Image copyright NASA Pluto may harbour a slushy water ocean beneath its most prominent surface feature, known as the "heart".
This could explain why part of the heart-shaped region - called Sputnik Planitia - is locked in alignment with Pluto's main moon Charon.
A viscous ocean beneath the icy crust could have acted as a heavy, irregular mass __that rolled Pluto over, so __that Sputnik Planitia was facing the moon.
Details of the work appear in the journal Nature.
Sputnik Planitia is a circular region in the heart's left "ventricle" and is aligned almost exactly opposite Charon.
"There's more mass in Sputnik Planitia than in surrounding regions - so somehow there's extra stuff there," said Prof Francis Nimmo, from University of California, Santa Cruz.
But there's a problem with this idea, because the feature is thought to be the result of an impact with another object at some point in Pluto's past.
"Sputnik Planitia is a hole in the ground, so there shouldn't be more weight, there should be less weight. If the story is correct, you have to find some way of hiding extra mass underneath the surface of Sputnik Planitia," said Prof Nimmo.
"If you take some of the ice beneath Sputnik Planitia and replace it with water, water is denser than ice... so you'd be adding extra mass. That would help Sputnik Planitia to have more mass overall."
The findings come from an analysis of data collected when Nasa's New Horizons spacecraft flew by Pluto on 14 July 2015.
The space probe is now continuing its journey into the Kuiper Belt, an icy region of the Solar System beyond the orbit of Neptune.
Prof Martin Siegert, from Imperial College London, who was not involved with the Nature studies, called the findings "fascinating".
"The ocean would be incredibly cold, and hyper saline (I think they said enriched in ammonia), so unlike water on Earth or Europa," he told the BBC News Website.
"It would certainly be an extreme environment! Perhaps the most extreme in the Solar System?"
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