Does our sun have a companion star whose orbit periodically causes comets to bombard Earth with disastrous consequences?
There is a reputable theory backed by considerable evidence to support this scenario. The companion star has been dubbed Nemesis, after the Greek goddess of retribution. It is sometimes called the death star.
The story of Nemesis began in 1977 when geologist Walter Alvarez collected rock samples in Gubbio, Italy, in a study of Earth's magnetism. The Gubbio rocks provide a good record of the transition from the cretaceous to the tertiary geological period 65 million years ago.
This transition marks the end of the age of reptiles, when dinosaurs ruled the Earth. The huge extinction of life at that time claimed nearly 75 per cent of all species on Earth, including the dinosaurs.
Alvarez found a thin layer of red clay sandwiched between the rocks of the two periods. The clay was unusually enriched in the element iridium. This iridium-rich clay layer at the cretaceoustertiary boundary was subsequently found in many places around the world.
Iridium is quite scarce in the Earth's crust, but higher concentrations are found in extraterrestrial objects such as asteroids and comets.
Alvarez concluded that the iridium anomaly was explained by the collision of a 10 km diameter extraterrestrial object with Earth, causing the massive extinction of life in which the dinosaurs perished. Such a huge collision would blow dust high into the stratosphere where it would encircle the globe.
An enormous fireball would trigger huge fires from which soot and debris would rise to blot out the sun, creating a long period of continuous frozen night. This would stop photosynthesis, the process whereby green plants use sunlight to make food, and all but the hardiest plants would quickly die off. Food chains would be destroyed causing a massive extinction of species.
In 1984, two scientists from Chicago, David Raup and John Seposki, having ca talogued extinctions of sealife over the past 250 million years, noted that the extinctions seemed to occur approximately every 25 to 30 million years. Various explanations were suggested. Volcanic eruptions were an obvious candidate, blowing massive amounts of dusty debris into the atmosphere and precipitating the freezing dark conditions already described. However, volcanoes cannot account for the high iridium and soot levels found in the clay layers.
Some scientists looked to space for an explanation. One possibility is comets, which have been described as frozen "dirty snowballs" with rocky centres. Beyond Pluto, the outermost planet in our solar system, and extending out more than 13 trillion km, is a vast swarm of comets called the Oort cloud, named after Jan Oort.
These comets slowly orbit the sun. Occasionally a passing star jerks some comets loose by gravitational attraction and hurls them into our solar system. Few ever reach the inner solar system where Earth lies because they get "vacuumed up" by the gravitational attraction of the giant Jupiter and Saturn.
However, if a large number of comets was to be jarred out of the Oort cloud, the probability of comets bombarding Earth would greatly increase. If the Oort cloud was regularly disturbed in this manner, Earth would receive periodic bombardments and life extinctions. This is the Nemesis star theory.
It is proposed that Nemesis is a companion star to our sun, both rotating around a common centre of gravity in a binary system. This is something like two dancers doing "a swing" in Irish dancing. More than 50 per cent of stars in the galaxy are partners in binary systems.
Some proponents of the Nemesis theory believe the companion star to the sun is a red dwarf, less than one-third the size of the sun and one thousandth as bright. Others believe that Nemesis is a brown dwarf star (i.e. an invisible star which never started to shine), or else it is a black hole.
Nemesis might travel in an elliptical orbit that, at its closest, brings it within five trillion kilometres of the sun and into the middle of the Oort cloud. At the moment, Nemesis would be at the farthest range of its orbit - about 30 trillion km from the sun. Every 25-30 million years, Nemesis enters the Oort cloud and Earth gets bombarded by comets.
If the Nemesis theory is correct, then comet impact craters on Earth should have birth dates which cluster at 25 million year intervals. Craters do not survive well on Earth, gradually getting eroded away by weather. However, 13 of the largest and most accurately dated craters, spanning the last 250 million years, show the 2530 million year periodicity.
The case for Nemesis is far from conclusive. For example, some palaeontologists deny that the fossil record shows mass extinctions every 25 to 30 million years. If the Nemesis star were found in the heavens, shown to be a binary star of our sun and its orbit shown to periodically disturb the Oort cloud, this would pretty much settle the case. A search is ongoing for such a star, but so far without success.
If the Nemesis theory is correct, the gradual mechanism of Darwinian evolution only operates over 30 million-year spans, and is then interrupted by cataclysmic change. The cataclysmic changes are massive lotteries in the history of life, producing a few winners and many losers. The giant reptiles lost out 65 million years ago, leaving small mammals to inherit the Earth. We are their descendants.
Finally, lest you worry overmuch about this matter, let me reassure you on one point. If Nemesis theory is correct, that deadly little star is not due to return again until the year 15 million AD.
William Reville is a senior lecturer in biochemistry and director of microscopy at UCC.