Underneath Pluto’s wide heart, there might be – or was until recently – a liquid ocean. According to a new study, if the dwarf planet was frozen solid, it should have cracks on its surface from when it contracted. But that's not the case.

Researchers from Brown University and the Planetary Science Institute created a thermal evolution model for Pluto with data from New Horizons. The NASA probe visited Pluto last July, but it didn’t find any sign that Pluto was contracting. Instead, it found that Pluto was expanding.

"Thanks to the incredible data returned by New Horizons, we were able to observe tectonic features on Pluto’s surface, update our thermal evolution model with new data and infer that Pluto most likely has a subsurface ocean today," said Noah Hammond, the study’s lead author, in a statement.

Based on the quick but extensive analysis of the dwarf planet, its surface is made of different types of ice (nitrogen, water, and methane). Sputnik Planum, the informal name of the heart, is believed to be a frozen nitrogen sea heated by radioactive processes and with floating water-ice hills and large cracks.

^{A close-up look at Sputnik Planum's features. NASA/JHUAPL/SwRI}

"What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion," Hammond said. “A subsurface ocean that was slowly freezing over would cause this kind of expansion.”

Standard water ice is actually less dense than water (hence why icebergs and ice cubes float), so a subsurface ocean freezing over would expand the planet. But if Pluto had been frozen for a very long time, the water-ice would have changed into a different type of ice due to the pressure. This ice, called ice II, is much denser than normal ice and would have made the planet shrink.

“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 hasn’t completely frozen.”

Although the results of the research are promising, the authors add a few words of caution. In the paper, published in Geophysical Research Letters, the team suggests that if the current model of Pluto’s layers is not correct, then it might have frozen over without forming ice II.

To form ice II, the surface shell needs to be at least 260 kilometers (160 miles) thick; current models place it, with a good degree of confidence, at 300 kilometers (185 miles). So Europa and Enceladus might not be the only small objects in the Solar System to have an underground ocean.

"That’s amazing to me," Hammond said. "The possibility that you could have vast liquid water ocean habitats so far from the Sun on Pluto – and that the same could also be possible on other Kuiper belt objects as well – is absolutely incredible."

^{Spectacular Image of the dark side of Pluto with mountains and clouds visible. NASA/JHUAPL/SwRI}