NASA scientists says 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 has become 20 per cent more reddened than earlier.Experts believe this is because of changes in Pluto's surface ice as it enters a new phase of its 248-year-long rotation.The new images are said to show frozen nitrogen brightening in the north and becoming darker in the south.