posted on Jul, 14 2010 @ 05:00 AM
For at least the last 500 million years, say Adrian Melott, an astrophysicist at the University of Kansas, and Richard Bambach, a palaeontologist at
the Smithsonian Institute, there has been a burst of extinctions every 27 million years.
Periodic mass extinctions have been posited before, and it has been suggested that it means the Sun has a huge, dark neighbour which orbits it every
27 million years, each time knocking a shower of comets out of the Oort Cloud at the fringes of the solar system and sending them crashing into Earth.
This hypothetical dark satellite was called "Nemesis".But the regularity of the extinctions and the timescale they occur over - the 500 million
years examined by Dr Melott and Dr Bambach is almost double that earlier studies have looked at - seem to rule that out.
This is because in the last 500 million years the Sun has had close encounters with many other, known stars. The gravitational pull of these stars
would have affected the orbit of Nemesis, causing it to lose the regular 27 million year cycle. The peak should either have been slowly changed by up
to 20 per cent, causing it to smear out over the aeons, or to suddenly change, giving two or more peaks on a single cycle. Instead, it is based around
a regular 27 million year cycle, with a 99 per cent chance that it's not random chance. The researchers say: "Fossil data, which motivated the idea
of Nemesis, now militate against it."
This implies that it is not a hidden celestial body, but something closer to home, since it is unlikely that anything in space would maintain such a
regular heartbeat over so long a time.
The extinctions range from utter catastrophe, devastating the majority of species on Earth, to smaller ones like the most recent, which destroyed 10
per cent of known species. And while they happen once every 27 million years, they do not arrive on cue, but up to 10 million years either side of the
However, there is no immediate panic. The last such event took place 10 million years ago, so there should be plenty of time to work out what is going
on before the next one comes.