The distant space rocks are all less than a hundred metres wide and form a vast band of debris beyond Neptune.
They are far too small to be seen with any telescope at that distance. But scientists have so far managed to spot 58 of them indirectly using an ingenious technique. From that small sample, they deduce that there must be around a quadrillion of the rocks – called Trans-Neptunian Objects – in orbit.
The scientists discovered the rocks by examining observations of a star called Scorpius X-1 from a Nasa satellite that observes with X-ray eyes. The star is the brightest such object outside the solar system.
Records from the Rossi X-ray Timing Explorer, pictured here, showed occasional, almost instantaneous fades in the star’s light as one of the rocks passed in front of it. The dips are a similar effect to an eclipse of the sun and are properly called occultations. They lasted between two and seven milliseconds.
Scorpius X-1 is thought to include a neutron star. The investigating astronomers, from Taiwan, ruled out any effect in the star’s own neighbourhood for the fades. Team leader Hsiang-Kuang Chang tells the journal Nature: “We conclude that occultation by objects in the Solar System is the most plausible explanation to these dip events.”
Fortunately for astronomers, the space rocks are too small to be planets and so don’t need names.