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Nasa scientists say that dwarf planet Pluto, on the edge of our solar system, is becoming increasingly red.

Images taken by the Hubble space telescope show that the planet is some 20% redder than it used to be.

Experts say they believe this is because of changes in Pluto's surface ice as it enters a new phase of its 248-year-long orbit.

The new images are said to show frozen nitrogen brightening in the north and becoming darker in the south.

"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.

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 our solar system, Pluto - at just 2,360km (1,467 miles) across - is smaller even than some moons.

But the red tinge is not thought to have had a marked effect on Pluto's temperature: despite any redness, Pluto's surface temperature is incredibly cold at -233C (-382F).