In July 2015, 85 years after discovering it as the ninth and final planet orbiting the Sun, humans saw Pluto in all its glory for the first time. The small, sometimes overlooked world was finally the star of the show.
Nasa's New Horizons spacecraft, about the size of a baby grand piano, was approaching its primary destination after a nine-year, nearly five-billion-km journey. As it hurtled towards Pluto at 50,000 km/h, it sent back tantalising images of ever increasing detail.
We've crossed the threshold, gone where no one has gone before
From a glowing speck, Pluto grew into a textured disk of light until, finally, on 14 July 2015, a fully-fledged world came into view. New Horizons got within 12,500km – less than an Earth's diameter – of the distant, icy dwarf planet. It was the closest any human-made object had ever come to Pluto.
The snapshot of a reddish world bearing a giant heart instantly became an icon. For a mission that was expecting the unexpected, New Horizons discovered an astounding place with dramatic and wide-ranging landscapes that would amaze even the most hardened of space explorers.
"We all thought it was going to be interesting, and we all hoped it was going to be diverse," says Bill McKinnon, a planetary scientist at Washington University in St. Louis and a member of the mission's science team. "It exceeded all of our expectations."
Already, the discoveries have reshaped our understanding of Pluto and also of the outer Solar System. Knowing what is hiding in these remote regions will help scientists piece together the history of how the Solar System formed.
"This mission is our gateway," McKinnon says. "We've crossed the threshold, gone where no one has gone before: into the Kuiper Belt."
For most of the time that humans have known Pluto, since Clyde Tombaugh discovered it in 1930, it represented the final frontier. It was the last planet, and the misfit that stood out from the rest. Whereas the other outer planets are giant, puffy balls of gas, Pluto is rocky, icy, and smaller than Earth.
At an average distance of almost six billion km from the Sun, Pluto was a mysterious, faint world, beyond the reach of the most powerful telescopes. Even the mighty Hubble Space Telescope could muster nothing more than a pixelated blob.
In the early 1990s, astronomers started to realise Pluto was not alone. They discovered that the outer Solar System was filled with a swarm of small, icy objects they eventually dubbed the Kuiper Belt, a ring beyond Neptune's orbit.
Then, in 2005, astronomers found that one of these objects, later named Eris, rivals Pluto in size. The discovery of something so big called into question Pluto's status as a unique and isolated planet. Pluto, it now seemed, might be just like Eris – a part of the Kuiper Belt. That is why Pluto never fit in with the other planets: it belonged to this other group of outcasts. After heated debate, astronomers made it official in 2006 and voted to demote Pluto to a dwarf planet.
While some lamented Pluto's exit from the planet club, the new classification signified a deeper understanding of the outer Solar System. Instead of the lone, distant oddball at the end of the planetary line, Pluto was the beginning of something more: a previously unknown and unexplored collection of ice and rock – and possibly new worlds.
It was a new vision of the final frontier, enticing enough that Nasa decided to send New Horizons to explore Pluto, its moons, and maybe another Kuiper Belt Object. The mission launched in January 2006, just a few months before Pluto's demotion.
You knew it was going to be weird, and you knew it would be different from everything we had ever seen
With spacecraft having visited every other planet in the Solar System, New Horizons would fill the last remaining gap of exploration. But despite more than four decades of encountering other alien worlds, scientists could not have foreseen what they would find on Pluto.
Now, more than a year after New Horizons showed us Pluto in all its detail, the spacecraft has sent back 80% of its data, with the rest coming in October 2016. Scientists are sifting through the torrent of information, piecing together a world of startling variety and complexity.
"You knew it was going to be weird, and you knew it would be different from everything we had ever seen," says Mike Brown, an astronomer at the California Institute of Technology, US, one of the discoverers of Eris, and self-described Pluto killer. "But there was just no way to predict what it was going to look like."
Pluto is far. Really far. While sunlight takes only eight minutes to reach Earth, the trip to Pluto is more than five hours. And even then not much light or heat gets there – the Sun is 1,500 times fainter than it appears on Earth. The average temperature is about -230 degrees Celsius. Such a frigid surface should be frozen and relatively dormant.
This was stuff that was over the top – people would've laughed at us
Still, astronomers had long known Pluto's surface was at least somewhat active. Pluto's extreme seasons lead to wild temperature swings, driving changes in the ices and its thin atmosphere.
"We knew the atmosphere and surface were coupled and things would be moving around on Pluto timescales," McKinnon says. "But it's another thing altogether to say there are moving glaciers, floating ice mountains, convection, or a vast frozen sea of solid nitrogen. This was stuff that was over the top – people would've laughed at us."
Indeed, New Horizons discovered a surface beyond anyone's imagination. Much of the giant heart that dominates the now-famous photo of Pluto turns out to be an enormous glacier, the biggest in the Solar System. It is a vast chunk of frozen nitrogen creeping across the surface.
More detailed images revealed polygonal shapes etched onto this glacial surface, called Sputnik Planum. Computer models and further analysis show that these shapes, as wide as 50km across, are churning convection cells. This is where the downward pressure from surface ice warms the ice underneath. Since heat rises, the warmer ice below flows upward and forces the surface ice downward, driving the ice into a circular motion called convection.
This motion is slow, cycling every half a million years or so. The ice itself only moves across a few centimetres per year. Still, the very existence of such an energetic process was shocking.
"The whole idea of glacial ices and flow, and current activity on Pluto was really contrary to previous thought," says Cathy Olkin of the Southwest Research Institute in Boulder, Colorado US, and one of the mission's deputy project scientists.
Beyond Sputnik Planum, jagged mountains of water ice punctuate more rugged terrain. Some are only about 100 million years old: remarkably young, given that Pluto and the rest of the Solar System is about 4.6 billion years old. Such vigorous geology was surprisingly recent.
Two of Pluto's highest mountains – Wright Mons and Piccard Mons, rising to about 4km and 5.6km, respectively – might even be volcanic, spewing out water ice, methane, nitrogen, ammonia, and other icy compounds. Unlike some of the other icy volcanoes, called cryovolcanoes, in the Solar System, such as the spouts on Jupiter's moon Ganymede and Saturn's moon Enceladus, these volcanoes have formed towering structures, more similar to the ones we think of on Earth, according to McKinnon.
Even the possibility of an ocean is cause for excitement,
But Pluto may have at least one intriguing similarity to some of these moons: a subsurface ocean of liquid water. Soon after Pluto formed, radioactive elements buried in the core might have melted some of the surrounding ice. Over time, as Pluto cooled, that liquid water would refreeze. Because ice occupies more volume than liquid water, Pluto would have expanded, forming cracks on its frozen surface – exactly the kind of cracks that New Horizons discovered.
But if the underground liquid completely froze over, recent computer models suggest that it would have formed a special kind of ice that is denser than both normal ice and liquid water. If that were the case, Pluto would have shrunk, leaving behind tell-tale fault lines across the surface. But New Horizons did not see any of these lines, suggesting some of the water inside Pluto is still liquid.
Much of the evidence is circumstantial, and before they can be sure, scientists will need to send a spacecraft to study Pluto's interior structure from orbit, which will not happen anytime soon. Still, even the possibility of an ocean is cause for excitement, not only for Pluto but also other Kuiper Belt Objects.
According to theory, more Pluto-sized worlds could be floating in the outer regions of the Kuiper Belt. And if Pluto has glaciers, cryovolcanoes, and oceans, then it is plausible that these other objects would as well.
If we look at Eris tomorrow, we'll probably find areas that look exactly like Pluto
As the first such object a spacecraft has visited, and one of the largest, Pluto is a sign of what else could be out there. For the first time, scientists have a concrete image of what some of these Kuiper Belt Objects may be like.
"If you accept this idea that Pluto has an ocean, there are probably a large number of icy worlds in the Kuiper Belt and beyond that possess these deep dark oceans," McKinnon says.
Even if other Kuiper Belt Objects do not have oceans, they could still bear a resemblance to Pluto.
"Each one will be different for slightly different reasons – but similar-looking," Brown says. "They will suddenly look familiar, instead of bizarre. If we look at Eris tomorrow, we'll probably find areas that look exactly like Pluto and vice versa."
The Kuiper Belt sits between about 35 and 45 astronomical units from the Sun; an astronomical unit (au) being the average distance between the Sun and Earth. Astronomers have scoured this region, uncovering only a handful of objects like Eris that are big enough to merit dwarf planet status.
In principle there could be a handful of Pluto-sized bodies out there
"But if you went out to three times the typical Kuiper Belt distance to 100 or 150 au, they could be hiding out there," says Wes Fraser, an astronomer at Queen's University in Belfast, Northern Ireland. "All you need is a darker surface than Pluto and a good distance, and you could hide objects that are that size."
It turns out that, despite such large distances, the surface temperatures at these far-flung worlds might only be about 10 degrees colder than Pluto, Fraser says. This means they could be as wonderfully dynamic and diverse. "Geologically speaking, if Pluto is an active body, I wouldn't see why a Pluto at 1,000 au wouldn't also be an active body," he says.
In principle, he says, there could be a handful of Pluto-sized bodies out there. Current theory says Kuiper Belt Objects of all sizes formed much closer to the Sun – including perhaps as many as a few thousand Pluto-sized ones.
But a series of violent gravitational interactions with gas giant planets scattered these chunks of ice and rock, sending some plunging into the Sun, and others out of the Solar System altogether. The ones that were left became the Kuiper Belt. Still others, including a few Pluto-sized objects, could have settled in the outer edge of the Kuiper Belt.
It was a fantastic voyage to the very edge of the Solar System, and we're not done
Future telescopes like the Large Synoptic Survey Telescope, scheduled to begin operation in 2023, will explore these regions of space, hunting for distant Kuiper Belt Objects. Being able to gauge what is out there will be crucial for testing today's theories for how the Solar System formed.
New Horizons' visit to Pluto was brief. But its mission continues, and on the first day of 2019, the spacecraft will zoom by a much smaller Kuiper Belt Object called MU69. "It's a cold, classical KBO," Olkin says. "What that means is it's primordial, very much a remnant of Solar System formation."
Because it is much smaller, MU69 is unlikely to have the glaciers, volcanoes, or oceans that Pluto might have. But it promises to contain more secrets about the Kuiper Belt and the origins of the Solar System.
"It was a fantastic voyage to the very edge of the Solar System, and we're not done," McKinnon says. "Even though Pluto is the last classical planet, in reality, there's this endless new world to be discovered and explored."
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