  We propose that the ``isolated'' planetary mass objects observed by Zapatero
Osorio et al in the $\sigma$ Orionis cluster might actually be in orbit around
invisible stellar mass companions such as mirror stars. Mirror matter is
expected to exist if parity is an unbroken symmetry of nature. Future
observations can test this idea by looking for a periodic Doppler shift in the
radiation emitted by the planets. The fact that the observations show an
inverse dependence between the abundance of the these objects and their mass
may argue in favour of the mirror matter hypothesis.
