IS THE solar system about to get another ocean? So far, besides Earth, six bodies are known or suspected to harbour oceans. These are Europa, Callisto and Ganymede (all moons of Jupiter), Enceladus and Titan (both moons of Saturn) and Triton (a moon of Neptune). The latest candidate is Pluto, the most famous inhabitant of the Kuiper belt, a girdle of asteroids that orbit the sun beyond Neptune.

Pluto’s claim to an ocean, argued this week in two papers published in Nature, is based on data collected in 2015 by New Horizons, a robotic spacecraft that zoomed past it in July of that year. The ocean in question, if it exists, is beneath Pluto’s surface. That makes it unlike Earth’s ocean, but like those of the other six bodies. To human sensibilities that is, perhaps, a funny sort of ocean. But add it to the other six and it is Earth’s surface ocean that looks anomalous, rather than Pluto’s buried one.

The argument for a Plutonic ocean—advanced by teams led by Francis Nimmo of the University of California, Santa Cruz, and James Keane, of the University of Arizona, Tucson, centres on Sputnik Planitia, a basin 1,300km across (see picture) caused by a collision in Pluto’s distant past. Pluto and its largest moon, Charon, are tidally locked. As they orbit their common centre of gravity, they always show each other the same face and, relative to the horizon, the position in the sky of either observed from the other never changes. The curiosity is that Sputnik Planitia lies almost exactly on the opposite side of Pluto from Charon, on the “tidal axis”, a line that runs through the centre of both bodies.

That is quite a coincidence—or, rather, in Dr Nimmo’s view it isn’t. He calculates the odds of it happening by chance as one in 20. He would therefore prefer to believe there is a physical explanation. And there might be. If Sputnik Planitia were an anomalously dense part of Pluto’s surface, and thus a concentration of mass, it would affect Pluto’s orientation with respect to its moon. That would cause Pluto to topple over until Sputnik Planitia lay at one of the two points at which the tidal axis intersects its surface.

Unfortunately, basins are characterised by the absence of mass rather than its presence. But Dr Nimmo is unfazed. He suggests that the huge quantities of material blasted out by the impact which created Sputnik Planitia would have reduced the pressure on Pluto’s crust, letting the subterranean water of a hypothetical ocean bulge closer to the surface. Since water is denser than most of the stuff found at or near Pluto’s surface, that upwelling would have increased the relative mass of Sputnik Planitia rather than decreasing it.

Though speculative, this idea is plausible. Water is common in the Kuiper belt, and Pluto in particular is thought to consist of a rocky core overlain by a thick mantle of ice. That rocky core will contain radioactive elements, the decay of which might provide enough heat to melt some of the mantle. Add a dash of ammonia, also common on Pluto, to lower the water’s freezing point, and Presto! you have an ocean.