What’s What in the Outer Solar System

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There’s a new minor planet in the solar system! Actually, that’s a lie, this minor planet, 2017 OF201, has been in the solar system probably since it formed. But now we know about it! This minor planet is also a trans-Neptunian object, and given its size it may turn out to be a dwarf planet, in which case some folks, but probably not many, may also consider it a plutoid.

That’s a lot of things for one object to be, but that’s actually not uncommon. That said, you may be wondering just what all these terms mean if one object can be all of them at once.

There are a lot of obscure vocab terms used to describe objects in the solar system, even if you just focus on the distant solar system. So today, inspired by the news of 2017 OF201’s discovery, we’re going to do a bit of a vocabulary lesson and talk about what some of these terms mean. Fair caution though, some of them have somewhat…fuzzy definitions. We’ll get a little pedantic at times. You’ve been warned. 

The Namekeepers

As a prologue, let’s start with where official space definitions and names come from. That’s the job of the International Astronomical Union (IAU), an organization of astronomy professionals from around the world. They’re the ones that give official names to individual objects in space, such as moons or asteroids. And they’re also the ones who define categories of things in space, such as what makes a planet.

Which, yes, does make them the folks who voted to reclassify Pluto in 2006. Calm down, Pluto folks, we’re not here to talk about that. That's what this post is for.

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Minor planet

This term actually has a bit of history, in that its official definition has been changed over time. Prior to 2006 anything orbiting the Sun that wasn’t a planet or a comet could be defined as a minor planet. The big IAU meeting of 2006, where an official definition of “planet” was decided upon, also technically led to the retirement of this term. 

Now if you’re orbiting a Sun and are not a planet or dwarf planet you are a “minor solar system body”. That said, the official organization in charge of curating positional measurements of all these things is still known as the Minor Planet Center, and it still refers to the things under its purview as minor planets, and all of the articles outlining the discovery of 2017 OF201 refer to it as a minor planet. So the old definition of “minor planet” as being anything orbiting the Sun that is not a planet or dwarf planet (and possibly still also not a comet? It’s unclear) is still in popular use.

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Planet

You know what, we should probably include this one just for clarity’s sake. Voted on in that epic 2006 IAU meeting, the official definition of a planet is something that orbits the Sun, is massive enough for gravity to pull it into a spherical shape, and gravitationally dominates its orbit.

Well…okay fine, the official definition actually refers to planetary bodies as things that gravitationally clear their orbit, but that wording is a little misleading. For instance, by the strictest reading of that definition Jupiter doesn’t qualify as a planet because it shares its orbit with a whole bunch of asteroids, the Trojans that the Lucy spacecraft is on its way to visit. You could even make the argument, being very pedantic, that Earth doesn’t qualify because of the Moon hanging around.

But the Trojan asteroids are under the control of Jupiter’s gravity, just as the Moon is under the control of Earth’s gravity. Jupiter and Earth are gravitationally in control of their orbits and everything sharing that orbital space, which is the intent of that planet definition. 

Dwarf Planet

“Dwarf planet” does not mean what it sounds like it means. Dwarf planets are not considered planets, not even small planets (which is what the words literally should mean!).

A dwarf planet meets the first two requirements for being a planet, being roughly spherical and orbiting the Sun, but they are not gravitationally in control of everything in their orbital space. One of them is smack in the middle of the asteroid belt while the others (including Pluto) are in the region of the Kuiper Belt (we’ll get to that later).

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There are five official dwarf planets: Ceres (the lone asteroid), Pluto, Eris, Haumea, and Makemake. But it’s kind of like the IAU chose those first five and then stopped caring about dwarf planets. It hasn’t designated a new official dwarf planet since 2008.

There are other objects we know to be more massive than Ceres, such as Quaoar and Gonggong, which are not counted as official dwarf planets, despite theoretically having the mass to make themselves round. And then there’s Haumea, which is an official dwarf planet and we now know looks more like a rugby ball than a sphere, thanks to its rapid rotation. Technically (pedantically) speaking, it probably shouldn’t count. 

So how many dwarf planets are there, actually? Unknown! For the record though, 2017 OF201 is probably big enough to make itself spherical and therefore count as a dwarf planet, if the IAU ever gets around to naming new ones.

Plutoid

This term isn’t that old but seems to never have found a following. It was decided in 2008 (by the IAU, naturally) that any dwarf planet orbiting the Sun farther out than Neptune could be designated a “plutoid”. This, of course, includes all the current official dwarf planets with the sole exception of Ceres.

That said, try to find any publications that use the term “plutoid”. It’s not even included in the IAU’s official glossary on their Office of Astronomy for Education page, and that glossary includes terms like “RR Lyrae star” and “Hubble-Lemaître Law”. It’s detailed. If the IAU isn’t even interested in this term anymore, it’s no wonder no one else seems to be.

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Kuiper Belt Objects

KBOs, as they’re known for short are, perfectly simply, members of the Kuiper Belt. Except of course you know it’s not quite as simple as that, because it never is. This belt is considered to begin at Neptune’s orbit, but even that is a little fuzzy because you have, for instance, unquestioned Kuiper Belt member Pluto scooting closer to the Sun than Neptune for part of its orbit. 

And the Kuiper Belt extends out to…well, out pretty far. Nobody quite knows, really, but in the interest of putting a number on it most astronomers put the outer edge of the main Kuiper Belt at 50 AU from the Sun. But that’s not where things end. It’s just where things start to get…scattered.

Scattered Disk Objects

Beyond 50 AU is the realm of the scattered disk. Scattered Disk Objects (SDOs) are Kuiper Belt Objects with more distant, less stable orbits. These orbits can be highly eccentric and highly inclined (tilted), and they got that way thanks to the gravitational influences of the gas giants, especially Neptune (the disk gets its name from the idea that these objects were gravitationally “scattered” by the planets). SDO orbits might bring them well within the bounds of the main Kuiper Belt, but they can also carry them really far out.

Is the scattered disk a separate entity from the Kuiper Belt? Depends who you ask. The Minor Planet Center distinguishes between KBOs and SDOs as separate types of objects, but other astronomers just see the scattered disk as an extension of the Kuiper Belt. Making things even more confusing, some astronomers call these things SKBOs—Scattered Kuiper Belt Objects.

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Trans-Neptunian Objects

Just to muddy the waters even further, almost everything listed above can also qualify as a trans-Neptunian object (TNO). All it takes to be a TNO is to have the average distance of your orbit around the Sun be greater than Neptune’s. 

This means that Pluto qualifies as a KBO, TNO, dwarf planet, and plutoid. Eris, the object whose discovery kicked off the “what is a planet, actually?” debate and led to that 2006 IAU definition-making party, qualifies as a dwarf planet, plutoid, SDO, and TNO. Quaoar, since it’s not officially a dwarf planet, would be a minor planet, KBO, and TNO. And we’re still not done!

Oort Cloud Objects

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Oort Cloud Objects belong to the most distant structure that is still part of our solar system, the Oort Cloud. These are essentially the same sorts of objects as KBOs and SDOs, but they orbit extremely far from the Sun, and the outer edges of the Oort Cloud (thought to be anywhere from 1-3 light years away from the Sun) represent the outer edges of the area of space under the gravitational control of the Sun—which is not the official definition of the edge of the solar system because…wait for it…there isn’t one. But it’s a sensible place to define as the edge. 

So where does the Oort Cloud start? Well…somewhere there’s a point where the scattered disk ends and the Oort Cloud begins. Where exactly that happens is uncertain. Take, for instance, the object Sedna.

Sedna is officially a scattered disk object, but it’s a very distant one, never coming closer to the Sun than 76 AU and getting out to nearly 1,000 AU at its farthest point. Even when it’s closest to the Sun, it’s still too far from even Neptune to really be gravitationally affected by it…which is kind of the definition of a scattered disk object.

Some folks argue that Sedna should more properly be classified as an Oort Cloud Object, in which case it could mark the boundary between the disk and the cloud. (Also, fun fact, Sedna is probably round and should probably also be called a dwarf planet.) 

Technicalities

Is your head spinning with all the vocab yet? What if I told you that sometimes things that start in one category of object can wind up in another. That can happen because definitions change (planet, minor planet, etc.) or because something happens to the object itself. 

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If an SDO/TNO/KBO, for instance, gets its orbit warped by the gravity of the gas giants, it might wind up orbiting much closer to the Sun, in the realm of the outer planets. In which case it’s likely going to get classified as a Centaur instead. If an Oort Cloud Object or KBO gets itself flung into the inner solar system far enough for the Sun to start to warm it up and start turning the ices on its surface to gas, that gas will form a long tail and the object will get reclassified as a comet.

And all this is just the outer solar system stuff! Don’t even get me started on the asteroids—that’s a whole ‘nother post. Just goes to show, it’s a wonderfully complex (and sometimes confusing) solar system we live in!