Caoimhín's Content

In today’s piece we are going to be taking a look at a different class of objects, the dwarf planets. Almost every month I briefly highlight a given planet, you can go back through the archives if you’d like to see some examples. Later this month we will be talking about Neptune, so I will have run out of planets in our solar system. Given that each planet has only gotten a brief overview, I will be going back to add more details about the planets in future videos. However, there is another class of objects, other than the planets, comets, asteroids and moons: Dwarf Planets. I will deal with various members of this class individually over the coming months.

To begin with, we almost have to start with Pluto. Pluto is certainly the most famous, a dwarf planet, and it is technically above the horizon it the moment. Early in August, Pluto will already be in the sky as the sunsets, reaching the South once the sky gets dark and Saturn rises into the sky. Saturn and Pluto can be seen to be on a similar line. Moving a little further into the month will bring the Moon into the sky, and the Moon will pass reasonably close to Pluto in the sky. This is thanks to the Moon moving close to the ecliptic in the sky, and it will pass close to, or occult, the other planets over the course of the month. While most of the planets are almost exactly on the ecliptic, Pluto orbits a little bit differently to the other planets in our solar system. Pluto’s orbit is slightly more inclined, so it goes above and below the disc of the ecliptic a little bit more than the other planets. More inclined and more elliptical orbits are common with small, distant objects. A Plutonian day is hundreds of hours, over 6 Earth days, and it’s year is similarly long. The sidereal period of Pluto is a little over 248 Earth years. Again, this is expected for such a distant object, and more distant objects will have even longer years. Given that Pluto is so small and so distant, it required reasonably sophisticated telescopes and techniques to discover. As such it was only discovered in the 1930s by C. W. Tombaugh. It was then named by an 11-year old girl, Venetia Burney Phair, who proposed the name Pluto in keeping with the other planets. Her grandfather told her about the discovery of the new planet over breakfast, and he was able relay her suggestion to some astronomers. These astronomers then relayed it to Lowell observatory where it was discovered.

When first discovered, Pluto was the only object like it that we knew of, a little icy ball of rock, smaller than even Mercury. As it was the only one, and astronomers were specifically looking for another planet, so it was considered a planet. Astronomers were looking for some object that could explain peculiarities in Neptune’s orbit, the ninth planet. Once Pluto was discovered it was considered the ninth planet, and astronomers started looking for a 10th planet. Even early after its discovery astronomers realised that it was quite small, and wouldn’t explain the quirks of Neptune’s orbit. Pluto is practically impossible to observe with out a truly massive telescope, the kind that requires a whole observatory. Pluto was only imaged reasonably clearly by the New Horizons mission quite recently. I say quite recently, it was 2015, which certainly feels recent to me. For this reason, Stellarium only provides a fuzzy ball for Pluto, but it’s spherical shape is an important characteristic, and most of our images of Pluto really are quite fuzzy. Along with Pluto we can see it’s moons, Nix, Hydra, Styx, and Kerberos. Kerberos is Cerberus, the famous three-headed dog that guards the gates to the underworld. Kerberos is usually Latinized it’s Cerberus, but Kerberos is closer to the original Greek. Nyx or Nix is one of the direct daughters of Chaos, related to nighttime and darkness. Nix is the official name of the moon, while Nyx is the usual way of writing the deities name, and I think Nyx with a Y goes better with Styx. Hydra is the many headed serpent fought by Hercules. Styx is the river of souls that passes through the underworld. Charon is the psychopomp, the ferryman that brings souls across the river Styx to Pluto, down to the the Plains of Elysium. Styx, as well as being a river, was a goddess in the Greek tradition. Furthermore, while Styx and Charon were incorporated into Romany mythology by poets such as Virgil, they probably weren’t originally part of the roman structure of the underworld. In this system, Charon is quite close to Pluto in size, similar to the Earth and the Moon. In fact, Charon is even closer in size to Pluto than our Moon is to us, they are comparably sized objects. This leads to them orbiting a common barycenter. Just like a big pair like the planet Jupiter and the Sun orbit a common point, so to do Pluto and Charon. A few of the other distant objects appear to have sizable moons as well, and may be closer to binary objects.

Moving onto the next dwarf planet, Eris. Eris is named after the Greek goddess of strife and discord, whose Roman equivalent is Discordia. Eris has a moon, Dysnomia, the daughter of Eris, the daemon, or daimon, of lawlessness. It’s name may be in reference to the disagreements surrounding Eris and the dwarf planet category, although it also spent three years unnamed while various people made proposals. Despite being discovered after Haumea and Makemake, two other dwarf planets, Eris is much closer in size to Pluto and a little more massive, making it far harder to put into a separate category. It is so similar to Pluto, either it would have to be classified with it as a planet along with all the other planets, or it would mean reclassifying things like Pluto and Eris into their own category. This is the category of dwarf planet, objects that are round and capable of maintaining their own shape (achieving hydrostatic equilibrium), but not after clearing out everything that goes around the Sun in their orbit like a planet does.

One of the best examples of an object that is clearly spherical, but not after clearing out its own orbit around the Sun, is Ceres. I’ve mentioned Ceres in previous videos, I was talking about minor bodies, the asteroids and the Asteroid Belt in particular. Ceres is in the asteroid belt, which means it shares its orbit around the Sun with a huge number of other objects. Ceres does hug pretty closely to the ecliptic, as many objects in the asteroid belt do, it doesn’t have as inclined or eccentric an orbit as the more distant objects. It is above the horizon just as the Sun sets at the moment, right in the middle of Sagittarius. If you can find the teapot shape of Sagittarius, Ceres is pretty much right in the center of it right now. Ceres is technically visible if you have a binoculars or telescope, unlike all of the other dwarf planets, which require massive telescopes to observe. Ceres is only in the asteroid belt, just about 2 AU from the Earth. Jupiter is closer to 6 AU while Mars is closer to 1.5 AU. At almost 1000 kilometers in diameter, it is a big ball of ice and rock. Thanks to its size and relative closeness, it’s been known about since the 1800’s, before the discovery of Pluto. Despite being big and round, it is in the asteroid belt and it was classified as an asteroid along with all the all the other asteroids for a long time. A couple of major asteroids had already been discovered, Juno, Pallas and Vesta, and along with them Ceres was briefly classified as a planet. The exact definition and terminology around “planet” hasn’t been fixed forever, different people have called different things planets at different times, with the demotion of Pluto only being the most recent change. Ceres is the biggest asteroid and is sometimes called the Queen of the Asteroid Belt, named after the Roman goddess of agriculture, which gives us the word cereal. Ceres was put into this category of dwarf planet after the category of dwarf planet was created for objects like Pluto.

Moving back out to the Kuiper Belt, the collection of rocks beyond Neptune, to the next dwarf planet Haumea. Haumea is a Hawaiian name, named for the patron goddess of the island of Hawai’i where the Moana Kea observatory is located. This observatory discovered its moons, Hi’iaka and Nāmaka. Haumea is a little bit more oblong than Stellarium shows, practically eggplant shaped. A far cry from Pluto’s long day, Haumea rotates every 4 hours, and this rapid speed stretches the dwarf planet out. Haumea is even more distant again, with an orbital period of almost 300 years. With improvements in telescopes and the knowledge that objects like this exist, the more we look the more of them we seem to find. However, because these objects are very distant, often increasingly distant from us, as well as the fact that they are rather small balls of ice and rock, they are difficult to image and measure. This means making sure that they fit all the necessary criteria for being a dwarf planet is difficult, so it does take a while for dwarf planets to get officially named and properly classified.

This brings us to Orcus. Orcus is now considered to be a dwarf planet, a similar size to Ceres and with its own moon, Vanth. Orcus is described by Stellarium as a plutino, which is a name that is given to objects similar Pluto. Given that little is known about many of these little balls of rock, the similarities used to group them are more how they orbit the Sun, rather than what they look like. This leads to categories like plutinos, that share a specific resonance with Neptune, and cubewanos which do not share a resonance but have reasonably circular orbits. Makemake would be an example of a cubewano, and there are other categories still. Orcus does not have even the close images that the New Horizon probe provided of Pluto. At a length of just 345 Earth years, its year is even shorter than Pluto’s. Pluto does still come closer to us at it’s closest, but is further away at its most distant. Pluto has a more eccentric orbit than Orcus, despite both of then sharing the same resonance with Neptune. Pluto’s orbit is eccentric enough that it sometimes gets closer to the Sun than Neptune’s closest to the Sun. That’s right, not just closer than some point in Neptune’s orbit, but closer to the Sun than Neptune ever gets. Of course Pluto also gets much further away from the Sun than Neptune’s furthest, and it’s generally further from the Sun than Neptune. Orcus is named after one of the deities associated with death in Ancient Rome as well, but not all of these distant objects are named after Classical cultures, such as Haumea and a few others.

Sedna is another dwarf planet and a member of another orbital category, the sednoids. This are objects with very eccentric orbits, with Sedna varying from “just” 76 AU from the Sun to 937 AU away, over 2000 times further from the Sun than Mercury. With such an incredible orbit, Sedna is almost like a comet that only visits the very edge of the solar system briefly, like a normal comet visits the inner solar system, every 10,000 Earth years in Sedna’s case. Most importantly, it’s another one of those objects that seems to be big enough to be round on its own, and therefore, big enough to be a dwarf planet in that same category as Pluto. In the video, I do try to find a couple of other dwarf planets and candidate dwarf planets such as Gonggong, while Makemake and Quaoar didn’t quite get mentioned this time. I do manage to focus in on a scattered disc object, 2007 UK 126, or 2007 UK₁₂₆. This string of numbers and letters is the identifier that this object has, as do all the other dwarf planets, and there may be a few that I know by name that are listed in Stellarium by identifier. Scattered disc objects are a loose group of objects beyond the Kuiper Belt and technically include the sednoids. Being very distant, this object has a long year, 626 Earth years. It isn’t a dwarf planet, but just like asteroids, this distant objects get names as well. This one is called Gǃkúnǁʼhòmdímà and it’s one of my favourites. It comes from the Juǀʼhoan language of southern Africa, making it a great example of the diversity of languages being represented in the naming of objects in space. While a line can often be drawn between a dwarf planet and some other objects, newly discovered objects may sit in candidacy for quite a while, often a year or two, while committees are held, arguments are made, votes are cast, and it’s decided whether they are or not a dwarf planet, and finally what their name will be.

I often end by expressing my hope that you get to see some of these objects in the sky, but other than Ceres, it isn’t very likely. If you have a big enough telescope or a binoculars at the right time of year, Ceres is technically visible, but only Ceres. All the other dwarf planets, they are very distant objects, the closest being as distant as Neptune and a small fraction of the size. All the same, I do hope that you enjoyed this brief overview of these incredibly distant object. Over the coming months I aim to deal with each one of them in isolation, or maybe a few inn pairs. If you’d like to catch those future entries then you can subscribe to this website and to my YouTube channel. Given that I’ll need a good year to accomplish such a plan, subscribing will also help to make sure it happens. It might take me a while to get them, but I hope that you come back for the next piece as well.