91av

THE recent discovery of planets apparently floating in space with no star to
orbit has baffled astronomers. Now a group of physicists in Australia has come
up with an extraordinary explanation: 鈥淭hey may not be isolated at all,鈥� says
Robert Foot of the University of Melbourne. 鈥淭hey could be orbiting invisible
蝉迟补谤蝉.鈥�

The isolated planets were puzzling because, in the conventional picture,
planets form only in dense discs of gas and dust swirling around newborn stars.
But recent observations turned up objects that look very like our own Jupiter,
wandering in the Sigma Orionis star cluster
(91av, 14 October, p 20).

The kind of invisible stars Foot and his colleagues Alexander Ignatiev and
Ray Volkas have in mind are made of 鈥渕irror matter鈥�. If such matter exists, it
would have been created alongside normal matter in the Big Bang and every known
particle would have a mirror counterpart
(91av, 17 June, p 36).

Theorists say mirror matter must exist because some characteristics of
particles wouldn鈥檛 have a mirror-image equivalent without it. Mirror matter
would interact with ordinary matter essentially only through gravity, so it
would be invisible.

In a paper submitted to Physical Review Letters, Foot and his colleagues say
that, if they are right, close observation of the isolated planets might reveal
periodic 鈥渨obbles鈥� as they orbit their parent stars. The astronomers who found
the planets say they have not yet seen any orbital motion. 鈥淥ur observations of
Sigma Orionis do not allow us to test Foot鈥檚 hypothesis,鈥� says Eduardo Martin of
the University of Hawaii in Honolulu.

Foot and his colleagues point out that the Universe could also contain
ordinary stars with mirror planets. This may explain the Jupiter-mass planets
which have been found orbiting very close to stars by detecting the star鈥檚
wobble. Ordinary planets could not form so close to their parent stars because
of the high temperatures. 鈥淏ut if they are mirror worlds they would be
essentially oblivious to the heat,鈥� says Foot.