Pluto isn't a planet, but is it a comet?

Pluto. Photo credit: NASA

Twelve years after it lost its planet status, scientists are now wondering if Pluto is actually a comet.

Or, to be more precise, 1 billion comets all mashed together.

In a new paper published on Saturday (NZ time), scientists propose the distant dwarf was formed "by the agglomeration of roughly a billion comets".

"We've developed what we call 'the giant comet' cosmochemical model of Pluto formation," said Dr Christopher Glein, who led the research.

Looking at data collected by the European Space Agency's Rosetta probe that landed on comet 67P, and NASA's New Horizons craft that flew by Pluto, they found "an intriguing consistency" in the amount of nitrogen in Pluto's large heart-shaped glacier and the comet.

Not only that, but the data suggests Pluto - 7.5 billion kilometres from the sun and smaller than our moon - once had an ocean.

"Our research suggests that Pluto's initial chemical makeup, inherited from cometary building blocks, was chemically modified by liquid water, perhaps even in a subsurface ocean," Dr Glein said.

The Rosetta mission is over, the probe having been deliberately crashed into 67P. New Horizons is on its way to an object in the Kuiper Belt, 1.6 billion kilometres beyond Pluto.