But when Christian Veillet and colleagues from the Canada-France-Hawaii Telescope looked at their images of the object, it appeared double. Further observations made on subsequent nights confirmed that the object was in fact a twin.
Astronomers have discovered a strange pair of objects orbiting each other at the edge of the Solar System.
They believe that the lumps of rock and ice, a few hundred kilometres in diameter, may be a pair of small planets.
But other experts disagree, saying the bodies are too small to be classified as planets.
The pair are situated among a group of at least 70,000 objects, known as the Kuiper Belt, that circle the Sun beyond the most distant planet Pluto.
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In January, astronomers attempted to find a specific object among this ring of comet-like objects. This body, designated 1998 WW 31, was first seen a few years ago.
But when Christian Veillet and colleagues from the Canada-France-Hawaii Telescope looked at their images of the object, it appeared double. Further observations made on subsequent nights confirmed that the object was in fact a twin.
Furthermore, images of the body taken by accident during observations of other objects and tracked down in astronomical archives also revealed it to be either a double or elongated in shape.
Astronomers say that there are similarities between the appearance of the pair and that of Pluto and its large moon Charon.
Some researchers believe that Pluto is too small and unusual to be a true planet. They think that Pluto and Charon should be regarded as the largest Kuiper Belt objects, and the first known example of a double planet.
The new pair may be the second known example of this phenomenon, they argue.