...or maybe a few hundred.

One of my favorite shirts honors the brave souls of the former planet Pluto, those billion voices which shouted out in agony and were suddenly silent as the International Astronomical Union’s space station destroyed– wait, no. That’s not what happened to Pluto. Pluto got demoted from the ranks of “planet” to “dwarf planet” a few years ago, much to the dismay of students around the country. But what does that mean, why did it happen, and what are these dwarf planets, anyway? Today I’d like to take you on a tour of some of our Solar System’s smaller inhabitants, and walk you through the strange lands of extreme cold, dark, and quiet on its farthest shores.

What dwarf planets are

Let’s start with the reason Pluto got demoted in the first place. The problem was simple: we kept finding other things in the Solar System which were officially “asteroids,” but which were all about the size of Pluto, or even a bit bigger. It was starting to seem silly to call some of them planets (and give them lots of extra attention) and others not when it was becoming clear that they were all kind of alike. So instead, we defined a new category for them, the “dwarf planets.” By the current census, our Solar System therefore contains four gas giants: Jupiter, Saturn, Uranus, and Neptune; four terrestrial (“rockball”) planets: Earth, Venus, Mars, and Mercury; and five dwarf planets: Eris, Pluto, Makemake, Haumea, and Ceres. And there are actually many more; the census of dwarf planets is only beginning. Astronomers estimate that there are between one and two hundred dwarf planets orbiting our sun. Mike Brown of Caltech, one of the foremost experts in this field, lists four more that are likely to be added to the list soon: 2007 OR-10, Quaoar, Sedna, and Orcus. (Yes, one of these still needs a name.)

So let’s take a look at what they are, where they are, and what’s going on with them.

First of all, there’s an official definition of a dwarf planet that makes it possible to actually argue about which is which. Planets, traditionally, have to do three things: they have to orbit the Sun, they have to be big enough that their own gravity makes them round, and they have to be able to clear out their orbits from debris. Dwarf planets are round and orbit the Sun, but can’t clear their orbits. (Things which are too small for their own gravity to round them out are asteroids) To get a really vivid example of debris-clearing, a few years ago, comet Shoemaker-Levy 9 smashed into the side of Jupiter. It was in Jupiter’s orbit; then, very dramatically, it wasn’t. Hanging around in Jupiter’s path of motion is not, in general, a good plan, because Jupiter weighs two septillion tons and is moving around the Sun at 29,000 miles per hour.

More generally, planets can clear debris out of their path in several ways. There’s simply smashing into the debris (and being the one to survive the impact), capturing the debris and turning it into a small moon, or most commonly, having your gravitational field simply fling the debris, with great force, out of your orbit. (Interplanetary spaceships use this sort of gravitational flinging a lot in order to get around) There’s a parameter you can measure about a planet called the Stern-Levinston parameter which describes how effective it is at getting rid of such junk; if it’s bigger than one, then the planet clears debris faster than it can accumulate and ultimately ends up with an empty orbit, and if it’s less than one, there will always still be junk floating around in its path. Jupiter has a value of about 1.3 billion; Earth, about 150,000. Pluto, on the other hand, only has a value of 0.003, and Makemake a mere 0.0002. Dwarf planets and regular planets are very different from each other in this way.

So that tells us what a dwarf planet is, and that definition is mostly useful so that we have clear categories we can talk about. If it orbits the Sun and has no clear surface, it’s a gas giant; if it has a surface, is round, and clears its orbit, it’s a rockball; if it’s round but doesn’t clear its orbit, it’s a dwarf planet; and if it isn’t even round, it’s an asteroid. (Anything big enough to clear an orbit is more than big enough to be round)

A Grand Tour of the Dwarves

So where do these dwarf planets live? We have our old sequence for the order of the planets, Mercury Venus Earth etc., with various mnemonic devices. And you may remember that there’s an asteroid belt in between Mars and Jupiter; there are actually lots of different belts and groups of asteroids, some of which I posted about recently. The asteroid belt itself is going around in a circle, but some of the other groups are being “shepherded” by the gravity of Jupiter and the Sun into other shapes; for example, the Hilda Group (cf. that link) forms a big equilateral triangle, with its peak directly opposite the Sun from Jupiter. This happens because of the opposite of debris cleaning: for points along this triangle, Jupiter’s gravitational force partially balances out the Sun’s, so that things sitting there don’t get flung violently into space. We see the asteroids of the Hilda Group in that triangle because all the asteroids who weren’t sitting on that triangle are now long-gone. Similar forces hold a lot of the dwarf planets in place, even as they fly close to gas giants.

Let’s take a tour of our five acknowledged dwarves, starting from the one closest to the Sun. To help guide us on our way, I’ve put together some animated maps. This first one shows the Inner Solar System; I’ve drawn the terrestrial planets in blue, the gas giants in red, and the five dwarves in grey.

This map is designed to spare your eyes by only showing you the planets. If you want to see a map that shows all of the asteroids, here’s a great one from the Minor Planet Center at the IAU. Don’t bother looking for Ceres on it; it’s lost in the haze of the asteroid belt.

Ceres is our nearest neighbor: it actually lives inside the Asteroid Belt, looming large over its neighbors. Its surface area is roughly the combined size of Alaska, Texas, and California; its gravity is a mere 0.028g’s, so you would weigh only 1/35th of your Earth weight. It has no atmosphere, but is covered in a layer of ice 100 kilometers deep; there is more fresh water on Ceres than there is on the Earth. Atop the ice is a layer of gray dust. It’s also the warmest of the dwarf planets, at a balmy 167K. (-160F, just above the freezing point of pure ethyl alcohol) If you stood on its surface, you would see that the Sun is only a third as large as it appears from the Earth; and at its closest approach, every 12 years or so, Jupiter would be large enough in the sky to be visibly round, and bright enough (apparent magnitude -4.1) that it would be visible in Earth’s sky at high noon.

The outer Solar System, Jupiter and beyond. Not only are the outer dwarves’ orbits elliptical, they’re at all sorts of crazy angles; by comparison, all the planets up to Neptune lie almost flat in a single disk, called the Ecliptic. If you want to see this region with all the known small objects in it, the IAU has a map of that, too.

All of the other known dwarves are much further out. The next three closest ones, Pluto, Haumea, and Makemake, live in the distant Kuiper Belt, beyond the orbit of Neptune. The Kuiper Belt is a region of comets and asteroids, orbiting the Sun from such a distance that you might not at first notice that it’s even your star. Pluto is the closest in of these three; it’s in what’s called a “plutino” orbit, shepherded around the Sun by Neptune in much the same way that the Hilda Group is shepherded by Jupiter. Pluto is much larger than Ceres, about the size of Russia; like Ceres, it has a rocky core surrounded by a thick layer of ice, but Pluto is so cold that this ice is capped by another layer of frozen nitrogen. (No, not liquid nitrogen. Frozen solid. 44K surface temperature.) It has a thin atmosphere, but as Pluto moves closer and farther from the Sun – its orbit is fairly elliptical, giving it quite extreme seasons – the atmosphere is prone to freezing out of the sky and falling as snow. It has five small moons of its own, Charon, Nix, Hydra, and the recently-named Kerberos and Styx. Standing on Pluto’s surface, the Sun would look like a distant, bright dot, casting about as much light at noon as the lights in your family room do at night.

Haumea, named for the Hawai’ian goddess of childbirth, is just slightly further out in the Kuiper Belt, likewise shepherded by Neptune. About half the size of China, we know relatively little about what it’s made of. Unlike the other dwarf planets, it isn’t spherical; Haumea seems to be rotating so rapidly about its own axis that it has stretched out into an ellipse. In fact, Haumea’s days last only 3.9 hours, which means that it spins fast enough that if you stood on its equator the centrifugal force from its rotation would almost entirely counteract its force of gravity! It has two moons circling it, so small that not only could you jump out of their gravity, but that you’d have to step carefully to avoid falling off accidentally.

Makemake, named for the fertility god of Easter Island (in part, a result of its having been discovered shortly after Easter of 2005; its original working code-name was “Easterbunny”), lives in the Kuiper belt as well, but is free of Neptune’s shepherding; like the inner asteroid belt, which can move unmolested in a circle, so do these unshepherded Kuiper Belt objects, the “cubewanos.” It orbits the Sun in a 309-year period, and is slightly smaller than Australia. It appears to have no atmosphere at the moment (we had a chance to see it as it passed in front of a star) but it may have one when it’s closer to the Sun. But we don’t know much about it; spectroscopy seems to suggest that it has far less nitrogen than than Pluto, and it could be that during its brief periods of having an atmosphere, its gravity was too weak to hold it in place, and so the nitrogen was ripped from it.

And finally we have distant Eris, largest and farthest of all the dwarf planets. It isn’t part of the Kuiper Belt; it lives in an even more distant region, the Scattered Disc. These objects, having been scattered roughly by the gas giants, are now in distant orbits, never coming closer to the Sun than 30AU, often highly elliptical and at steep angles from the Ecliptic. (The AU, or astronomical unit, is the average distance from the Earth to the Sun, about 100 million miles) No longer shepherded by any of the planets, but not far enough to be entirely free of their reach, they are prone to being scattered yet more. Eris, the only agreed dwarf planet in this region, has an orbit at a nearly 45° angle to the rest of the planets. It is nearly twice as far from the Sun as even Pluto; an Eridian year lasts over 560 Earth years. The brightest that the Sun ever gets in the sky is a dim twilight illumination, but the surface of Eris is better-lit than you would expect from this, because its icy surface is so polished that it is the second-most reflective large object in the Solar System, more reflective than a perfect field of fresh snow. (Its albedo, the fraction of light which it reflects, is 96%; by comparison, the brightest Greenland ever reaches is 80%)

Its lone moon, Dysnomia, contains a secret hint about the argument over the planet’s naming: the official acceptance of a name was delayed by arguments over just what a dwarf planet was, so it became widely known by its working code-name, Xena. Since International Astronomical Union rules require planet names to come from mythology, that name was ruled out, but its moon was then named for the goddess Eris’ daughter, “Lawlessness” – and also for Lucy Lawless, who played Xena in the TV show.

I won’t list all of the other likely dwarf planets, but one interesting one is Sedna, which is even further away than Eris: there is even some argument about whether it should be considered part of the Scattered Disc, or whether it should be considered part of an even more separated group, the Detached Objects. It flies in a wildly elliptical orbit, never coming closer to the Sun than 76AU (nearly twice as far as Pluto) and going as far out as 937AU. Its years last over 11,000 Earth years, and its surface may well be (it’s hard to tell) dark red, as red as Mars. At the farthest point of its orbit, the Sun would only be a dot in the sky, no brighter than a crescent moon on Earth. And Sedna is likely not alone; we suspect that there are another 40 or so similar-sized objects floating around at this great distance. These are the outer shores of our Solar System.

Which brings us to the end of our tour of the Solar System as we now understand it. Our revised list of planets, in order from the Sun, is Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris. If we add in the most likely next candidates, we get Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Orcus, Pluto, Haumea, Quaoar, Makemake, 2007 OR-10, Eris, and Sedna.

I invite you to think up some good mnemonics for these. If you manage to work in “2007 OR-10,” I’ll be particularly impressed.

Further reading:

Your one-stop shop for the minor planets is The Minor Planet Center, the branch of the International Astronomical Union that handles minor planets. They have all the latest catalogues, as well as a whole page of cool animations.

If you want to read more, Wikipedia is a great place to start, especially with this introductory article about the dwarf planets as a whole. The links in the rest of this article will point you at more about the individual planets and regions, and our quest to map our Solar System.