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Last year, NASA鈥檚 New Horizons mission found evidence that there was once a massive subsurface ocean on Pluto, and that it might still exist in some form underneath the planet鈥檚 surface.

New evidence, found by a team of researchers from Brown University and the Planetary Science Institute, suggests that , and that it is still liquid.

The study鈥檚 lead author, Brown graduate student Noah Hammond, worked with colleagues to analyze data from the New Horizons mission and develop a thermal computer model of Pluto鈥檚 evolution.

Astronomers have known for a long time that our former ninth planet, since demoted to dwarf planet, has an icy surface. Nitrogen, methane, and carbon dioxide ices comprise the surface of the planet, while astronomers say it likely has a rocky core.

But what is in the middle? Is it a completely frozen ocean, or is there a chance it remains liquid?

Research by Mr. Hammond and his colleagues, accepted last week for publication in the journal Geophysical Research Letters, suggests that the dwarf planet鈥檚 .

If Pluto鈥檚 subsurface ocean had frozen, researchers say, the whole planet would have contracted. Yet the massive mountains and deep faults on the dwarf planet鈥檚 surface suggest that Pluto has been expanding, a fact consistent with an unfrozen subsurface ocean.

鈥淲hat New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion,鈥� Hammond said in a Brown University press release. 鈥淎 subsurface ocean that was slowly freezing over would cause this kind of expansion.鈥�

Due to low temperatures and high pressure on Pluto鈥檚 surface, a different type of ice would have formed, had the planet frozen fully. That form of ice, Ice II, has a dense crystalline structure, and can only form if the planet鈥檚 outer shell is at least 260 meters thick. Pluto鈥檚 outer shell of ice is at least 300 meters thick, and yet the planet does not seem to have shrunk, indicating that ice II has not formed.

鈥淲e don鈥檛 see the things on the surface we鈥檇 expect if there had been a global contraction,鈥� Hammond said in a press release. 鈥淪o we conclude that ice II has not formed, and therefore that the ocean hasn鈥檛 completely frozen.鈥�

The nitrogen and methane ices on Pluto鈥檚 surface could also be keeping the underwater ocean warmer.

鈥淭hose exotic ices are actually good insulators,鈥� said Hammond in a press release. 鈥淭hey may be helping Pluto from losing more of its heat to space.鈥�

Researchers say that that this research has implications for the search for new planets. If a liquid ocean can exist beneath Pluto鈥檚 frigid surface, watery planets could exist elsewhere in the universe, despite considerable distance from their stars.

鈥淭hat鈥檚 amazing to me,鈥� Hammond said. 鈥淭he possibility that you could have vast liquid water ocean habitats so far from the sun on Pluto 鈥� and that the same could also be possible on other Kuiper belt objects as well 鈥� is absolutely incredible.鈥�