The icy, mottled world is undergoing seasonal changes

Images taken by the Hubble Space Telescope have revealed some unusual and colourful changes to the surface of Pluto.

Nasa says the dwarf planet on the edge of our solar system is becoming increasingly red.

Its illuminated northern hemisphere is also getting brighter.

Nasa's scientists believe these are seasonal changes - as the planet heads into a new phase of its 248-year-long seasonal cycle.

"These changes are most likely consequences of surface ice melting on the sunlit pole and then re-freezing on the other pole," Nasa's Space Telescope Science Institute said in a statement.

The overall colour is probably a result of ultraviolet radiation from the distant Sun breaking up methane on Pluto's surface. This, scientists say, would leave behind a red carbon-rich residue.

But some astronomers have expressed shock at the changes.

"It's a little bit of a surprise to see these changes happening so big and so fast," said Marc Buie, of the Southwest Research Institute. "This is unprecedented."

In 2006, astronomers stripped Pluto of its status as a full planet, downgrading it to a dwarf planet.

Further away and considerably smaller than the eight other "traditional" planets in the solar system, Pluto - at just 2,360km (1,467 miles) across - is smaller even than some moons.

Nasa said: "The Hubble pictures underscore that Pluto is not simply a ball of ice and rock but a dynamic world that undergoes dramatic atmospheric changes."