Pluto moon 'has no atmosphere'

Nasa is sending a spacecraft to Pluto and Charon

A rare astronomical event has allowed scientists to show that Pluto's moon - known as Charon - has no atmosphere.

This could dismiss claims Charon is a planet twinned with Pluto and provide further insight into their formation.

Two groups of scientists made observations of the moon as it eclipsed a distant star in the July of 2005.

The size of any atmosphere around Charon was figured out by observing how gradually the star disappeared and reappeared during the eclipse.

When Charon passed in front of a star - in an occultation event - astronomers knew it would either cut the light off gradually or very quickly.

If the moon had no atmosphere then the light would be cut off sharply. However an atmosphere would cause a gradual cut off.

In South America, two groups, headed by Amanda Gulbis and Bruno Sicardy, were watching through some of the world's largest telescopes.

In reports published in Nature, they both agreed Charon had little or no atmosphere, with any atmosphere present being thinner than on Pluto or even on our moon.

Similar observations were made 25 years ago and it left some astronomers thinking it might have a thin atmosphere. Coupled with Charon being over half the size of Pluto, there was an argument for Charon being classified as a planet.

January launch

The new observation could settle the debate over Pluto and Charon being twinned planets.

The two teams also made a more accurate measurement of Charon's size, with a radius between 603 and 606km (375 and 377 miles). Combining this with data from the Hubble telescope, the research groups concluded just over half of Charon is rock.

The long wait between occultations illustrates the difficulties in exploring the far reaches of the Solar System. For example, little is understood about how Pluto and Charon were formed.

It is unknown whether they formed together or if Pluto captured Charon from the Kuiper Belt, the vast region containing icy objects beyond Neptune's orbit.

Pluto and its moon are also the target of a US space agency (Nasa) mission. The New Horizons spacecraft is expected to be launched in their direction on 17 January.