The Solar System Doesn’t End At Neptune

A new day, a new obsession.

I was helping my daughter revise for her Science exams when we came across the astronomy chapter in her textbook. I realised that a lot of the material that she is learning is pretty much the same things that I was learning when I was in school, apart from the fact that we now have only eight planets in the solar system instead of nine (Sorry Pluto, you will always be a planet in my heart).


Truth of the matter is our solar system has way, way more celestial bodies in it than just the terrestrial planets, the asteroid belt, the gas giants and moons. To stop teaching about the Solar System at Neptune is a missed opportunity. #justsaying

I think that our Science syllabi in school can be more interesting and fascinating if we put in more chapters or topics to do with astronomy and the study of our Solar System. You know, make it less about which astronomer discovered which planet at which year (memory work) and make it more about how they did it, what were the consequences of their discoveries, what tools they used to explore the night skies and how far technologies have come in this modern world when it comes to finding very, very far and very, very small objects in the sky.

I’m no Science textbook author, but if I were, I would put into textbooks things I’m putting into this blog post now.

But first, let’s recap what is already in the textbooks.

Planets & Moons

We have our Sun, and orbiting it are the terrestrial planets Mercury, Venus, Earth and Mars. Then there’s the asteroid belt between the rocky planets and their gas giant neighbours, Jupiter, Saturn, Uranus and Neptune.

Image by Jonny Lindner from Pixabay

Now, moons. How many moons are there in the solar system? A staggering number of them. Apart from Mercury and Venus which have no moons, the rest of the planets have natural satellites orbiting them.

Earth has one, Mars has two, while the other planets have … a lot. At the time of this writing, Jupiter has 79, Saturn has 82, Uranus has 27 and Neptune has 14. You can see a pattern here, something along the lines of the larger the planet, the more moons they can hang on to.

That’s not how it works with Pluto.

Let’s talk about Pluto

Pluto has 5 moons. Five. There’s Charon, the largest of Pluto’s moon, pretty much half the size of Pluto. As you can see in the video below, they’re locked in this dance where they face each other; there’s a binary planet thing going on there.

Then you have the two larger ones, Nix and Hydra (the one that looks like it is drunk and spinning out of control). Between them is the smaller Kerberos and lastly, you have Styx which was found by New Horizons, the probe which travelled for 9 years just to check out Pluto. It has since moved on, and is en route to a Kuiper belt object, Ultima Thule. Read more about the New Horizons mission here.

I’m sure you know that Pluto is no longer a planet. I’m sure you know that it is a dwarf planet but did you know that it is a Kuiper belt object?

What Are Kuiper Belt Objects?

If you find asteroids in the asteroid belt then you will find Kuiper Belt Objects (KBO) in the Kuiper belt. Although Pluto was discovered in 1930, the Kuiper belt where it originated from was not found until the year 1992.

This is primarily because the belt is just so darn far away, beyond the orbit of Neptune. It’s a region that is cold and dark and full of small objects made of mostly rock, water ice, ammonia and methane.

That’s a pretty crowded neighbourhood beyond Neptune

The Kuiper belt starts at the edge of the orbit of Neptune, 30 AU from the Sun, and extends to around 50-55 AU from the Sun (reminder: 1 AU is the distance between the Earth and the Sun).

Sometimes KBOs are also known as trans-Neptunian objects (TNOs) although this definition covers pretty much everything that is found beyond Neptune.

Finding the Kuiper Belt

In 1943, long before we had the technology to locate the belt, astronomer Kenneth Edgeworth suggested the existence of objects beyond Neptune. Gerard Kuiper predicted a “belt of icy objects” near the edge of the solar system in 1951. This is why sometimes this region is known as the Edgeworth-Kuiper belt.

In 1992, after searching for 5 years, astronomer David Jewitt and graduate student Jane Luu, announced their discovery of the first Kuiper Belt object: 1992 QB1 also known as 15760 Albion.Since then there had been thousands more of such objects found.


We know that Pluto is a dwarf planet but it’s also a Kuiper Belt Object. In fact, it is the largest KBO in the group, with a diameter of 2377 KM, and technically it’s the first KBO every discovered.

Pluto, together with Makemake (1430 KM), the second brightest KBO after Pluto, and oblong-shaped Haumea (1595 KM), are the only Kuiper Belt Objects that are also dwarf planets.

Read more about Kuiper Belt Objects here, here and here.

Eris, the Dwarf Planet from Beyond

At this point we know that the Kuiper Belt is very far away from the centre of our Solar System. And yet, beyond that we have a region called the scattered disk which extends up to 1000 AU from the Sun!

Now from this region, scientists found Eris. It was initially thought to be bigger than Pluto but eventually we discovered that it has a moon, and that in fact, it’s only about the same size as Pluto (2326 KM).

Eris and her moon Dysnomia, far beyond Pluto’s orbit

At its closest to the Sun, it’s 38 AU away; at its furthest, 96 AU! Remember that Neptune is at 30 AU – sunlight takes more than 4 hours to get to Neptune. Eris’ orbit is tilted by 45 degrees and it takes around 557 years to go once around the Sun.

Eris plays an important part in our understanding of the solar system because its discovery was one of the reasons Pluto got demoted to dwarf planet status. Had they decided to keep Pluto as a planet, Eris would have been the tenth planet in the solar system.

So based on the IAU definition of a planet released in 2006, our solar system now has 8 planets and 5 dwarf planets: Pluto, Makemake, Haumea and Eris. Wait, that’s four. What’s the fifth dwarf planet?

Ceres, King of the Asteroids

Ceres is the largest object in the asteroid belt, a region between Mars and Jupiter. It only has a diameter of 945 KM but because of its proximity to Earth, it was a lot easier to find than say, Pluto or Eris.

In fact, Ceres was discovered back in 1801 by Giuseppe Piazzi, and like Pluto, it was declared a planet. At the time, the Solar System had only 7 planets; Neptune had not been found yet. Ceres would become the fifth planet from the Sun for about 50 years before the discovery of more objects like it force astronomers to reclassify Ceres as an asteroid instead of a planet.


After Pluto was demoted to dwarf planet status, Ceres was once again reclassified as a dwarf planet. Amongst the 5 dwarf planets, Ceres is the only one within the inner Solar System. It’s also the first dwarf planet to have been visited by a dedicated space probe, Dawn back in 2015.

See pictures sent back from Dawn and read more about Ceres here.

Sedna, A World of Possibilities

Reading about Sedna fills me with frustration and with a new appreciation for the work astronomers do. You see, Sedna is widely considered a dwarf planet but it isn’t officially one, mainly because it is just too far to observe.

To be a dwarf planet you need to meet two requirements: you need to orbit the Sun, and you need to be big enough for your own gravity to give yourself a round shape. Unlike the definition for a planet, a dwarf planet need not have cleared the neighbourhood around its orbit.

Sedna is big enough to see, but is it big enough to have its own self-rounding gravity? The short answer is, we don’t know. That’s because Sedna has an incredibly elongated orbit.

Pluto orbits the Sun once every 248 years; Eris, once every 557 years. Sedna takes 11,400 years for one trip around the Sun. At its farthest from the Sun, Sedna would be 937 AU away. When it was found in 2015, it was 86 AU away, still a great distance from us. While it is big enough for us to sit up and take notice, but we just can’t see enough of it to be sure that it is a dwarf planet.

What we can study however is its orbital path, which many believe alludes to the existence of another giant planet beyond the Kuiper belt. It is believed that this trans-Neptunian planet would be able to explain the eccentric orbits of Sedna and other objects like it in the Kuiper belt. But until we find it, this is just a theory. Fingers crossed, we will one day find out why Sedna travels the way it does.

So, are we done? Nope.

Trojan Asteroids

In October 2021, NASA is launching Lucy, a space probe mission that will head for Jupiter trojans. #saywhatnow?

See it in action here

Jupiter trojans are a type of trojan asteroid (depicted in green in this pic above). There are two groups of asteroids that flank Jupiter on both sides while orbiting the Sun. They are found in stable orbits, at two Lagrangian points 60 degrees ahead (L4) and behind (L5) of Jupiter. Jupiter has millions of trojans larger than 1 KM across.

These trojans are the fossils left over from failed planet formations, remnants from when the Solar System was in its planet-forming phase. Lucy will be aiming for multiple Jupiter trojans during its 12-year mission to study and send back data that can help scientists understand more about planet formation.

Apart from Jupiter, at the moment of this writing, Neptune has 22 trojans, Mars has nine, Uranus has two, Earth has one, and Venus had one temporary trojan. Two of Saturn’s moons have trojan moons also known as co-orbital moons. Just to be clear, these aren’t moons that go around moons, they are just moons that orbit in a locked pattern (60 degrees ahead and 60 degrees behind) a fellow moon. Tethys has Calypso and Telesto, while Dione has Helene and Polydeuces. #betyoudidn’tknowthat


So far, all the objects that we have previously discussed, dwarf planets, asteroids, trojans, Kuiper belt objects and trans-Neptunian objects, are grouped under the category “minor planets”. The only one that has not been discussed is Centaurs. And that’s because centaurs are complicated.

Centaurs are small bodies in the outer solar system region, found between Jupiter and Neptune. They behave like, and thus are sometimes categorised as, comets and asteroids on top of being centaurs. It’s believed that they are strays that have escaped the Kuiper belt object inward towards the outer Solar System area.

They have chaotic orbits that are often eccentric (like stretched out ellipses) and unstable. Let’s talk about what those two things mean:

  • Eccentric orbit: Earth has a near circular orbit, so it has an orbital eccentricity of near zero. On the other hand, Halley’s comet has an orbital eccentricity of 0.97, a highly elliptical orbit. Because of the perturbations from the gas giants, many centaurs (though not all) are found to have eccentric orbits. So far, the centaur with the most eccentric orbit is Asbolus at 0.62.
  • Unstable orbit: Centaurs have dynamical lifetimes. This means their orbits will change over time. Some centaurs may move into different orbits after a period of time. If they get flung into the inner Solar System, the end up as Jupiter family comets (short-period comets). If they get flung outward, they may leave the Solar System entirely.

Below is a great image of what is in our outer Solar System (you can check out the source here). You have Jupiter (J), Saturn (S), Uranus (U) and Neptune (N) and the yellow Sun right in the middle. The blue flecks are Kuiper Belt Objects while the yellow ones are objects in the scattered disk region. The green flecks that are inside the orbits of the gas giants are the centaurs.

By User: Eurocommuter – Plotted by a program written by the author, CC BY-SA 3.0 (link)

Yes, the Solar System isn’t as empty as we thought. It’s just really big.

Gravitational Perturbations

Why are the orbits of centaurs so unstable? Well, as the orbits of centaurs cross the orbits of the four gas giants, centaurs are subject to the gravitational pull of the much larger planets. This extra force exerted by the planets is called gravitational perturbation and in the long run, it is this force that will cause the orbits of centaurs to change.

One wonderful story that illustrates the significance of perturbation is the discovery of Neptune.


Neptune was discovered in the mid-1800s and was the only planet in our solar system that wasn’t discovered from direct observation. It was simply too far and too small to be observed. But astronomers know that mathematically, it’s there.

Neptune’s position in the sky was calculated based on the understanding that something large, possibly a planet, had been causing irregularities in the orbit of Uranus. French astronomer Urbain Le Verrier did the math then sent the calculations to the Berlin Observatory for confirmation. Based on his calculations, Neptune was subsequently discovered in under an hour, and only a degree from the predicted location. #mathisgreat

If Neptune can exert such an influence on a fellow gas giant, imagine the chaos Neptune and the other gas giants can cause when it comes to smaller bodies like 2060 Chiron, a centaur that’s only 206 KM across. Chiron’s orbit crosses between Saturn and Uranus and lasts around 50 years. It also exhibits comet-like activities, such as having a coma when its orbit is near the Sun (at its perihelion).

If you look up info about other centaurs and check out their orbits, it will probably look something like this.

And if you look further, say at Chariklo, you might find more interesting information, like how this centaur has not one, but two rings around it.

Chariklo is a centaur slash asteroid that has an orbit between Saturn and Uranus. It looks something like this. #iknowitdoesntlooklikemuch #waitforit

Chariklo via the Hubble Space Telescope in 2015

Discovered in 1997, Chariklo is the largest known centaur so far, with a diameter of around 300 KM. In 2014, a group of scientists found evidence of rings on Chariklo by observing it during an occultation. An occultation occurs when the object you want to study crosses in front of a star, thereby blocking its light from reaching you. The longer the period of the block, the larger the object.

During the stellar occultation of Chariklo, the light magnitude from the start dipped not once, but 5 times, indicating 2 distinct ring systems (and outer and an inner) around Chariklo.

Read more about this process here.

I’ve gotten to this point in my post when I discovered that scientists are still at odds on how to define centaurs. I mean, if you think about it, they’re everywhere and then millions of years down the road, they’re just not where they were anymore. They’re not doing that to annoy you, that’s just how they roll. But you got to admit, centaurs are just too cool to leave out of this post.

Finally Wrapping Up

The thing with astronomy is that the objects we find inside our Solar System don’t have to fall neatly into boxes for us. Try as we might to classify this object or that, we will constantly be making new and surprising discoveries that will push the boundaries of our definitions and force us to rethink and recategorise and reboot our understanding of the cosmos for as long as we are willing.

This is precisely why No. 1, astronomy is a must-learn topic in Science class. And No. 2, we need to and should update our textbooks to give our children the opportunity to learn about all these “new” discoveries beyond Neptune.

If you want the next generation to be fascinated with Science, keep astronomy in Science class and it will do the rest.

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