Something is moving beneath Pluto's icy skin - and it might be liquid water

Pluto, our lonesome ex-planetary cousin in the Kuiper Belt, might have a secret buried kilometers below its surface: an ocean of liquid water.

When New Horizons flung itself past Pluto in 2015, it returned a stream of stunning images. One of the biggest surprises in those photos was that the distant icy body remains tectonically active. That is to say: Pluto's surface is cracking and expanding as though the entire ball of ice and rock is taking a long, slow breath.

Initially, nothing we knew about the dwarf planet could explain that active geology. Pluto is old, frozen, and locked into a stable orbit with its moon Charon - nothing should have caused shudders below its surface.

But, as Maddie Stone highlighted in Gizmodo, a new paper offers liquid water as a compelling explanation for this mystery.

Under a layer of ice, rock, and other materials - all of which add up to a couple hundred kilometers thick - liquid water could flow between Pluto's shell and its hot, radioactive core.

Such an ocean would freeze slowly, according to the new research. As it turned inch-by-inch into ice it would take up more space, forcing Pluto's shell to grow and crack. (Frozen water has greater volume than liquid water of the same mass.)

Our earlier assumption about the planet - that it's all solid - doesn't appear to stand up to this kind of analysis.

Based on the model the researchers used, that ocean is likely still there, buried out of sight.