IT WAS the evolutionary theory of its age. A revolutionary hypothesis that undermined the cherished notion that we humans are somehow special, driving a deep wedge between science and religion. The philosopher Giordano Bruno was burned at the stake for espousing it; Galileo Galilei, the most brilliant scientist of his age, was silenced. But Nicolaus Copernicus's idea that Earth was just one of many planets orbiting the sun - and so occupied no exceptional position in the cosmos - has endured and become a foundation stone of our understanding of the universe.

Could it actually be wrong, though? At first glance, that question might seem heretical, or downright silly. But as our cosmic horizons have expanded over the centuries so too has the scope of Copernicus's idea. It has morphed into the Copernican, or cosmological, principle: that nothing distinguishes the position of Earth's galaxy from any other place in the entire universe. And that idea, some cosmologists point out, has not been tested beyond all doubt - yet.