Pluto as seen by the New Horizons probe. Photo: NASA/JHUAPL/SwRI

Pluto's ocean may have been hiding below the dwarf planet's icy shell for billions of years since not long after the world formed.

Why it matters: Understanding how Pluto formed during the early days of the solar system is key to getting a broader picture of how objects like the distant world came to be and why its part of space looks the way it does now.

What they found: A new study in the journal Nature Geoscience suggests Pluto's early days were likely warmer than some models have suggested, meaning the world came to be over the course of about 30,000 years instead of millions.

  • That hot start means Pluto's subsurface ocean has probably been around since just after its formation and is not a more recent result of the radioactive decay of elements within the dwarf planet heating it up.
  • Pluto would have shown signs of compression on its surface if it started off cold and then an ocean melted within it, according to one of the study's authors, Carver Bierson, but New Horizons data shows just the opposite.
  • “We see lots of evidence of expansion, but we don’t see any evidence of compression, so the observations are more consistent with Pluto starting with a liquid ocean," Bierson said in a statement.
  • The new study also shows it's possible that other large dwarf planets like Makemake and Eris could have their own subsurface oceans as well.

Flashback: When New Horizons flew by Pluto in 2015, the complicated geology of the world shocked everybody.

  • Instead of the cold, dead dwarf planet they were expecting, researchers and the public were treated to photos of a diminutive world alive with icy mountains the size of the Rockies and plains of frozen nitrogen, forcing scientists to re-examine long-held theories about planetary formation.
  • "Pluto is a key clue to how the entire early solar system evolved," planetary scientist William McKinnon, who was not involved in the new study, told Axios.

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