Nasa's New Horizons probe flew close to Pluto last year, revealing clues the dwarf planet might once have had a liquid ocean sloshing around under its icy crust.

The ocean, which may have hosted alien life, was thought to have frozen over millions of years ago.

But new analysis shows the ocean beneath the surface is still liquid now - raising hopes that signs of life could yet be discovered on the dwarf planet.

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This dramatic image from the New Horizons spacecraft shows the dark, rugged highlands known as Krun Macula (lower right), which border a section of Pluto’s icy plains. Evidence for plate tectonics suggests there is still a liquid ocean beneath the planet's surface

THERE COULD BE ALIENS BENEATH PLUTO'S CRUST, SAYS BRIAN COX Alien life may be lurking beneath Pluto's crust, according to physicist Brian Cox. His comments, made last year, come after the historic flyby of the dwarf planet by New Horizons, which uncovered huge glaciers and mountains made of water ice. These features hint at the possibility of subterranean seas on the dwarf planet warm enough for organic chemistry to thrive, said Cox. The probe 'showed you that there may well be a subsurface ocean on Pluto,' Cox told The Times. '[This] means - if our understanding of life on Earth is even slightly correct - that you could have living things there. The latest research appears to back up Cox' claims. Advertisement

'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, a graduate student at Brown University, and the study's lead author.

The study found 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 Pluto contracting. In fact, New Horizons showed signs Pluto has been expanding.

The pictures showed Pluto was much more interesting than anyone had expected, and much more than just an icy rock.

It has an exotic surface made from different types of ices - water, nitrogen and methane. It also has mountains hundreds of meters high and a vast heart-shaped plain.

The dwarf planet also has giant tectonic features, sinuous faults hundreds of miles long as deep as 2.4 miles (4 kilometres). It was those tectonic features that got scientists thinking that a subsurface ocean was a real possibility for Pluto.

'What New Horizons showed was that there are extensional tectonic features, which indicate that Pluto underwent a period of global expansion,' Mr Hammond said.

The New Horizons spacecraft spied extensional faults on Pluto, a sign that the dwarf planet has undergone a global expansion possibly due to the slow freezing of a subsurface ocean. A new analysis by Brown University scientists bolsters that idea, and suggests that ocean is likely still there today

Pluto has mountains hundreds of meters high and a vast heart-shaped plain (pictured). This is the last and most detailed image sent to Earth before the New Horizons spacecraft’s closest approach to Pluto on July 14 2015

'A subsurface ocean that was slowly freezing over would cause this kind of expansion.'

Scientists, such as physicist Brian Cox, suggest that if Pluto does have an ocean beneath its surface, there could be a chance that it would be warm enough for organic chemistry to thrive.

'Our model shows that recent geological activity on Pluto can be driven just from phase changes in the ice – no tides or exotic materials or unusual processes are required,' said Amy Barr, senior scientist and co-author on the study.

'If Pluto's most recent tectonic episode is extensional, that means that Pluto may have an ocean at present.

'This lends support to the idea that oceans may be common among large Kuiper Belt objects, just as they are common among the satellites of the outer planet.'

The scientists think there may have been enough heat-producing radioactive elements within Pluto's rocky core to melt part of the planet's ice shell.

Nasa's New Horizons spacecraft took this stunning image of Pluto only a few minutes after closest approach on July 14, 2015. It shows sunlight filtering through and illuminating Pluto's complex atmospheric haze layers. It also appears to show a cloud - which Nasa describes as ' an intriguing bright wisp measuring tens of miles'.

Artist's impression of the New Horizons spacecraft that completed its first flyby last July. The latest finding from the craft's data shows the way it interacts with the solar wind is completely unique, and surprisingly it makes it seem more like a larger planet than a comet

NEW HORIZONS' NEW MISSION The spacecraft that gave us the first close-up views of Pluto now has a much smaller object in its sights. New Horizons is now track to fly past a recently discovered, less than 30-mile-wide object out on the solar system frontier. The close encounter with what's known as 2014 MU69 would occur in 2019. It orbits nearly 1 billion miles (1.6 billion kilometers) beyond Pluto. Nasa and the New Horizons team chose 2014 MU69 in August as New Horizons' next potential target, thus the nickname PT-1. Like Pluto, MU69 orbits the sun in the frozen, twilight zone known as the Kuiper Belt. MU69 is thought to be 10 times larger and 1,000 times more massive than average comets, including the one being orbited right now by Europe's Rosetta spacecraft. On the other end, MU69 is barely 1 percent the size of Pluto and perhaps one-ten-thousandth the mass of the dwarf planet. So the new target is a good middle ground, according to scientists. The team plans to formally ask Nasa next year to fund the mission extension for studying MU69. Scientists promise a better name before showtime on January 1, 2019. Advertisement

Over time in the Kuiper belt, the melted portion would eventually start to refreeze.

Ice is less dense than water, so when it freezes, it expands. If Pluto had on ocean that was frozen or in the process of freezing, extensional tectonics on the surface would result- which is what New Horizons saw.

There are not many other ways on Pluto to get such features.

One way might have been through a gravitational tug of war with its moon, Charon. But the active gravitational dynamics between the two have long since wound down, and some of the tectonics look fairly fresh in geological terms. This is why many scientists believe an ocean is the most likely cause of the plate tectonics.

'We have been waiting a long time to see the surface of Pluto, and it did not disappoint,' Ms Barr said.

'Many people thought that Pluto would be geologically "dead",' that it would be covered in craters and have an ancient surface.

'Our work shows how even Pluto, at the edge of the solar system, with very little energy, can have tectonics.

'We are grateful to the New Horizons team for working so hard to guide the craft to Pluto and return the beautiful images that motivated our study. They have provided another piece in the puzzle of the comparative planetology of icy worlds.'

The researchers say there's good reason to believe that the ice shell is more than 161 miles (260 kilometres).

Their updated model suggests that Pluto's ice shell is actually closer to 186 miles (300 or more kilometres) thick.

The nitrogen and methane ices that New Horizons found on the surface also point to the dwarf planet having a thick ice shell.

'Those exotic ices are actually good insulators,' Mr Hammond said. 'They may be helping Pluto from losing more of its heat to space.'

Taken together, the new model bolsters the case for an ocean environment in the furthest reaches of the solar system.

'That's amazing to me,' Mr 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.'

Nasa's New Horizons science team were happy to see the spacecraft's images of Pluto in July last year. The Swap data will continue to be analysed for many years to come the scientists said, until they get to grips with the implications of the data