If mass extinctions have a 26-million-year cycle, then there is a driving force that disturbs the planet at a regular interval of 26 million years. Where is that force in the universe? Where is 'the big clock' that triggers mass extinctions?
It cannot be the sunspot cycle; the number of visible sunspots - the freckle-like, dark, cool regions on the Sun's surface - varies in a regular cycle reaching a maximum about every eleven years. It is not planet Pluto, the most distant of the nine planets, which takes only 247 years to circle the Sun. It is not even the wobble of the Earth's axis, which does not always point exactly at the same spot in the sky but traces out a small circle in a 26,000-year cycle. The 'clock' is certainly not in the solar system. However, the whole solar system makes a complete circle around the centre of the Milky Way in about 250 million years. This is known as the 'cosmic year', but it does not fit the bill either. We do not know of any astronomical event that has a cycle of 26 million years.
The boldest, yet quite feasible, idea to explain the 26-million-year cycle came from the American astronomer Richard Muller and his colleagues. They proposed that, like many other stars, the Sun has a companion star. This companion star moves in an elliptical orbit around the Sun, taking 26 million years to complete one orbit. Once in every 26 million years the companion star comes closer to the solar system, where it passes through the Oort cloud of comets.
During each passage through the Oort cloud, the Sun's companion star disturbs a large number of comets, sending them towards Earth. A shower of comets that lasts thousands of centuries bombards Earth. The increasing dust in the atmosphere darkens the skies. As the temperatures on the ground plunge, most of the animals and plants perish.
The researchers said: 'If and when the companion is found, we suggest it to be named Nemesis, after the Greek goddess who relentlessly persecutes the excessively rich, proud and powerful.' The eminent palaeontologist Stephen Jay Gould proposed that it be named Siva, for the Hindu god who periodically destroys and creates the world.
Popular science magazines were, however, quick to dub it the 'death star'. During the mid-1980s the Nemesis story was very popular and controversial, and was widely covered by the popular press. One newspaper even described it as 'having everything but sex and the Royal Family'. It is rare for a major daily newspaper to write an editorial opinion on a scientific theory. However, in 1985 The New York Times wrote an editorial, 'Miscasting the Dinosaur's Horoscope', on the Nemesis theory. It rejected 'the alleged repeating pattern of mass extinctions' and suggested that 'astronomers should leave the astrologers the task of seeking the cause of earthly events in stars'.
The idea of a 26-million-year cycle raises the question: when is the end coming? The good news is: it's at least 13 million years away. The last time Nemesis brushed the comet cloud was about 13 million years ago. It is now about 2 light years (18,921 billion kilometres) away from the Sun, nearly half the distance to the second nearest known star, Alpha Centauri.
If there is a companion star, why have astronomers not yet seen it? Nemesis is believed to be a brown dwarf. Brown dwarfs are too small to achieve the hydrogen burning that powers stars; therefore, they are too faint to detect. No optical or infrared telescope has yet detected a brown dwarf.
In 2001, nearly two decades after the publication of the Nemesis theory, physicists Robert Foot of the University of Melbourne and Zurab Silagadze of the Budker Institute of Nuclear Physics in Russia suggested that Nemesis had proved elusive because it is made of mirror matter and so is invisible. 'It's very hard to prove, but fun to speculate', according to Silagadze.
If it is not Nemesis, the Sun's companion star, behind the 26-million-year cycle, then it must be a distant unknown planet - Planet X. The tenth planet is believed to be three to five times heavier than Earth. It is gaseous like Jupiter, and takes 1,000 years to orbit the Sun. At present it is three times as far out as Pluto, that is, about 15 billion kilometres from the Sun (Earth is only about 150 million kilometres away). Some scientists have proposed that the orbit of Planet X continuously shifts because of the gravitational tug of the other planets. Every 26 million years the shifting orbit disturbs the Oort cloud, producing a comet shower on Earth. No search - even by space probes and the Hubble space telescope - has yet given any hint of the existence of Planet X.
The idea of a 26-million-year cycle in mass extinctions is an attractive one: it suggests that most of them will have similar causes. If we could find the culprit for one, we would have caught a serial killer. Most scientists now reject the idea. However, Raup believes that the idea is still 'alive and well' despite the lack of an astronomical clock. The proposal is still on the table, he says, awaiting new data or new ways of looking at old data. If we move away from the idea of a single cause for all the mass extinctions, we can limit our search to the one that concerns us most - the death of the dinosaurs.
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