What's Hiding In The Kuiper Belt? | Unveiled

What’s Hiding in the Kuiper Belt?

Some maps of the solar system stop at Neptune, or perhaps Pluto. But those charts only tell part of the story! The truth is that the space beyond the major planets is a place of perfect mystery, and there are parts of it that we didn’t even know existed until recently. So, what other secrets await far from the light of the sun?

This is Unveiled, and today we’re answering the extraordinary question; what’s hiding in the Kuiper Belt?

The Kuiper Belt is sometimes called the “third region of the solar system” and, simply put, is a vast sea of icy bodies and other chunks of space debris. It begins just beyond the orbit of Neptune and exists between 30 and 50 Astronomical Units away from the sun - with one Astronomical Unit being the average distance between the sun and the Earth. The Kuiper Belt has two-hundred times more mass than the inner Asteroid Belt, to the point where some astronomers believe that if it wasn’t for Neptune’s strong gravity, the Belt might not even have existed at all… the debris inside it could have instead formed into another planet. So, it’s thanks to Neptune that the Belt takes its current shape, but while the ice giant has potentially prevented another whole planet from forming, there’s plenty of other interesting stuff out there!

The Kuiper Belt is named after Gerard Kuiper, one of the first and leading astronomers to theorise that there could be additional objects in the solar system beyond Neptune. It wasn’t Kuiper who discovered the Belt, however, seeing as he died in the 1970s almost two decades before it was confirmed. We only truly knew that the Kuiper Belt existed in 1992, when some ground-breaking discoveries of TNOs were made. A TNO is a Trans-Neptunian Object, which is basically any celestial body further out than Neptune but still within the sun’s heliosphere. The first Trans-Neptunian Object ever discovered was Pluto back in 1930, which is still the largest one we’ve found – though Eris is the most massive. Famously, Pluto was once deemed a planet, but it was re-categorised in 2006 as a dwarf planet.

There are other designations within the TNO category, however, including specific KBOs – Kuiper Belt Objects – something that Pluto also is. Even further out, there are Scattered Disc Objects and Oort Cloud Objects. The Kuiper Belt is the closest to the sun of these three classifications, but there’s a lot of overlap between it and the Scattered Disc - the objects of which orbit between 30 and 100 AU away. The objects in the Oort Cloud orbit the sun at a much greater distance of between 2,000 and 200,000 AU. A long, long way away from even the Kuiper Belt!

The Kuiper Belt and the Oort Cloud are both thought to be main sources of comets. Long-period comets that orbit further from the sun come from the far-out Oort Cloud, but short-period comets - which typically burn out much more quickly - originate in the Kuiper Belt. KBOs become comets when Neptune’s orbit interferes with them too much, flinging them towards the sun and Jupiter (which also has a large, gravitational effect, and can potentially direct these Kuiper Belt missiles towards the inner planets!)

As well as comets, there are plenty of other dwarf planets lurking in the Kuiper Belt, as well as the previously mentioned Pluto and Eris. And there are moons, too. Pluto is one such KBO to have moons; including its largest, Charon. But the likes of Eris, Haumea and Quaoar also have moons and satellites of their own - incredibly distant worlds that make the likes of our own Moon or even Mars feel really quite close!

And there are objects even stranger still hiding in the darkness, including just beyond the Kuiper Belt, like the mysterious Sedna and Niku. Sedna has one of the most varied orbits of all TNOs, orbiting between 76 and 900+ AU along a route that’s estimated to take 11,000 years to complete. Niku is perhaps even more unusual. Its name translates as “rebellious” in Chinese, and it gets its rule-breaking reputation because it has a “highly inclined orbit”; it’s tilted almost 110 degrees above the solar orbital plane, meaning that it’s positioned above most of the sun’s other satellites. The reason why objects like Niku, Sedna, and even Pluto have such unusual orbits isn’t conclusively known, but there are more than a few theories…

Despite Neptune’s strong gravity allegedly making the formation of another planet impossible, the idea that there is an extra planet is prevalent. So prevalent, in fact, that there are numerous theories for hidden planets of various sizes - and many where the Kuiper Belt is key. The proposal originally came about in the early 1900s because of perceived discrepancies within the orbits of Neptune and Uranus. The discovery of Pluto supposedly explained those discrepancies, until it was found that Pluto’s gravity wasn’t strong enough to affect the outer giants. So, supposedly, something else was… In time it turned out that the original hypothesis was flawed by miscalculations, but that hasn’t put other Planet Nine theories to bed.

In 2017, studies held by the University of Arizona’s Lunar and Planetary Laboratory suggested that there could potentially be another planet that we haven’t discovered yet - though the Kuiper Belt might not stretch far enough to contain it! Again, the unusual orbits shown by KBOs were what drove scientists to investigate. According to some estimates, if the hypothetical Planet Nine does exist, it should have a mass around ten times that of Earth and to be up to 800 times further away from the sun - far beyond the Belt. It’d be a bona fide “Super-Earth”, though(!)… were we ever to observe anything that even closely matches it.

Along with Planet Nine, there’s the even more mysterious Planet X theory. Planet X differs from Planet Nine because it’s said to be much smaller, with an estimated mass similar to Mars. It ties into a popular conspiracy theory called the Nibiru cataclysm, which says that Earth is one day going to be destroyed when an unknown, additional planet crashes into it. It could be, according to the theory, that this ominous planet, Nibiru, is a rogue planet hiding in the outer solar system - perhaps in the Kuiper Belt. The majority of actual scientists and astronomers are quick to dismiss the Nibiru narrative, mind you, so there’s no need to gulp in fear at the sky just yet!

Not everything about the Kuiper Belt amounts to mysterious hidden worlds, however! There are some simpler explanations for the orbital anomalies, like the theory of “collective gravity”. This relatively new idea suggests that the Kuiper Belt could be being changed and distorted by a gravitational “snowball effect” caused by smaller objects; meaning that the collective gravity of all the other, definitely non-planet TNOs could be enough to pull bodies like Sedna and Niku seemingly “out of place”. Bizarrely though, mass is still a problem, here. While “collective gravity” could be happening, researchers also speculate that the Kuiper Belt needs a lot more mass than we currently know it has for this effect to take hold. Whether that mass is a new planet or something a little less radical remains to be seen. We are discovering new KBOs and TNOs of varying sizes all the time, though… so perhaps one day - when we know all there is to know about the Belt - we’ll be able to account for everything in balance, after all.

That day might seem as though it’s incredibly far away but, despite the extreme distance between us and the Kuiper Belt, NASA is trying to investigate it as best they can - most notably with the New Horizons probe. Launched in January 2006, New Horizons passed Jupiter in 2007 and finally reached the Kuiper Belt to flyby Pluto in 2015. With the Pluto mission complete, and because it still had enough fuel and its systems were in good condition, its goals were extended in 2016 to study one (or more) of the mysterious KBOs… and it arrived at Ultima Thule in 2019; a peanut (or snowman) shaped body made up of two small planetesimals stuck together. Ultima Thule is today the furthest object in the solar system ever to be visited by a spacecraft from Earth!

There’s a lot going on in this distant region of the solar system, and there’s a lot left to discover! But that’s what’s hiding in the Kuiper Belt.