“The problem is that Sputnik Planitia is a hole in the ground, so there ought to be less mass, not more,” says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz. “If this is right, you have to come up with a way of hiding that extra mass.”

That mass could come in the form of a slushy subsurface ocean, Nimmo says. When the enormous impactor pummeled Pluto, it would have excavated some of the planet’s ice shell. The ocean beneath the now-thinned crust would well up, filling the void. Water is denser than ice, so Pluto’s mass would now be unevenly spread out. The entire dwarf planet would be unbalanced, as though it were heavier on one side. (We know something similar happened on the moon.) Over time, this would reorient Pluto’s spin until it eventually balanced itself again. That would be what brought Sputnik Planitia to its current location, directly opposite Charon.

The temperatures and pressures within Pluto are right for a slushy, viscous ocean to exist, according to coauthor Richard Binzel, a planetary scientist at MIT. The sea might also contain ammonia, a known antifreeze, according to Nimmo. Pluto is 40 times more distant from the sun than Earth is, but it can warm itself from within using radioactive elements in its rock-ball core. This internal radiator can heat its sea for another billion years or so. Charon might have had a water ocean of its own, but it’s so small, and light on radioactive elements, that the ocean would have frozen solid two billion years ago.

The research suggests that many other distant worlds in the Kuiper Belt might also hold inner oceans of water, or other liquids.

“The only places where you don’t get much water is in the very innermost solar system,” Nimmo says. “The outer solar system is really very rich in water.”

Above that slushy sea, Pluto’s frozen heart is full of nitrogen snow, which might have also played a role in reorienting the dwarf planet in the eons after the collision.

Pluto is lying on its side, so its poles get more sunlight than its equator. As it slowly journeys around the sun—one orbit takes 248 Earth years—nitrogen and other gases freeze in the permanently shadowed areas, then evaporate into gases again, and then re-condense. This nitrogen snow might pile up over billions of years, and eventually, a heavy nitrogen glacier in Sputnik Planitia could overwhelm the planet’s shape, says James Keane, a doctoral student at the University of Arizona.

Whether the fault of subsurface water or surface snow, or both, the result is the same: The center cannot hold. Pluto has to reorient itself.

That phenomenon is called true polar wander, and it’s common on rocky worlds: Scientists have studied it on Earth, its moon, and Mars. True polar wander is different than the 23-degree tilt in Earth’s axis that gives our planet seasons. When true polar wander happens, the planet’s spin axis doesn’t tilt, but instead the planet’s crust shifts. This would be like keeping Earth’s tilt the same, but sliding continents around so that New York moves to the North Pole. Or like holding a peach in your hand, and moving the skin around, but not the flesh or the pit.