as the extinctions, fitting the 26 million year pattern. The idea of comet storms explains other observations well, too.

There's just one problem: Nemesis itself has not been found yet.

All the stars visible to the naked eye have been ruled out. Nemesis, if it exists, must be a small, dark star, lost in a sea of much brighter stars. There are skeptics who doubt it exists, but Muller carries on searching for it undaunted. He is convinced the Nemesis theory best fits the evidence.

And if he turns out to be right, you can bet a comet will come barrelling into us, just as Shoemaker-Levy did on Jupiter. The next such impact is scheduled for something like 13 million years from now. Set your alarm clock.