Second moon spotted circling cigar-shaped world

Astronomers have spotted a second moon around a massive, cigar-shaped world at the fringes of the solar system. The discovery suggests multiple moons orbit many large, distant objects – but their unusual orbits raise questions about just how they could have formed.

The moons orbit a rocky body called 2003 EL61, which is a particularly bizarre member of the Kuiper Belt – a vast ring of icy objects beyond Neptune. 2003 EL61 rotates once every four hours – faster than any other Kuiper Belt Object (KBO) – and is shaped like a cigar that stretches 2000 kilometres on its longest side – nearly the diameter of Pluto.

In January 2005, a team led by Mike Brown at Caltech in Pasadena, California, US, discovered it is orbited by a moon that may be 300 km wide. The moon takes about 49 days to orbit 2003 EL61 at a distance of 49,000 km.

Now, the same team has spotted another, fainter, moon in three of five earlier images taken using the Keck Observatory in Hawaii. The new moon may be just 150 km across and appears to orbit once every 34 days at a distance of 39,000 km. But the new moon travels in a plane that is tilted by about 40° to the orbital plane of the bigger, brighter moon.