See, due to the celestial body's high pressure and low temperatures, a freezing ocean would have turned into ice II -- a crystalline form of ice that's more compact than what we're used to. Ice II would have occupied a smaller volume and wouldn't have led to the expansion of Pluto's surface. "We don't see the things on the surface we'd expect if there had been a global contraction," Hammond said. "So we conclude that ice II has not formed, and therefore that the ocean has't completely frozen."

It's not an absolute certainty at this point, of course. If Pluto's ice crust is thinner than 260 kilometers (around 850,000 feet), then the ocean would have frozen over without forming ice II. Scientists believe the crust is thicker than 260 kilometers, though, and might even be close to 300 kilometers.

Francis Nimmo of the University of California at Santa Cruz told New Scientist that Pluto and other rocky planets could be more hospitable to life than watery moons like Jupiter's Ganymede. In case Pluto does have a seabed, it could have what it takes to sustain some type of life form. Of course, we wouldn't know for sure until space agencies start sending robots to dive into extraterrestrial bodies of water.