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THE Solar System may contain an extra planet made of a mysterious dark
substance called mirror matter, say physicists. The mirror world may even have
caused several mass extinctions on Earth.

Astronomers first suggested in the mid-1980s that a massive planet or star,
dubbed Nemesis, might exist鈥攂ut it has never been spotted. Now, Robert
Foot of the University of Melbourne, Australia, and Zurab Silagadze of the
Budker Institute of Nuclear Physics in Novosibirsk, Russia, suggest that Nemesis
has proved elusive because it鈥檚 made of mirror matter and so is invisible.

Mirror matter has been appearing in physicists鈥� hypotheses since the 1950s.
It was dreamed up to explain an apparent lack of symmetry in nature: for
instance, neutrinos always spin in the same direction as a left-handed
corkscrew, never the other way. Nobody knows why. But the mystery would be
resolved if there were invisible 鈥渞ight-handed鈥� mirror particles to restore
symmetry.

If mirror matter exists, however, it would be very hard to spot. It doesn鈥檛
interact with ordinary matter and doesn鈥檛 emit radiation. The only way it
betrays itself is through gravity. In fact, it鈥檚 a candidate for the huge
amounts of dark matter in the Universe鈥攎atter that is only detected
through its gravitational pull.

Nemesis was first proposed to explain an apparent regular period of some 26
million years in the timing of mass extinctions on Earth. A distant planet or
star in a 26-million-year eccentric orbit could periodically stir up the Oort
Cloud of comets that orbit the Sun far beyond Pluto. This would result in a
catastrophic comet shower in the inner Solar System every 26 million years,
causing the cataclysms on Earth. However, all searches for the planet have drawn
a blank.

Foot and Silagadze鈥檚 hypothesis offers a new line of investigation. If
Nemesis is an invisible mirror planet, or even a mirror star, they say, it could
be detected by its gravitational lensing effect: its gravity would slightly bend
light from distant stars and galaxies. This tiny effect could be measured by
NASA鈥檚 Space Interferometry Mission, which is scheduled for launch in 2009. 鈥淭he
hypothesis is testable in principle,鈥� they write. But Max Tegmark, a cosmologist
at the University of Pennsylvania, is unconvinced. 鈥淢y guess is that this
particular speculation will turn out to be incorrect,鈥� he says.

Piet Hut of the Institute for Advanced Study in Princeton, one of the
originators of the Nemesis theory, says that the periodicity argument for
Nemesis鈥檚 existence has become weaker. 鈥淭here鈥檚 no convincing reason any more to
believe in Nemesis,鈥� he says, 鈥渂ut there鈥檚 also no convincing reason not to
believe in it. It鈥檚 an open question.鈥�

Some circumstantial evidence hints at the existence of mirror matter, but the
jury is still out
(New Scientist, 17 June 2000, p 36).
Nevertheless, Tegmark says: 鈥淲e should certainly not dismiss things out of hand. We need to
consider every possibility to explain the mystery of dark matter in the
鲍苍颈惫别谤蝉别.鈥�