The study, led by a Brown University PhD student Noah Hammond, found that if Pluto's ocean had frozen into oblivion millions or billions of years ago, it would have caused the entire planet to shrink.
But there are no signs of a global contraction to be found on Pluto's surface. On the contrary, New Horizons showed signs that Pluto has been 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 Hammond.
The pictures New Horizons sent back from its close encounter with the Kuiper Belt's most famous denizen showed that Pluto was much more than a simple snowball in space.
It has an exotic surface made from different types of ices - water, nitrogen and methane.
It has mountains hundreds of metres high and a vast heart-shaped plain.
It also has giant tectonic features - sinuous faults hundreds of kilometers long as deep as four km.
It was those tectonic features that got scientists thinking that a subsurface ocean was a real possibility for Pluto.
There may have been enough heat-producing radioactive elements within Pluto's rocky core to melt part of the planet's ice shell.
If Pluto had on ocean that was frozen or in the process of freezing, extensional tectonics on the surface would result, and that's what New Horizons saw.
But if Pluto had an ocean, what is its fate today? Could the freezing process still be going on, or did the ocean freeze solid a billion years ago?
The new model showed that because of the low temperatures and high pressure within Pluto, an ocean that had completely frozen over would quickly convert from the normal ice we all know to a different phase called ice II.
Ice II has a more compact crystalline structure than standard ice, so an ocean frozen to ice II would occupy a smaller volume and lead to a global contraction on Pluto, rather than an expansion.
"We don't see the things on the surface we did 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."
The updated model suggests that Pluto's ice shell is actually closer to 300 or more km thick.
In addition, the nitrogen and methane ices that New Horizons found on the surface bolster the case for a thick ice shell.
"Those exotic ices are actually good insulators. They may be helping Pluto from losing more of its heat to space," Hammond noted in a paper in press in the journal Geophysical Research Letters.