http://www.newsweek.com/pluto-giant-comet-according-new-research-946206

Over a decade ago, Pluto was downgraded from a planet to a dwarf planet. Now, scientists have a new theory about the celestial body: that it is a huge comet.  

Scientists previously thought that Pluto was formed like a planet 4.6 billion years ago, after gas and ice bound together with a rocky core to create an enormous sphere in the Sun’s protoplanetary disc.