People can call Pluto a planet if they want, according to an expert Reuters

New evidence of complex organic molecules has been spotted in the surface of Pluto by the Hubble Space Telescope, a study has found.

Scientists at the Southwest Research Institute in Colorado, found that substances on the dwarf planet's surface are absorbing more ultraviolet light than expected.

These substances could be organic and made up of complex hydrocarbons or nitrogen-containing molecules.

Study leader Alan Stern believes the research could explain Pluto's colour. It is thought that the chemical species could be produced by the interaction of cosmic rays with surface ice.

"This is an exciting finding because complex Plutonian hydrocarbons and other molecules that could be responsible for the ultraviolet spectral features we found with Hubble may, among other things, be responsible for giving Pluto its ruddy colour," he said.

Probe New Horizons was launched in 2006 to make the first ever visit by space craft to the planet, which is one third of the size of Earth's moon. The probe will pass within 8,000 miles of the dwarf planet in July 2015.

Pluto was re-defined as a dwarf planet inl 2006, when the International Astronomical Union found that it failed to meet the third criteria to be classed as a planet - to have become gravitationally dominant within its orbit.