Halley’s Comet as seen in 1910. Image: The Yerkes Observatory

Stars with Long Hair

From antiquity, long before we knew what comets are made of, we knew them as stunning phenomena in the sky. The full, original name “star comet” comes from the Greek meaning “star with long hair.” By definition, the essence of a comet is the “long hair”, the tail. If a solar system body doesn’t display this “hair” each time it goes round the sun, it’s not a comet. (That’s why we had to invent the separate name asteroid for bodies that don’t have the “hair”.) We later figured out what comets are made of. They are clumps of ice mixed with dust, flying so near the sun that they are evaporating. The ice can be a mixture of different compositions:  frozen water, ammonia, and methane, for example.  The tail is the evaporating ice and dust.  Since they do evaporate, they must be relatively short-lived bodies. There must be a source for new ones, a reservoir of icy bodies far from the sun, or else they would be all gone by now.

The original meaning of “comet” is “star with long hair”. On the left, Chris Hemsworth is classified as a comet. On the right, he is not classified as a comet. You can discover more comets and non-comets by clicking on the picture.

Some of those bodies far from the sun have been discovered, and they are not classified as comets. They need something to kick them out of their cold orbits so they can move closer to the sun, start evaporating, and grow tails before they can become comets.  (And if they are too large, like Pluto, then they still wouldn’t grow tails because their gravity will retain the gas and dust as an atmosphere.)  These faraway, icy bodies are classified as Trans Neptunian Objects (TNOs). There are various types of TNOs: Kuiper Belt Objects, Scattered Disc Objects, Extended Disc Objects, and Oort Cloud bodies. Some of these TNOs are called dwarf planets and some are called small solar system bodies, but none of them are called comets because they don’t have the “long hair”.

Possible internal structure of Ganymede, an icy moon of Jupiter. Image credit: NASA/JPL-Caltech

Some bodies that were once TNOs have been kicked into orbits that come closer to the sun, crossing the orbits of one or more giant planets. but they still don’t come close enough to the sun to form a healthy tail. They form only lesser tails at times. They seemed to be half asteroid, half comet, so they were classified as centaurs. (Centaurs in Greek mythology were half human, half horse.) The centaurs of our solar system are icy bodies but they are not comets.

Some of the moons of giant planets are icy worlds and they are supremely interesting:  Europa, Ganymede, Titan, Tritan, and more.  Like the TNOs and the centaurs, they may be icy but they are not comets.

To be sure, some icy bodies are comets, but not all of them are comets.  Icy bodies can be many things, including centaurs, moons, and even planets if they are big enough.  Being icy doesn’t imply that Pluto is a comet.

Planets can be any material or state

Model of the internal structure of Uranus. Note that the icy mantle is believed to be a supercritical fluid, not a solid. Source: Wikimedia

Planets can be made out of any material.  They can be rock, metal, gas, ice, or peanut butter. If space aliens have polluted the Milky Way, there might be a planet formed entirely of plastic bottles and K-cups that floated into a clump off the shoulder of Orion.  Someday we will recycle that planet.  Closer to home, planets like Mercury, Venus, Earth and Mars are made out of rock. Others, like Jupiter and Saturn, are made out of gas. Still other planets are made out of volatiles that have higher melting temperatures, such as methane, ammonia, and water. Planetary scientists call these volatiles ices even when they are in a gas or liquid state, as they are in Neptune and Uranus. So yes, Neptune and Uranus are made out of ices and thus they are classified as ice giants, not gas giants.  And yes, they are planets. Bodies made of ices can be planets.

Planets can also be in any physical state. Some planets can be in the solid state, like Mercury. Others can have surfaces that are in the liquid state, like Earth’s oceanic surface. Still others can be in the gaseous state, like much of Jupiter.  It doesn’t matter which state the matter is in.

The geophysical definition of a planet is simply this:  to be a planet, a body must have enough mass to pull itself into a round shape by its own gravity, and it must not have so much mass that it initiates nuclear fusion (for that would render it a star). Composition and physical state do not matter.  Icy worlds as large as Pluto easily meet this geophysical definition of a planet.

Ice and rock are not fundamentally different

In fact, the distinction between ice and rock is rather arbitrary.  It isn’t a fundamental boundary in physics. When we defined some materials as ices and others as rocks, we drew the line by picking a melting temperature rather close to our body temperature. Why did we do this?  We were influenced by the mental conception of ice that we developed here on Earth, where water gets hard and feels cold to our bodies in the winter, then it gets fluid and feels warmer to our bodies in the summer. We started calling the cold version of this water ice. Compounds that stayed solid all year round, on the other hand, we called rock. Thus, the distinction between ice and rock originated from our human experience.  There is nothing fundamental about that.

Jeffrey Kargel wrote,

By the geologic definitions of mineral and rock, ice can be both, as can the Solar System’s more volatile condensed solids, such as ammonia dehydrate and nitrogen ice. These minerals melt, respond to stress by fracture and flow, react chemically with one another, weather when exposed at satellite surfaces, and generally do many of the things that Earth’s silicate rocks do, although they do so in unique ways. Some of the basic geologic processes of icy satellites, such as volcanism and tectonism, differ little in their fundamental physics compared to the processes that affect Earth’s silicate rocks… [1]

Also, rock can evaporate like ice and form a tail when a rocky body is close enough to the sun.

If you want to say that rocky worlds can be planets but icy worlds cannot, then to justify it you need to invent some property of ice that makes it fundamentally different from rock. But there is no such property. These materials all exist on a continuum and we drew an arbitrary, human-centered line between them. Planets can exist on both sides of that arbitrary line.

Pluto is not primarily an icy body, anyway

Possible internal structure of Pluto. Source: NASA / Pat Rawlings via Wikimedia

It is perfectly acceptable for a planet to be made out of ice, either fluid ice like Uranus and Neptune or solid ice like the outer layers of Pluto. Nevertheless, this discussion is basically moot. Although Pluto does have an icy exterior, most of its mass is rock. Even if the ice were completely removed from the rock, the amount of remaining rock would be enough mass to qualify Pluto as a planet according to the geophysical definition. Pluto’s ice is just icing on the cake. Adding icing to a cake doesn’t take away the cake. Adding ice to a planet doesn’t take away the planet. It just makes it a bigger, more interesting planet.

Summary

There’s no reason to say icy worlds are comets simply because they are icy. There’s no reason to say icy worlds cannot be planets.  Icy worlds are beckoning, and we will probably find that many of them are everything we could want a planet to be.  They can be complex with dynamic atmospheres, subsurface oceans, cryovolcanism, even liquid lakes on the surface.  I can’t wait for the first close-up views of an icy, Kuiper Belt planet:  Pluto, along with its five known moons this July. It will be awesome!

Reference: Jeffrey S. Kargel (1998), “Physical chemistry of ices in the outer solar system.” In B. Schmitt, C. de Bergh, and M. Festou (Eds.), Solar System Ices, Vol. 1. (pp. 3-4). Dordrecht: Springer.

Author information

Phil Metzger

Phil Metzger is a physicist/planetary scientist who works on technologies for mining the Moon, Mars, and asteroids; for developing extraterrestrial spaceports; and starting for robotic industry in space. He recently took early retirement from NASA, where he co-founded the KSC Swamp Works. He is now with the planetary science faculty at the University of Central Florida. Subscribe to the email list to get notified of updates to this blog!

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The trajectory of the New Horizons spacecraft as it encounters Pluto (along the inclined orbit) and passes into the Kuiper Belt. Image credit: NASA

There are some very strong hints that the planets haven’t stayed in their original orbits and the giant planets tossed around the smaller ones.  These hints will help us understand several cool things about the structure of the Kuiper Belt including how Pluto got into an orbit that is so tilted and resonant with Neptune.  (Orbital resonance was described in a prior post, at this link.)

A Little Orbital Mechanics

Orbital mechanics is the strange physics of how bodies orbit one another. Let’s say you are in a spaceship in low Earth orbit trailing the Space Station, and let’s say you are trying to catch up to it.  You might think you need to go faster, right? Actually you need to slow down. Attempting to slow down makes you you swing closer to the Earth so your potential energy is reduced causing you to actually go faster, and also your orbital path is shorter. You then orbit the Earth in less time than the Space Station, so you come out ahead.  Attempting to speed up makes you go slower and attempting to slow down makes you go faster. Strange, right?

Here’s another weird thing about orbital mechanics:  the combination of real and apparent forces can make you orbit an empty location in space. Centrifugal force is an apparent force you feel when you are spinning.  If you spin a bucket of water over your head, the water feels the centrifugal force holding it in the bucket when it is upside down so it doesn’t fall out onto your head.  When your spaceship is in a circular orbit around the Earth, the centrifugal force the spaceship feels just exactly balances the gravity from the Earth, so it never moves closer to nor farther from the Earth. That balance gives it a circular orbit.  Another apparent force is the Coriolis force.  In the Wizard of Oz, when Dorothy’s house was spinning in the twister, if she tried to walk across the room she would have been thrown toward the wall.  We call that tendency to swerve off a straight line when you’re in a spinning reference frame the Coriolis force.  If you are orbiting the Earth then you will feel the gravity of both the Earth and the Moon, and these two real forces plus the apparent forces will balance each other in five special locations.  These are called the Lagrange points. If you are very close to a Lagrange point but not exactly right at it, then the real and apparent forces will tend to push you back toward the Lagrange point, and they can even make you go into orbit around the Lagrange point. Two of the five Lagrange points are stable so you really will orbit them. The other three are almost stable so you can go around and around for a long time before you finally break free.  This is very cool, because you are orbiting a point in space where nothing exists.  There is no actual gravity from that point to make you orbit it.

Now all of this was to introduce the next concept.  In a recent post I talked about orbital resonances and how the planets give little kicks to each other. Pluto is in a 2:3 resonance with Neptune, so every time Pluto orbits the sun twice, Neptune orbits the sun exactly three times. Therefore, the kicks from Neptune don’t hit Pluto randomly but in an orderly pattern. It turns out this pattern of kicks is stable like the Lagrange points. If Pluto’s orbit wasn’t exactly in the 2:3 resonance — let’s say it completed slightly more than two orbits for Neptune’s three, then the kicks from Neptune would tend to pull Pluto along faster, and since going faster makes it swing farther from the sun it would actually slow down and take longer to go around, which would move it back toward the exact 2:3 resonance. And if Pluto were completing slightly less than two orbits for Neptune’s three, then the kicks would tend to pull Pluto backward as if to slow it down, but that would make it swing closer to the sun so it actually speeds up and moves forward closer to the exact 2:3 resonance. Unlike the Lagrange points, these resonances are not specific locations in physical space. Instead, they are locations in “parameter space.” When the parameters of the orbit (i.e., the semi-major axis, the eccentricity, and so on) are close enough to what they need to be for resonance, then the forces of orbital mechanics will push the planet through this parameter space closer to that special set of parameters. Cool, right?

That’s enough orbital mechanics. Now, what does this have to do with the origins of the Kuiper Belt?

Clues in the Solar System

A young star with a protoplanetary disk. Credit: ALMA (NRAO/ESO/NAOJ); C. Brogan, B. Saxton (NRAO/AUI/NSF)

The planets formed out of a flat disk of gas and dust like a giant Frisbee spinning around the newly forming sun.  Therefore, they orbited in approximately the same, flat plane; we call it the ecliptic plane.  We can find the ecliptic plane when we look at the planets in the night sky. They will all be in the same great circle that seems to go around the Earth, all following approximately the same narrow path crossing the same constellations year after year. That path is the side view of the ecliptic plane. We see it as a side view because the Earth is in the plane along with the other planets.

When planetary bodies get too close to each other they can scatter out of the plane. We call the resulting tilt of their orbit relative to that plane the inclination of the orbit. Consider two cars going the same direction down a long, straight highway. If the cars collide just slightly off-center, they can careen completely off opposite sides of the road, even though they were going pretty straight down the road to begin with.  The slightly off-center alignment gets exaggerated by the crash. Likewise, the planets were not all exactly in the ecliptic plane, but pretty close. When they swing near each other, the gravitational “crash” exaggerates their slightly off-center alignment, throwing them in opposite directions out of the plane, giving them both larger inclinations. Also, if a heavy truck collides with a tiny car, the car will get scattered farther off the road than the truck will.  Likewise, if a smaller body like Pluto interacts with a heavier one like Neptune, Pluto will get the larger inclination as a result.  By looking at the inclinations of the many bodies of our solar system, we can get a clue as to how much scattering has taken place.

The semi-major axis is like an average distance from the sun, so the first four triangles on the left are Mercury, Venus, Earth and Mars, followed by the circles that are in the main ateroid belt, and so on out to the TNOs. TNOs are Trans-Neptunian Objects, including the Kuiper Belt, the Scattered Disc, and the Detached Objects. The smaller bodies can be far out of the ecliptic plane, showing they were scattered by the larger ones. Even the classical planets are not perfectly in the plane.

Another clue about scattering in our solar system is the eccentricity of planetary orbits. Eccentricity is defined so that a circular orbit has an eccentricity of zero, while an extremely elliptic orbit approaches the limiting value of one.  Note in the next plot how the TNOs tend to fall on the same arc. That’s because they all have perihelions (closest approaches to the sun) in a narrow range of values near Neptune, which scattered them, but depending on how much they were scattered they can travel varying distances from the sun, and that distance gives them both a higher eccentricity and a higher average distance from the sun (semi-major axis) so the two are mathematically related. Note also the large eccentricity of Mercury, showing how it has been scattered by other bodies.

High eccentricity also indicates that bodies have been scattered.

Migration of the Giants

To explain these and many other clues, scientists have come up with models of what apparently happened in the solar system’s past.  Two recent models, the Nice Model and the Nice 2 Model, have been highly successful at explaining many of these things.  (“Nice” is pronounced like “niece” since it refers to the city in France.)

According to these models, several billion years ago the four giant planets were in more circular orbits with smaller inclinations.  Uranus and Neptune were closer to the orbit of Saturn at that time, and the Kuiper Belt was vastly more dense than it is today and much closer to the sun, just outside the orbits of the giants.  Then, Jupiter and Saturn slowly drifted into a 1:2 resonance with each other. That was brought about through the gravitational tugging of the many small bodies around them. Once they reached this resonance, Jupiter and Saturn kicked each other into more eccentric orbits. That made them start kicking Uranus and Neptune, flinging them farther out, plunging them into the heart of the early Kuiper Belt. Then Uranus and Neptune began flinging most of the bodies of the Kuiper Belt far away, out into the Oort Cloud or completely out of the solar system. The survivors — those that were not tossed as violently — are what we see as the modern Kuiper Belt.  Though they weren’t thrown as far, they were still tossed into higher inclinations and higher eccentricities, explaining the shape of modern Kuiper Belt.

Click for larger view. Simulations by Wikipedia User:AstroMark (whom I guess is Mark Booth). Left: the solar system before the 1:2 resonance. The four circles are the orbits of Jupiter, Saturn, Neptune and Uranus. (Yes, Neptune was closer to the sun than Uranus back then according to about half of the simulations.) Middle: Uranus and Neptune have switched places and have plunged into the Kuiper Belt. Right: the majority of the Kuiper Belt has been scattered away. Image source: Wikimedia CC BY-SA 3.0

Although I used the word “plunged”, the migration of Neptune and Uranus took tens of millions of years. As they moved, their orbital periods were getting longer. Inevitably, Neptune’s period got close to resonance with one Kuiper Belt body after another.  Some of those bodies were very close to the special location in parameter space for the 2:3 resonance, so the forces of orbital mechanics pushed them even closer to it and they went into orbit around that point. As Neptune kept migrating outwardly, this location in parameter space also moved, but orbital mechanics kept pushing the resonant bodies so they all migrated together. Like a wave moving toward the beach sweeping up the surfers in its path, Neptune swept many bodies into this resonance. This explains why there are so many plutinos:  they didn’t form in that 2:3 resonance; they were swept there one-by-one as Neptune’s influence rolled through the Kuiper Belt like a giant wave.

This 2:3 resonance is not the only one that swept up planetary bodies during the gas giants’ migration. Other resonances are populated as well.  Many of these resonant bodies are likely to be confirmed as dwarf planets when we get a better measurement of their sizes.

Summary

When the New Horizons spacecraft flies past Pluto on its historic encounter, here are some cool things to remember:

1. Pluto likely formed much closer to the sun, along with the other Kuiper Belt Objects. This may help explain why Pluto is mostly rock instead of ice.
2. Pluto (like most Kuiper Belt Objects) has a high inclination and high eccentricity because it was scattered through gravitational encounters with Neptune (and Uranus?) billions of years ago. Its orbit was affected by these encounters more than Neptune’s was affected simply because of their differences in mass.
3. Pluto, and the other bodies in 2:3 resonance with Neptune (the plutinos), were likely swept into this resonance as Neptune migrated away from the sun several billion years ago, and once they got into the resonance they rode the wave until Neptune finished migrating.
4. Many clues about our solar system are in the Kuiper Belt, so the New Horizons mission may be the first of a new wave of planetary spacecraft going out to this third zone of the solar system.

If you enjoyed this post, please share it with others! Do you have questions or comments? Please let me know, below.

Note: this article was amended. Originally it stated that objects farther from the sun have higher velocities yet take longer time to complete an orbit only because the distance is longer. Thanks to reader Robert who pointed out my error. In fact, objects go more slowly the farther they are from the body they are orbiting, their velocities scaling as v=GM/Sqrt(R), where G and M are the gravitational constant and mass of the object they orbit, and R is the semi-major axis, sort of like an average orbital radius. Velocity decreases as R increases. Therefore, as you thrust to try to “speed up”, you are fighting “uphill” against gravity the whole way and you are actually slowing down.  The lost kinetic energy and the new work from your thrusting both go into gravitational potential energy.

Author information

Phil Metzger

Phil Metzger is a physicist/planetary scientist who works on technologies for mining the Moon, Mars, and asteroids; for developing extraterrestrial spaceports; and starting for robotic industry in space. He recently took early retirement from NASA, where he co-founded the KSC Swamp Works. He is now with the planetary science faculty at the University of Central Florida. Subscribe to the email list to get notified of updates to this blog!

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Orbital Resonance

When you pump your legs in resonance with the swing’s natural frequency, the energy of your leg motion is consistently transferred into the energy of the swinging. Image: Edd Prince via Wikimedia, CC BY 2.0

The planets’ orbits are affected by something called resonance.

When you were a child you learned how to swing on a swing set by pumping your legs with just the right timing to make it go. You were learning how to achieve resonance between your legs and the swing. Once you had that special timing figured out, the energy of your legs got transferred into the motion of the swing so the more times you pumped, the higher the swing went.

You probably tried messing with the resonance of your legs, too.  You may have discovered that you can pump your legs twice as fast for every swinging motion and it still sorta works. That is a 2-to-1 resonance, denoted as 2:1.  Other resonances could work, though they are not as efficient. You may have also noticed that changing the timing of the leg pump can slow the swing back down and make it stop. That’s because you are making the energy go in the opposite direction, from the swing back into your legs. And if you pump your legs at random times then it makes the swing go all wonky but it doesn’t slow down or speed up overall.

Planets going around the sun can behave similarly. Each time a faster planet passes a slower one, their gravities kick each other like legs pumping a swing. (This is how astronomers discovered Neptune. They could see something was giving little kicks to Uranus’ orbit.) As long as the planets are in orbits where the kicks happen at random times, then it’s like wonky pumping on a swing set. Those planets will wobble a bit but basically stay in their orbits. However, when planets are synchronized, then the kicks will happen at the same place each time around the sun, and one planet can start speeding up while the other slows down.

Check out the video that shows a simple experiment you can do at home to help your mom or dad understand resonances and get them ready to enjoy the Pluto flyby. (You can explain to them how it illustrates Pluto’s orbital resonance with Neptune. That will be discussed, below.)

Resonance in the Asteroid Belt

At a certain distance from the sun, an asteroid completes exactly three orbits for every one orbit of Jupiter (denoted as 3:1). The asteroid in that orbit gets regular kicks from Jupiter like legs pumping on a swing set. This causes the asteroid’s orbit to change year-by-year until it’s in a much different orbit. In addition to this 3:1 resonance, there is a 5:2 resonance, and 7:3, and others at various distances from the sun.  This resonance effect has produced gaps in the asteroid belt. Wherever an asteroid would have been in resonance with Jupiter, Jupiter kicked it away. These are called the Kirkwood Gaps.

The Kirkwood Gaps can be seen in this representation of the main asteroid belt. The outermost orbit in this diagram is Jupiter. Credit: Wikimedia

Number of asteroids at each distance from the sun. Distances that produce resonance with Jupiter are not populated with asteroids, because Jupiter’s synchronized pumping has already pushed them all away. Credit: Alan Chamberlin, JPL/Caltech via Wikimedia

Resonances can do more than empty an orbit, however. They can also fill an orbit.  At the outer edge of the asteroid belt there is a class of asteroids called the Hildas. They have a 3:2 resonance with Jupiter, so they orbit the sun three times when Jupiter orbits just twice. Their orbits are elliptical so by swinging closer to and farther from the sun they pass Jupiter at a safe distance the swing back near Jupiter’s path again while Jupiter is elsewhere around the sun. Thus, they avoid strong kicks from Jupiter that would push them out of the resonance. They do, however, get little kicks from Jupiter, and the kicks actually tend to shepherd the Hildas back into these special orbits.

The following video is in a rotating reference frame. In other words, the “camera” looking down at the solar system is rotating along with Jupiter so it seems that Jupiter (the big dot on the left) is standing still. The Hildas are actually in elliptical orbits around the sun, but in this rotating reference frame they appear to move between three corners of a triangle. Studies have shown that Jupiter probably collected the Hildas by shepherding them into this 3:2 resonance as it migrated closer to the sun billions of years ago.

Resonance in the Kuiper Belt and Beyond

In the outer solar system beyond Neptune, the planetary bodies may been grouped into the following classes based on the type of orbit they have. These are called dynamical classes.

Relatively Stable Orbits

Classical Kuiper Belt Objects (Cubewanos)

The cubewanos are relatively stable because they keep a modest distance from Neptune and they aren’t in any resonance, so the little kicks from Neptune are randomized, like wonky kicking on a swing set.  The kicks make them wobble, but it doesn’t add up to anything consistent. Nevertheless, they may be randomly nudged over time into more unstable orbits from which they will be scattered away.  They are playfully called cubewanos because the first one discovered was numbered 1992 QB1, and the QB1 was given an “o” at the end to make it sound similar to plutino or twotino: “Q-B-One-oh.”

Quaoar and Makemake are two of the dwarf planets that are cubewanos. In fact, a recent list of known Kuiper Belt Objects shows 29 of the cubewanos are probably large enough to be dwarf planets and another 102 are possibly large enough to be dwarf planets. (We need better measurements to know for sure.) You might think that makes it a crowded place in the Kuiper Belt, but actually it is not. The distances are vast that far from the sun, and the area of the ecliptic plane per dwarf planet is more than 1000 times greater in the Kuiper Belt than the area per planet in the inner solar system. More than 1000 times!  It is the inner solar system that is crowded. (In my opinion, these Kuiper Belt bodies actually have cleared their orbits and should be classified as planets. More on this, below.)

Resonant Bodies

Haumea is an example of a resonant body. It is in a 7:12 resonance with Neptune. Resonances can be any combination of small integers, like 2:3, or 3:5, or 4:7. We know of 27 resonant bodies that are probably large enough to be dwarf planets, and 66 more that are possibly large enough.

The orbital path of Orcus in a view that rotates to keep Neptune (N) stationary. Note how Orcus keeps a great distance from Neptune. The small orange circle near the center is the orbit of Mars, showing the tiny size of the inner solar system.  Dancing around the giant planets simply cannot fit there. Credit: kheider via Wikimedia

Plutinos

This is a subclass of the resonant bodies. They include Pluto, the dominant member of the Kuiper Belt, hence the name. They are in a 2:3 resonance with Neptune, so every time Neptune goes around the sun three times, they go around twice. There are lots and lots of plutinos.  Orcus and Ixion are two more examples. We know of 19 plutinos that are probably large enough to be dwarf planets, and 32 more that are possibly large enough.

Twotinos

This is another subclass of the resonant bodies. They are in a 1:2 resonance with Neptune, hence the name. They are considered the outer edge of the Kuiper Belt, meaning that they have an average distance from the sun that is larger than the other Kuiper Belt bodies. (But they are in elliptical orbits so they all move into and out of the main bulk of the Kuiper Belt; it’s an abstract edge.) Recent data show three known twotinos are probably large enough to be dwarf planets while 30 more are possibly large enough.

Tostitos

These bodies are a snack, not technically part of the Kuiper Belt, and they are not in any known planetary resonance except with the Earth.  They may be eaten during the Pluto flyby. We have never found a Tostito large enough to be a dwarf planet. #NerdHumor

Detached Objects

These bodies are not considered part of the Kuiper Belt. They avoid Neptune’s gravitational bullying by staying far away from its territory.  They might swing into the heart of the Kuiper Belt at their closest approach to the sun, but never close to the orbit of Neptune, and their farthest distance from the sun is way, way out there. Thus they do seem rather detached, but many planetary scientists are still attached to them.  Seven known detached objects including Sedna are probably large enough to be dwarf planets, and another 10 are possibly large  enough.

Unstable Orbits

Scattered Disc Objects

These bodies are not considered part of the Kuiper Belt. They are in unstable orbits and they are right now in the process of being pushed around by Neptune.  Their orbits may pass through the Kuiper Belt, but they are typically very tilted and elliptic.  Twenty two scattered disc objects are probably large enough to be dwarf planets, including Eris and Salacia. Another 97 are possibly large enough.

Where are the Kirkwood Gaps of the Kuiper Belt?

When you look at the distribution of main belt asteroids versus their distance from the sun (above), you see very obvious gaps wherever there is a resonance. The resonances actively push the asteroids away. But when you look at the known Kuiper Belt objects versus their distance from the sun (below), you don’t see these gaps. Instead, you see clustering of bodies at the resonances. Why the difference?

Click for larger version. Each circle represents a known body beyond Neptune. The location of each circle in the horizontal axis is the semi-major axis of the body, sort of like an average distance from the sun (measured in Astronomical Units, or AU). The vertical location tells the tilt of the body’s orbit relative to the solar system’s ecliptic plane. The size of each circle is an estimate of the size of the body. Resonances are annoted near the top of the figure. You can see that bodies tend to be clustered at some of the resonances. Dark red bodies are plutinos. Blue are cubewanos. More color coding and information is given at this link. Image source: Wikimedia

We think the difference is because of Saturn. Main belt asteroids have to contend with gravitational kicks from both Jupiter and Saturn, and it is hard enough to satisfy one bully at a time, nearly impossible to satisfy two. For the Hildas, they are close enough to Jupiter that it is able to keep shepherding them into the 3:2 resonance despite Saturn’s disturbances. Bodies in other, weaker resonances, however, were not so lucky. When Saturn bumps them, their timing with Jupiter is not quite right so Jupiter kicks them out of the orbit entirely.  In the Kuiper Belt, bodies only need to satisfy Neptune’s bullying.  Uranus is so far away — the distances in that part of the solar system are so much greater — that it doesn’t have enough effect.

Another Reason Why Pluto is a Planet

Here are the statistics on the dwarf planets discovered so far in the outer parts of the solar system.

Dwarf planet dynamical classes

Why don’t we see these different kinds of orbits among the planets in the inner solar system?

The cubewanos are similar to the major planets of the inner solar system, so this is the one class we do see here. This type of body is in relatively stable orbits because they are far enough from the nearest giant planet and also not in resonance with it, so the weak gravitational kicks are random and average out.  There are so many more of these in the outer solar system than the inner because they have more room to orbit without disturbing one another.

The resonant bodies of the Kuiper Belt are relatively stable because they are in highly elliptic orbits that take them far from Neptune every time the giant planet passes by, thus avoiding big kicks. This type of motion simply doesn’t fit as a stable orbit in the small space of the inner solar system. We expect that Mercury will eventually go into a secular resonance with Jupiter (a different kind of resonance than discussed here), and this will push it to a more elliptic orbit. There’s no room for it to stay like that in the inner solar system. It will collide with one of the other planets and be destroyed, or it might thread the needle and escape to the outer solar system, but then it would no longer be defined as a planet according to the IAU’s current definition.

The detached objects are stable simply because they are so far away. Needless to say, that cannot exist in the inner solar system since the space is too small.

The scattered disc objects are in the process of being moved by Neptune. They can exist in their highly elliptical orbits for a long time in the vast space of the Kuiper Belt. The similar objects that exist in the inner and middle solar system (asteroids, comets, and centaurs) cannot exist as long before colliding with a planet, so they must be continually replaced from the Main Asteroid Belt, the Kuiper Belt, and the Oort Cloud. These transient bodies are mostly small, simply because small bodies are so much more common than large ones. On the rare occasion that a larger body begins scattering in the inner solar system (like Mercury may one day, or like Theia may have done in the past) the chances are it won’t survive long before it strikes a planet.  So it’s not surprising we see no planet-sized bodies in the inner solar system belonging to this dynamical class. Again, the space is just too small.

Unfortunately, we based the definition of a planet on our experience in the inner solar system, where planetary dynamics are not diverse. These more diverse dynamical classes are capable of coexisting in the the vast space of the Kuiper Belt, and it was this fact that made the IAU declare the KBOs are not true planets. When we say the inner solar system planets have “cleared their orbits”, what we really mean is that there isn’t enough room inside the inner solar system for these other dynamical classes to exist. They only exist where there is room for them, and that’s by definition. It’s not a property of the Earth or Mars that its orbit is clear. It’s merely the fact that dynamical diversity doesn’t fit within their distances from the sun.  It’s not a deficiency of Pluto that it’s orbit hasn’t been “cleared”.  It’s just that the outer solar system is much bigger than the inner, so it supports a more diverse phenomenology.

So in the final analysis, the IAU definition of a planet is tantamount to this:

To be a planet, a body must: (1) orbit the sun, (2) be gravitationally round, and (3) exist in a location where interesting dynamical classes cannot fit.

There’s no such thing as clearing an orbit where the interesting dynamical classes can fit, because if they can fit, then they exist.  And they will continue to exist until the sun burns out.

I don’t find this a very satisfying definition of a planet, one that determines a priori that the outermost zone of the solar system shall have no planets in it, simply because it can have so many. I would rather let nature show us which of our words describe it best. Planets means wanderers in the original Greek, referring to the lights in the sky that wandered through regular paths against the background of stars. And wander they do in the Kuiper Belt! Their wandering is so interwoven it could even be called a dance. Near the sun it is too crowded for the wanderers to dance, so they circle round and round like donkeys at a grist mill, till finally Jupiter pushes them from their tracks and they die in the escape or survive to the dance beyond. Out there, bodies rarely collide, and they move with a collective complexity that would make choreographers proud.  To my thinking that makes them consummate wanderers, all the more planet-like, not less.  It makes their wandering more beautiful, more interesting, and more surprising, like everything in nature when we finally see it for the first time.

If you found this interesting, please share it!  And I would love to discuss this with you so please comment below.

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Phil Metzger

Phil Metzger is a physicist/planetary scientist who works on technologies for mining the Moon, Mars, and asteroids; for developing extraterrestrial spaceports; and starting for robotic industry in space. He recently took early retirement from NASA, where he co-founded the KSC Swamp Works. He is now with the planetary science faculty at the University of Central Florida. Subscribe to the email list to get notified of updates to this blog!

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Hurrah! It’s the time of year we have all been waiting for. Roll out the red carpet, not for the stars, but for the Kuiper Belt Objects (KBOs). It’s THE 2015 KUIPER AWARDS!

You’ve heard of the Grammys and the Emmys, but these are the Kuipers. Sadly, none of the Underworld-themed KBOs were nominated this year, mainly because we already looked at them in the previous blog post (here).

So without further ado, here are the Kuipers!

The Strangest Way to Choose Leaders Award

Which Kuiper Belt Object is associated with the strangest way to pick a culture’s leaders? (Envelope, please)

And the winner is…Makemake!

Motu Nui, the island a mile off the coast of Easter Island where champions had to swim to retrieve the first seasonal tern egg. Foreground: birdman petroglyphs at the top of the steep sea cliff. Credit: Alejandra Edwards  License: CC BY-SA 3.0

Makemake (pronounced Mah-kay-mah-kay) was a god of the Rapa Nui culture that lived on Easter Island in the south Pacific. Around the 1600’s, the island was deforested by over-use and wars erupted over the dwindling resources. The islanders found a way to stop this warfare by picking one ruler over the island each year. It’s called the birdman cult and was a strange contest dedicated to Makemake. It continued until 1867.  In the springtime (September in the southern hemisphere) the candidates would select their champions. The champions had to swim a mile through sharks to a tiny island, and those who made it would wait until the population of terns laid their first eggs. Whoever got the first egg would swim back with it, climb a dangerous sea cliff, and give the egg to his candidate who became the island’s new leader. That’s right. On Easter island they had an Easter egg hunt to pick their government. It’s sort of like our modern system.

Makemake is a large, planet-sized Kuiper Belt Object (KBO). Maybe one day, people will gather on Makemake for an annual egg hunt.  A robotic “tern” will fly to another body whose has orbit brought it near to Makemake, and the tern will hide eggs there. The champions, dressed as the Rapa Nui, will race over in their spacecraft, and the one who locates and returns an egg first gets to lead Makemake’s scientific outpost for the next year. (Note to future generations: you really should do this.)

The Least Like a Comet Award

Neil deGrasse Tyson famously said that the KBOs are comets and so is Pluto. I don’t believe Pluto is like a comet (as explained here), but there’s another KBO that is even less like a comet than Pluto. Which one is it?

(Envelope, please.)…The winner is Haumea!

Haumea received her name from the Hawaiian goddess of fertility and procreation. Like Pluto, she was a large and differentiated body, meaning that the heavier materials had sunk to her core while the lighter materials like ice stayed on the surface. Apparently Haumea underwent a tragic collision with another KBO earlier in her life, which blew off the thick layer of ice. This made Haumea what she is today, a heavy body with hardly any ice, and therefore very much unlike a comet.

Best Supporting Actor Award

The Best Supporting Actor Award goes to the member of a binary KBO (that is, two KBO’s that orbit each other) that is the smaller body of that pair. Which KBO has best supported its dominant partner? (Envelope, please.)

The winner is Nunam!

Inuit man in an 1854 photograph. Credit: National Maritime Museum, London.

You’ve probably never heard of the KBO Nunum, mainly because it is so closely associated with its dominant partner Sila that their names are spoken together as the hyphenated Sila-Nunam. In the minor planet data base they share their number together as “79360 Sila-Nunam”. That’s unusual. Most binaries are named for their primary member while the secondary is listed as a moon. Sila and Nunam were given Inuit names. The Minor Planet Center says, “Sila is the Inuit god of the sky, weather, and life force. Nunam is the Earth goddess, Sila’s wife. Nunam created the land animals and, in some traditions, the Inuit people (in other traditions Sila created the first people out of wet sand). Sila breathed life into the Inuit.”

The Least Personable Award

The Least Personable Award goes to the KBO that best demonstrates the characteristic of not having any personality. And while it might surprise you, many of these chunks of rock and ice out there are quite personable. For example, Neil deGrasse Tyson tells us that Pluto is happy since it was called a dwarf planet. But actually Pluto wants to be classified as planet, again, so it’s not happy, but it is still very personable. So which KBO is not personable? (Envelope, please.)

The Least Personable Award goes to…Chaos!

Quark-gluon plasma is thought to have filled the universe in the first moments after the Big Bang. Credit: NASA/WMAP Science Team

Chaos is known for being unpersonable because it has no personality at all.  It was a formless primordial entity in Greek mythology from which everything else came into existence. Imagine something like Quark Gluon Plasma. Not a great dinner guest:  penultimately disheveled, lacking in wit, yet strangely popular among cosmologists.  Well, to each his own.

Chaos is probably a dwarf planet, and we will know for sure when we get a better measurement of its size.

Best Scientific Reinterpretation Award

The Best Scientific Reinterpretation Award goes to the KBO who tempts us to reinterpret its story with cool science. (Envelope, please.)

The winner is Deucalion!

Deucalion is probably too small to be a dwarf planet, but that didn’t hold him back. He was named after a flood hero from Greek mythology. As retold by the Roman poet Ovid in Metamorphoses, no life survived the flood apart from the couple Deucalion and Pyrrha. They went to the temple of Themis, who told them, “… throw behind you the bones of your great mother!” Dismayed, they finally guessed that “great mother” might mean the Earth and her “bones” might be rocks. Giving it a try they threw rocks over their shoulders, and when they fell upon the Earth they transformed amazingly into living people.

Wow. I am SO tempted to reinterpret their story with cool science. I can’t resist. A long time ago, Deucalion and the other bodies of the Kuiper Belt bodies threw rocks over their shoulders. Those rocks are what we call comets, and when some of them fell onto the barren, lifeless Earth they brought the water and complex organic molecules that eventually became living people. There. I did it.

“Deucalion and Pyrrha”, Painting by Peter Paul Rubens in 1636. Via Wikimedia Commons.

The Minor Planet Center didn’t tell us who suggested the name Deucalion, and I don’t know whether this scientific spin-doctoring was in their minds when they suggested it, but it does kind of work, doesn’t it?

The Most Under-Represented People Award

The Most Underrepresented People Award goes to the KBO that all by itself represents the most people of the Earth. (Envelope, please.)

The award goes to Varuna!

For now, Varuna represents all the people of sub-arctic Asia since there is no other KBO named for any such Asian culture.  China and India alone contain over 1/3 of the people of Earth, so the paucity of Asian-named KBOs is shocking.  The Minor Planet Center says, “Varuna is one of the oldest of the vedic deities, the maker and upholder of heaven and earth. As such he is king of gods and men and the universe, and he has unlimited knowledge.”

Varuna is probably large enough to be a dwarf planet.

The Cross-Cultural Award

The Cross-Cultural Award goes to the KBO whose namesake was part of very diverse cultures. (Envelope, please.)

Oh, we have a tie! The winners are a binary pair, Logos and Zoe!

Logos and Zoe were named for concepts in Gnosticism, a loose set of religious and philosophical beliefs that spread through both the Persian and Roman Empires (both East and West) during the 2^nd through 4^th centuries. According to the Minor Planet Center, “Logos and Zoe are a pair in a rich pantheon of paired emanations of the deity in the gnostic traditions and are part of the creation myth in this tradition.”

They are too small to be dwarf planets, so they are a binary pair of small solar system bodies.

The Most Fictional Award

The Most Fictional Award goes to the KBO whose namesake is more fictional than all the rest. Now how can one mythological figure be more fictional than all the rest? Let’s see. (Envelope, please.)

The winner of the Most Fictional Award goes to the binary pair, Borasisi and Pabu!

Kurt Vonnegut in 1972. Credit: WNET-TV/PBS via Wikimedia Commons

This binary pair of small solar system bodies was named for characters of a mythology found in Kurt Vonnegut’s novel Cat’s Cradle. The novel tells of Bokonon, a man on an island who invents this mythology and clearly tells everybody that it is false but that they should follow it, anyway, for practical benefits. “All of the true things I am about to tell you are shameless lies,” he says. The people end up following him. He tells them that Borasisi and Pabu are the sun god and moon god, respectively.

Now let’s remember that Bokonon and his fellow islanders were themselves just made-up characters in a novel.  This is striking. It’s not just that nobody ever believed in Bokonon and Pabu, but that nobody ever believed in the people that never believed in them, either.

It is fiction-squared!

The Most Difficult for Outsiders to Pronounce Award

(Envelope, please.) And the winner is…Teharonhiawako and Sawiskera!

Teharonhiawako and its large satellite Sawisker were named from the Haudenosaunee (Native American Iroquois) story of the twin brothers, one good and one evil, born of the Sky Woman. The good brother Teharonhiawako helped form both sky and earth from his mother’s body after the evil Sawiskera caused her death. They are small solar system bodies.

A group of Iroquois people photographed in 1914. Source: Wikimedia Commons

The Most Original Award

(Envelope, please.) The winner of the Most Original award is…Aboriginal.  It is Altjira!

Altjira was the primary deity of the Arrernte, an Aboriginal Australian people. Altjira created the world during the Dream-time (a dimension outside of time), but when the other gods went back to sleep, Altjira ascended into the sky.

Altjira is a small body and is part of a binary KBO, but the smaller member of the pair has not been named.

A group of Arrernte performing the Welcoming Dance

Best Childhood Memories Award

This award goes to the KBO that evokes the most special childhood memories. Now this is entirely unfair, because I’m going by my own childhood memories and not yours. But while the Kuiper Awards are fun, nobody ever said they would be fair. (Envelope, please.)

The winner of the Best Childhood Memories Award goes to…Quaoar!

Yes, Quaoar, even though it was discovered when I was an adult, brings back great childhood memories. Quaoar was named after a deity of the Tongva people of Southern California. The Minor Planet Center wrote:

Quaoar is the great force of creation in the diverse myths of the Tongva, the indigenous people of the Los Angeles basin. Quaoar has no form or gender and dances and sings Weywot, Sky Father, into existence. Together, they create Chehooit, Earth Mother, and the trio bring Tamit, Grandfather Sun, to life.

If you or your children haven’t read ‘Island of the Blue Dolphins’ yet, click this link and order a copy. You’re welcome.

One of my favorite books of childhood, Island of the Blue Dolphins by Scott O’Dell, tells the true story of a young Tongva woman stranded on an island alone for 18 years. Her resourcefulness and closeness to nature inspired me all my life to be like her. She was the last surviving member of the Nicoleño tribe, and after rescue to the mainland not a soul in the world could understood her speech, and she couldn’t understand a soul in the world. Yet she continues to inspire many people. Quaoar is now an island in space. If you were stranded there in a broken spaceship, would you have the resourcefulness and understanding of nature to survive?

If you loved Island of the Blue Dolphins, then maybe Quaoar will be one of your favorite KBO’s, too.

Most Related to Neptune Award

This year, the Most Related to Neptune Award goes to…(envelope, please)…Salacia!

Salacia is not actually a Kuiper Belt object but rather a Scattered Disc Object, bodies that were probably part of the Kuiper Belt at one time until scattered by Neptune’s gravity into more eccentric orbits. Salacia was named for the goddess of salt water who married Neptune, so she wins this award. And I do hope Salacia actually wanted Neptune to scatter her out of the Kuiper Belt. Salacia is probably a dwarf planet and her moon Actaea was named for a sea nymph.

Ancient Roman mosaic of Neptune and Salacia discovered in a house in Pompeii, which was buried in volcanic ash in 79 AD. Photo credit: Wolfgang Rieger

Least Related to Neptune Award

Of all the bodies we know so far, which one is least related to Neptune? (Envelope, please.)

And the winner is…Sedna!

Animation that illustrates how far Sedna is from the sun. The animation begins at the inner solar system then zooms out. Credit: NASA/JPL-Caltech/R. Hurt (SSC-Caltech) via Wikimedia Commons

Now Sedna isn’t a member of the Kuiper Belt, either. Sedna spends so much time at great distances from the sun that her orbit is hardly affected by Neptune’s gravity at all. Thus, she is classified as a detached object.  She might even be part of the inner Oort Cloud. Shunning Neptune as she does, she wins the award.

Because it is so cold that far from the sun, leader of the discovery team, Mike Brown, wrote on his website, “We feel it is appropriate to name it in honor of Sedna, the Inuit goddess of the sea, who is thought to live at the bottom of the frigid Arctic Ocean.” He suggested this could establish a new Arctic naming theme for similar bodies.

Most Like a Spiky Fish-Man with Crab Claws Award

The Most Like a Spiky Fish-Man with Crab Claws Award this year goes to…(envelope, please)…Phorcys!

Phorcys on a Roman mosaic at the Bardo National Museum, Tunis, Tunisia.

Phorcys was a spiky fish-man with crab claws. He had red skin, human hands, and a long, fat tail, plus two appendages with crab claws poking from his abdomen like an alien baby busting free. He had hair like Isaac Newton, horns like the Vice Chancellor of Star Wars, and skin that was covered with spikes. He was married, so all I have to say is that his spouse really knew how to pick ‘em. He ruled over dangers that lurk in the deep.

Phorcys is not really a KBO, but rather a scattered disc object that crosses the orbits of Neptune and Uranus then swings far beyond the outer edge of the Kuiper Belt.

Another view of our handsome fellow with the alien-baby crab claws protruding from his gut. Source: Theo Greek Mythology.

Most Longsuffering Spouse Award

The Most Longsuffering Spouse Award goes to…(envelope, please)…Ceto!

Ceto is a longsuffering spouse because she is married to a spiky fish-man with crab claws. Yes, her husband is Phorcys. Ceto really knew how to pick ‘em. She was one of the sea goddesses of Greek mythology. She and her spiky husband (shown together in the picture, above) are best known for their fantastical children: nymphs of the sunset, snake-haired Medusa who could turn you to stone by looking you in the eye, three girls who took turns sharing one eye and one tooth between them, the fish-woman on your Starbucks cup, a snake-woman, a dragon-boy, and the mom of a cyclops. Imagine poor Ceto driving those kids to soccer practice and piano lessons. “Medusa, stop looking at your brother! Enyo, give your sister that eye back, right now! She needs it to do her homework!”

Ceto and Phorcys are binary KBOs, with Ceto being the larger of the two. They are small solar system bodies. I think it’s interesting that Ceto is related to the word cetus for sea monsters, from which we get the word cetaceans for whales, dolphins and porpoises. Orcus, one of the plutinos, is related to the word orca, so we have two KBOs related to whales.

Astronomer’s Choice Award

This year’s Astronomer’s Choice award goes to the KBO whose namesake did the most for the advancement of astronomy. (Envelope, please.)

The winner is Varda!

Varda and Manwë in Valinor. In the legendarium of J.R.R. Tolkien, Varda made the stars and put the sun and moon in the sky. Image copyright by Ted Nasmith. Used by permission.

Varda is from J.R.R. Tolkien’s mythology where she is the queen of the Valar (the angelic powers) that rule over the world. She did a huge service for astronomy because she created the stars, and astronomy would have be the most ridiculed profession in the world if there was nothing for astronomers to look at. Imagine all the astronomers staring through giant telescopes at nothing and publishing papers about it. Tolkien mentioned some of the constellations Varda created including “the Sickle of the Valar.” He gave us enough clues to realize it is the Big Dipper. Another constellation she made is “the Swordsman of the Sky.” Any guesses what real-world constellation that could be?

Varda’s handmaiden was Ilmarë, the leader of the Maiar, and they were the second-tier of angelic powers ruling the world. They included wizards like Gandalf.

Varda and Ilmarë are a binary pair of KBO’s and possibly both large enough to be dwarf-planets. Like Pluto and Charon they may be a double dwarf planet! I loved Tolkien’s writings and I love double planets, so I think Varda and Ilmarë are super cool. Did you know we’ve got this stuff in our solar system?

The Most Like Manwë Award

Book cover of The Simarillion, illustrated by Ted Nasmith. Available at this link.

The Most Like Manwë award goes to the KBO that most reminds us of J.R.R. Tolkien’s character Manwë. (Envelope, please.)

And the winner is… Manwë!

Manwë was the chief of the Valar and he was married to Varda. You might think it’s unfair to have an award that Manwë wins every year. But the Kuipers are like the awards at my kids’ soccer clubs. Everybody gets a trophy just for showing up, and if I can’t think of an award for Manwë then I have to invent one.

Since Manwë ruled the Valar, he was basically the Jupiter or the Zeus of Tolkien’s mythology. Sadly, however, the KBO named Manwë is not very much like the planet named Jupiter. It is too small even to be a dwarf planet, so it is probably bumpy and irregular, not round like Pluto. Nevertheless, as a Tolkien fan I’m happy to see his mythology represented in the Kuiper Belt.

The Most Tolkienish Award

‘Maedhros’s Rescue from Thangorodrim’. Copyright Ted Nasmith. Used by permission.

The Most Tokienish Award goes to the KBO that best represents the writings of J.R.R. Tolkien. (Envelope, please.)

The winner is Thorondir!

Thorondir is part of a binary pair of KBOs. The larger member of that pair is actually Manwë, this year’s winner of The Most Like Manwë Award. Thorondir beat out the other KBOs as the Most Tolkienish because it is a giant talking eagle, and what is more Tolkienish than a giant talking eagle? At critical moments in Tolkien’s stories, when all hope was lost, the heroes would cry, “The eagles are coming! The eagles are coming!” The eagles only came when all hope was lost because they served the Valar, and the Valar were aloof and wanted the people in Middle Earth to work things out for themselves and thereby make better novels. And the Valar never actually promised to save the people of Middle Earth, so it was never a sure thing that they would send the Eagles, anyhow. But as Samwise Gamgee once said in the Lord of the Rings:

It’s like in the great stories Mr. Frodo, the ones that really mattered. Full of darkness and danger they were, and sometimes you didn’t want to know the end because how could the end be happy? How could the world go back to the way it was when so much bad had happened? But in the end it’s only a passing thing this shadow, even darkness must pass.

And so it is, my friends, that even the Kuiper Awards must pass and the world must go back to the way it was before. Can it? Will this overly long post ever come to an end?

The eagles are coming! The eagles are coming!

The end.

Summary

We live in an amazing solar system, which is getting bigger and more amazing every year as we learn more about it. In July the New Horizons spacecraft will get the first up-close look at Kuiper Belt bodies as it visits Pluto and Charon.

Author information

Phil Metzger

Phil Metzger is a physicist/planetary scientist who works on technologies for mining the Moon, Mars, and asteroids; for developing extraterrestrial spaceports; and starting for robotic industry in space. He recently took early retirement from NASA, where he co-founded the KSC Swamp Works. He is now with the planetary science faculty at the University of Central Florida. Subscribe to the email list to get notified of updates to this blog!

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The Underworld Gathers

The Underworld is gathering in the Kuiper Belt. The background is an artist’s impression of the view from the surface of Pluto. Foreground credit: – Leopoldo Aurioles (adjusted color and cropped), CC BY-SA 4.0. Background credit: ESO/L. Calçada, CC BY-SA 4.0.

Pluto

Pluto and its moons, a six-body system. Image Credit: NASA, ESA, Mark Showalter (SETI Institute)

Pluto was discovered in 1930. An 11 year old girl living south of London, Venetia Burney, heard the news at the breakfast table and suggested, “They could name it Pluto” [1]. The darkness around the distant planet reminded her of the dark Underworld in Greek mythology. What a difference one sentence from a child can make! Her grandfather loved the name and sent a telegram across the ocean to the Lowell Observatory (where the planet was discovered), and the astronomers there loved it, too. Ever since then the Underworld has been gathering in the Kuiper Belt. So if you visit the Kuiper Belt, be sure to dress appropriately. It’s cool to go as a member of the Underworld like Asajj Ventress or Ronan the Accuser, but dressing as Elsa or Barney would be a mistake.

Charon

The second body discovered in the Kuiper Belt is Pluto’s largest moon, or perhaps its binary partner. Jim Christy, the discoverer, decided to name it after his girlfriend Char (short for Charlene) and appended the –on just to make it sound more sciencey. (True story.) Completely unknown to Christy at the time – and to his great surprise – Charon is the name of an Underworld figure, the ferryman who takes departed souls across the river Styx into Pluto’s realm. It was perfect. It’s going to be an Underworld party in the Kuiper Belt!

Rhadamanthus

Rhadamanthus is likely too small to be a dwarf planet. It was named for a mythological king of Crete, a demi-god born of Zeus and the mortal woman Europa. Homer said he dwelt after his death in the Elysian Fields, a paradise on the western edge of the world, but later writers made him one of the judges of the dead in the Underworld.

Ixion

Ixion bound to the spinning wheel. Fragment of a relief from the Side Museum in Side, Turkey.
© Ad Meskens / Wikimedia Commons, Used by permission.

Judging by its brightness, Ixion is probably large enough to be a dwarf planet. It is classified dynamically as a plutino, meaning that every time Neptune goes around the sun thrice, Ixion goes around it twice, just like Pluto. This is a stable type of orbit so Neptune doesn’t fling plutinos out of the Kuiper Belt. (At one time, astronomers thought that Rhadamanthus was a plutino, too, but more precise analysis says that it isn’t.)

In Greek mythology, Ixion was not a god, but was a king like Rhadamanthius. While upright king Rhadamanthius got into the Underworld by being good, evil king Ixion got there by being bad.  He tried to take the wife of Zeus and unwittingly fathered the race of Centaurs. For punishment he was bound forever to a flying, flaming, spinning wheel. Ouch. The later myths say this wheel is flying in the depths of Tartarus, part of the Underworld.

Nowadays, centaurs are small bodies that orbit in the zone of the giant planets, Jupiter through Neptune. The centaurs’ orbits are unstable because the gas giants will absorb them or fling them far away, usually within a few million years. Simulations tell us that one new centaur must arrive in this zone from the Kuiper Belt every 125 years (or so) to maintain the observed population. Therefore, bodies in the Kuiper Belt like Ixion really are fathering the centaurs. However, if you see Ixion floating around the Underworld party on his flaming, spinnng wheel, you probably shouldn’t mention the centaurs to him. He is still a bit sensitive about the subject.

Typhon and Echidna

Typhon and Echidna are small solar system bodies and a binary pair. Although they cross the orbits of Uranus and Neptune like centaurs, they also swing to the farther parts of the Kuiper Belt so they are classified as scattered disc objects (SDOs). Typhon, whose father was a personification of Tartarus itself, was the most terrible of monsters in Greek mythology. He was as tall as the stars, had a hundred dragon heads and shot fire from his eyes – oh, I can’t go on, it’s just too terrible. At first Typhon defeated Zeus and ripped all the cartilage from his body. Yikes. But Zeus recovered and trapped Typhon under a volcano where he keeps himself busy doing crafts with molten rock. Typhon had married the half-woman, half-serpent Echidna, who was his half-sister by the same dad. Together, Typhon and Echidna had lots of monstrous children. Doubtless they will be at the Underworld party, so if you value your cartilage, mind your manners when you meet them.

 Orcus and Vanth

Orcus is almost certainly a dwarf planet. Dynamically it is a plutino along with Ixion and Pluto but with a unique feature: when Pluto is closest to the sun, then Orcus is always farthest from the sun, and vice versa. For this reason, Orcus is considered to be the Anti-Pluto. Orcus is the name of the Etruscan version of Pluto.

Orcas’ moon might be a dwarf planet, too. Naming it was put to a public vote by the discovery team’s leader Mike Brown. His blog’s readers picked Vanth: an Etruscan guide of the dead into Hades, somewhat like Charon but female. Thus, the anti-parallel with Pluto and Charon is complete.

I like the name Orcus the best out of the entire Kuiper Belt. I think it’s because Orcus sounds like orca, a killer whale. The similarity is not a coincidence; orca comes from the same root word as Orcus. We know this word in English today as ogre, a monster from Hades. J.R.R. Tolkien used the Anglo-Saxon version of the word, orc, for the race of goblins in The Hobbit and The Lord of the Rings. Whales were once thought to be monsters so they, too, were named orcs. Therefore, when you attend the Underworld Party, it would be appropriate to dress as a killer whale or an orc or the Great Goblin himself, but don’t go dressed as Legolas who killed orcs. That would be orcward.

Orcs and Orcas are ogres from Orcus. Say it three times fast. Orc image: Wesnoth community artists via Wikimedia Commons, CC BY 3.0. Orca image: Ed Schipul via Wikimedia Commons, CC BY-SA 2.0.

Nix and Hydra

In 2005, two groups of researchers were looking at Pluto and discovered (at pretty nearly the same time) that it has two more moons in addition to Charon. Working together they came up with the names Nyx and Hydra. Their initials suggest the name New Horizons, a spacecraft that was already on its way to Pluto and is due to arrive this July.

Heracles slaying the Lernean hydra. Statuette by J. M. Félix Magdalena. Photograph by Jomafemag, used by permission.

Hydra was a many-headed snake-like monster in Greek mythology. It guarded the underwater entrance to the Underworld. Most of the myths say it had nine heads. Some astronomers argued against approving this name because nine sounded like a subversive attempt to say that Pluto is the ninth planet again! (We really are subversive like that, so don’t scoff.)

Nyx is the goddess of the night in Greek mythology. She was born of Chaos and lived in Tartarus in the depths of the Underworld. She was also the mother of Charon. The Latinized version of her name is Nox, from which we get the word nocturnal. The International Astronomical Union (IAU) agreed to these names except that Nyx was changed to Nix to avoid confusion with an asteroid of the same name. They reported that Nix is the “Egyptian spelling” of Nyx, but I can’t find evidence that the (Hellenistic) Egyptians spelled it that way. There is another report that it is actually the Spanish spelling. In any case, Nix in every language means that it’s nighttime in the Kuiper Belt, so party on.

Kerberos and Styx

In 2011 and 2012, two more moons of Pluto were discovered. Yes, my friends, the Pluto system is an amazing six body system, if not larger! The public was asked to choose names for these new moons through an Internet vote, and the top three choices were Vulcan, Cerberus, and Styx. The IAU disapproved the name Vulcan because it is already associated with the (disproven) planet Vulcan, and the (hypothesized) Vulcan asteroids close to the sun. The IAU changed the spelling of Cerberus from the Latin version to the original Greek Kerberos because the Latin already belonged to an asteroid. Styx was approved without change.

Etching showing Hercules fighting Cerberus (Kerberos), the three-headed dog. Credit: Los Angeles County Museum of Art.

You will recall that the river Styx is the boundary of Hades in the Underworld, but it is also the goddess that rules over the same river. Kerberos is the three headed dog that keeps people from entering or exiting the Underworld by patrolling the river’s banks. Kerberos will remind Harry Potter fans of the three-headed dog named Fluffy that guarded the hatch to the Sorcerer’s Stone, a stone that allowed one to escape death. (J.K. Rowling knows her Greek mythology.) Maybe in the ultra-low gravity of Kerberos there will be a covering of snow and dust so that the moon actually is Fluffy. In any case, coming to an Underworld Party dressed as one of the more sinister Harry Potter characters would be the bomb.

So many Underworld names! But this is not entirely coincidence. Following the lead of young Venetia Burney, the IAU made it policy to use Underworld names for plutinos (and so the moons of plutinos will be named for the Underworld, too).

Huya

Huya is a plutino discovered by Venezuelan astronomers who proposed the name Juyá from the indigenous Wayuu culture of that region. (The IAU changed the spelling to Huya.)  According to everything I could find, Juyá isn’t an Underworld being in Wayuu mythology but is rather a rain god – a phenomenon that happens above the Earth, not beneath it. The Minor Planet Center Circular that assigned the name says,

Huya is one of the most representative gods in the infra-world [underworld] of the Wayuu Indians of Venezuela. As such it is associated with the rain and winter, and lives in the celestial altitudes beyond the sun.

I was confused when I read this, but after some reflection I think I understand. Apparently the Wayuu do not have a concept of an Underworld similar to that of the Greeks or Romans, and I’m guessing the closest analog they have to the cold, dark halls of Hades would be the shadowy underwater world and the coldness of winter. Thus, Huya being a rain god and associated with winter is “one of the most representative gods” of the Underworld – i.e., the best analog from that culture. Since the IAU’s objective in choosing the naming themes wasn’t to omit cultures from the Kuiper Belt but rather to celebrate their diversity, including Huya as the Wayuu interpretation of the Underworld contributes to this fascinating tapestry. Welcome to the Underworld party, Huya!

Friends of the Underworld

There are a lot more bodies out there in the Kuiper Belt with names that don’t have Underworld connections, and there are many more that aren’t named at all. However, I think a couple more guests should be invited to the Underworld party.

Eris and Disnomia

Eris and its moon Disnomia were discovered in 2003 and 2006, respectively, by Mike Brown and co-workers. Because Eris’ size rivaled that of Pluto, it precipitated the events leading to the IAU redefinition of a planet. That led to some strife and discord among astronomers, so Brown named this body after Eris, the Greek goddess of strife and discord, and its moon after Disnomia, the daemon of lawlessness. Eris is not a plutino, and neither Eris nor Disnomia are Underworld figures. The names don’t fit into some other theme of the outer solar system, either (see next post). That’s not surprising, since they are scattered disc objects where a naming convention has not been specified. So for now, Eris and Disnomia are lonely. But strife, discord and lawlessness fit the behavior of most Underworld supervillains, so I think they should be invited to the party. Don’t you?

Summary

So there is an Underworld party in the Kuiper Belt. New Horizons is about to send us the first close-up pictures from the party. Of the dozens of known plutinos, only four have been given Underworld names so far. There are six moons between them so that makes ten. Rhadamanthus has an Underworld name, too. To round out the party we invited two pairs of Scattered Disk Objects that have Underworld names (or at least some nominal connection to the Underworld). In the next post we will leave the party and look at two other naming themes out there in the far reaches of our most excellent solar system home.

Diagram showing the named bodies of the outer solar system (except for the smallest moons). Color text indicates the major naming themes.

Footnotes:
[1] Schilling, Govert. The Hunt For Planet X: New Worlds and the Fate of Pluto. Springer Science & Business Media, 2010., p. 37.
[2] SDO = Scattered Disc Object

This post was modified by adding Huya and by updating the graph at the end.

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Phil Metzger

Phil Metzger is a physicist/planetary scientist who works on technologies for mining the Moon, Mars, and asteroids; for developing extraterrestrial spaceports; and starting for robotic industry in space. He recently took early retirement from NASA, where he co-founded the KSC Swamp Works. He is now with the planetary science faculty at the University of Central Florida. Subscribe to the email list to get notified of updates to this blog!

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