Practically everyone alive today grew up learning some way of remembering the nine planets of our Solar System in order: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Beginning in the 1990s, however, two revolutions in astronomy happened all at once: the uncovering of trans-Neptunian objects and the discoveries of worlds around other stars. These discoveries compelled us to rethink our definition of a planet, culminating in the official 2006 definition of “planet” by the International Astronomical Union (IAU). Pluto was out. But with many people unhappy with the IAU’s definition, the story doesn’t end there.

When we first discovered Pluto, it was crazy to think it was anything other than a planet. The asteroid belt was known — thousands of small, rocky bodies that were most definitely distinct from planets — but Pluto was thought to be larger and more massive than even Earth to start. We thought it exerted a significant gravitational tug on Neptune, but those observations turned out to be flawed. We thought it was going to be at least as big as the inner, rocky worlds, but it’s less than half the size of even Mercury. And as the bounty of frozen worlds from the Kuiper belt, the scattered disk and even (perhaps) the Oort cloud have come in, we’ve learned that Pluto isn’t all that special by comparison with the rest of the Solar System. It was just first.

At the same time that we were discovering the first additional Kuiper belt objects, we were also finding the first exoplanets. The first planets we found were the easiest type to find: the largest-mass worlds orbiting close in to their parent stars. As techniques and technology improved with the passage of time, we began to find large numbers of worlds of all different masses at a variety of orbital distances from their stars. These exoplanet solar systems have proved to be vast, varied and rich, showing us that our Solar System is both nothing special and not even necessarily the norm. The diversity of what’s out there is tremendous.

So who gets to be a planet? And who determines it? The IAU’s original, 2006 definition was as follows:

1. It needs to be in hydrostatic equilibrium, or have enough gravity to pull it into an ellipsoidal shape.
2. It needs to orbit the Sun and not any other body.
3. And it needs to clear its orbit of any planetesimals or planetary competitors.

As you can see, it’s problematic from the start. For one, it doesn’t define “planet” for any star system other than our own. For another, “clearing its orbit” seems pretty subjective and dependent on what else is out there. (If you were to place Jupiter too distant from the Sun, it would fail to clear its orbit; would it therefore stop being a planet?) And even if you replaced “the Sun” with “its parent star,” it isn’t like we can measure exoplanetary systems well enough to tell whether their orbits are cleared or not. The definition isn’t precise enough.

On the other hand, planetary scientists have proposed a geophysical definition for a planet:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameter.

But this poses a set of problems, too. Large moons (like ours) and asteroids would become planets. Charon, Pluto’s moon, would be a planet. In fact, over 100 known objects in our Solar System alone would become planets. In our efforts to include Pluto, we include every non-stellar object more massive than about 0.01% of Earth’s mass. The big problem is it’s far too inclusive.

Solar systems come in huge varieties. The recently discovered system around TRAPPIST-1 looks more like Jupiter and its moons than like our Sun and its revolving worlds, but these seven Earth-sized bodies should definitely be planets. They meet the geophysical definition, but that’s only the starting point. They also — and you may feel strongly or less-strongly about some of these — meet the following requirements:

• They orbit their parent star.
• They dominate their orbits in terms of mass and orbital distance.
• They would clear out any debris in their orbit in well under 0.1 billion years.
• And their orbits, barring any outside influences, will be stable as long as their star exists.

What’s perhaps most remarkable is that we can make a simple, mathematical relationship between a world’s mass and its orbital distance that can be scaled and applied to any star. If you’re above these lines, you’re a planet; if you’re below it, you’re not. Note that even the most massive dwarf planets would have to be closer to the Sun than Mercury is to reach planetary status. Note by how fantastically much each of our eight planets meets these criteria… and by how much all others miss it. And note that if you replaced the Earth with the Moon, it would barely make it as a planet.

When it comes to planetary status, geophysics isn’t enough. In astronomy, the three rules of real estate also apply: location, location, location. There’s something very meaningful about our place in the Solar System that makes Earth a planet and Pluto not-a-planet. If we’re being honest about our Solar System and the number of planets within it, there are very clearly eight objects that are different from all the others.

There’s a limit to how far you can move Earth away before we become a dwarf planet or even a rogue (or orphaned, i.e., star-less) planet, and it’s important to have a real definition that means something wherever we look.

After all, the next decade should teach us whether there’s a larger-than-Earth mass out in the distant Solar System: what’s been dubbed Planet Nine. If this world does exist, we should be able to determine its mass and its orbital parameters. It will be an interesting object, a tremendous discovery and a fascinating world whether it makes it as a planet or not. But will it meet not just the geophysical but the astronomical criteria for being a planet? That’s a scientific question that should have a right (and wrong) answer. It’s up to us to draw the planetary line correctly, unswayed by our own biases and opinions.

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.”

A possible ice volcano on Pluto’s surface

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.

Pluto is more diverse and dynamic than imagined

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’s haze layer shows its blue colour, as revealed by this New Horizons picture

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.

Pluto’s vast nitrogen ice plains are known as Sputnik Planum

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.

These convection cells are where older surface ices are replaced with fresher material

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.

New Horizons revealed Pluto’s giant heart

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.”

Pluto is just one of countless icy bodies

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.

This image shows the dark, rugged highlands known as Krun Macula

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.”