Over 50 dwarf planets and counting...

The fateful decision

In 2006, the International Astronomical Union (IAU) defined a planet as any object that (1) orbits the Sun directly, (2) is round, and (3) is gravitationally dominant within its orbit. Currently, the only objects in the solar system that make the cut are the eight classical planets—Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. For objects that satisfy the first two conditions, but not the third, the IAU established a new class of objects: dwarf planets. Objects that only satisfy the first requirement, such as asteroids and comets, were given the blanket classification of “small solar system body”—“small body” for short.

The new dwarf planet classification originally included the once-planets Pluto and Ceres, as well the newly discovered Eris, which we now know to be slightly smaller but more massive than Pluto. Within the next few years, two more dwarf planets were added to the mix: Haumea and Makemake.

One of the arguments for creating a separate class for dwarf planets was a practical one: If Pluto was a planet, then similarly-sized Eris would have to be as well. If Eris was a planet, what about an object that was eighty percent the size of Pluto? What about seventy percent? Sixty? Fifty? One solution—and one which was even briefly adopted—was to say that any object that was round and orbited the sun was a planet. The problem with this approach, according to some astronomers, was that this led to a solar system with dozens of planets—at least fifty, by estimates at the time, and possibly hundreds.

Ultimately, the IAU decided to distinguish between gravitationally dominant planets and dwarf planets, in part to allow the number of planets in the solar system to stay constant, while the number of dwarf planets could increase as they were discovered, much as the number of asteroids and other small bodies has done for hundreds of years.

A half-executed plan

The only problem—apart from public outrage at Pluto's “demotion”—was that there were some issues with how to implement the new dwarf planet classification. Because the IAU went ahead and started declaring official dwarf planets, they set themselves up as the official gatekeepers of dwarf planet status. But after starting off with three dwarf planets and later adding two more, something went wrong: They stopped adding new ones. It has now been the better part of a decade since Haumea and Makemake were given official dwarf planet status, and no new dwarf planets have been added since then. Bewilderingly, this isn't because there haven't been more dwarf planets discovered; it almost seems as if they've just moved on to other things.

A reasonable proposal

At the very least, it makes sense to assume that since Ceres is round and therefore a dwarf planet, anything larger than Ceres will be a dwarf planet as well. Simply using this criterion, Quaoar and Sedna, which were both discovered before 2006, and 2007 OR10, which was discovered in 2007—and is likely bigger than both Haumea and Makemake—should all be classified as dwarf planets.

To take things a step further, the best information we have tells us that icy objects in the solar system—like moons of the giant planets and objects out past Neptune—can be assumed to be round if they are larger than about 400 km across. The smallest known round object in the solar system is Saturn's moon Mimas, at 396 km across, but a few larger rocky bodies such as the asteroid Vesta (525 km) and Neptune's moon Proteus (420 km) are known not to be round. (Since the moons mentioned do not orbit the Sun directly, they cannot be considered dwarf planets, and are used here for comparison only.)

To determine the number of objects that currently should be classified as dwarf planets, we need to know how many of them are round. We can assume that Vesta is not round only because it is a rocky body, and rocky bodies require more gravitational force to be made round than icy bodies. (In fact, the best estimates put the lower roundness threshold for rocky bodies at about 800 km—twice the threshold of icy bodies.) Knowing this, we can use a very conservative lower threshold of 500 km to determine which icy objects (i.e., objects beyond Neptune) are likely to be round, and therefore likely to be dwarf planets. This threshold gives us the following results, with currently accepted dwarf planets starred:

Likely and official dwarf planets

From largest to smallest, with official dwarf planets marked with an asterisk (*)

Pluto*	2,374 km
Eris*	2,328 km
2007 OR10	1,535 km
Makemake*	1,447 km
Haumea*	1,403 km
Quaoar	1,046 km
Sedna	1,032 km
Ceres*	946 km
Salacia	883 km
2002 MS4	865 km
Orcus	828 km
2013 FY27	796 km
v774104	750 km
2005 UQ513	728 km
Varuna	721 km
2002 AW197	714 km
2014 UZ224	710 km
2015 RR245	708 km
Varda	694 km
2004 GV9	692 km
2003 AZ84	691 km
2005 RN43	685 km
2006 QH181	683 km
2002 UX25	683 km
Ixion	657 km
2007 JJ43	654 km
2010 KZ39	637 km
2001 UR163	636 km
2007 UK126	608 km
2010 RF43	607 km
Chaos	606 km
2003 UZ413	597 km
2008 ST291	591 km
2012 VP113	588 km
2002 TC302	588 km
2002 XW93	584 km
2005 RM43	580 km
2013 FZ27	575 km
2010 FX86	560 km
2010 RE64	560 km
2002 XV93	555 km
2014 UM33	540 km
2004 XR190	535 km
2003 VS2	522 km
2004 TY364	520 km
2010 VK201	520 km
2008 OG19	512 km
2014 FC69	504 km
2007 JH43	503 km
2003 QX113	503 km
2005 UQ513	500 km
2014 FT71	500 km

Even using this fairly conservative estimate gives us 52 known dwarf planets orbiting beyond Neptune, plus the asteroid dwarf planet Ceres, bringing the count of known dwarf planets up to 53. In addition to these, more dwarf planets are discovered all the time, including a new one announced just a few days ago. (It's 2014 UZ₂₂₄, which at 710 km across is well above our already-conservative threshold.)

Things that surprised me (and might surprise you!) about space

As I've mentioned before, I really like space. Even so, until a few years ago, I really didn't know that much about it. I knew about the nine planets I'd learned about in school (more about that later); I knew the Sun was a star; and I had heard something about scientists discovering some sort of outer asteroid belt out by Pluto. I'd seen some of the headlines about newly discovered would-be planets like Quaoar and Sedna, but I didn't really know much about them.

For some reason, in the last year or two, space has become one of my major interests. It might have something to do with last year (2015) being such a great year for space—after all, we had spacecraft visit two of the five official dwarf planets last year, including poor old Pluto. Like a lot of other people, I decided to learn a little bit more about Pluto's “demotion”, and in the process, I started learning a lot more about the whole solar system. A lot of the things I learned surprised me, and I continue to be surprised by new things I learn, and by new correlations I hadn't put together before.

Here are a few of the things that surprised me the most:

There are moons that are bigger than planets

That's right. When I was learning more about Pluto, I was surprised to find out that Pluto is actually smaller than the Moon. I was even more surprised to find out that our Solar System actually has seven moons that are larger than Pluto. But what surprised me the most was that there are two moons that are actually bigger than the planet Mercury. Jupiter and Saturn each have dozens of moons, and most of those moons are nothing to write home about. But Jupiter's largest moon, Ganymede, and Saturn's largest moon, Titan, are both bigger than the planet Mercury. Ganymede has a thin oxygen atmosphere and Titan has a fully formed atmosphere, clouds, rivers, seas, sand dunes, and a fully functional water cycle (except with methane instead of H₂0). On top of that, Jupiter's second largest moon, Callisto, is only one percent smaller than Mercury. If these moons were orbiting the Sun instead of Jupiter and Saturn, they would be full-blown planets. So would our Moon. Go figure.

Not all planets are created equal

This might seem obvious after the last section, but bear with me. I'm pretty sure that I knew that there were different sizes of planets, but I didn't realize that they were entirely different kinds of planets. What do I mean by this? Well, Earth, Venus Mars and Mercury are all rocky inner planets. They have solid surfaces that you can land a robot on, and maybe even land people on. Jupiter, Saturn, Uranus, and Neptune are all giant planets, and don't have solid surfaces. Even among the giant planets there are two different kinds: Jupiter and Saturn are both gas giants, while many scientists describe Uranus and Neptune as ice giants instead. But none of them have solid surfaces. This means that there's really no way to actually go to Jupiter. You can fly there, you can orbit it, but you can't land on it. It's not that landing there would kill you (even though it would); it's that there is literally nothing to land on. The closest you could get would be landing on one of its (planet-sized) moons.

The reason the giant planets don't have a solid surface is because they are made mostly out of clouds and gas. But what does it mean to say that the rocky planets and the giant planets are all “planets” if half of them have solid surfaces and half of them don't? It really hit me as I was learning more and more about this that my understanding of planets was severely lacking. This, along with the fact that all four of the giant planets have planet-sized moons, really brings home the fact that the giant planets are essentially and fundamentally different from the inner planets. Understanding this helps me to understand how it is the dwarf planets can be thought of as so different from the rest of planets, since the planets themselves are so different from each other.

The giant planets only kind of orbit the Sun

In school we learn that the planets orbit the Sun, and that a long time ago people thought everything orbited the Earth, but later on they realized they were wrong. Well the truth is, they were wrong, but it's still more complicated than simply saying that the planets orbit the Sun. The inner planets definitely orbit the Sun directly, meaning that the center of their orbits is actually inside the Sun. But the outer planets actually orbit a point in space outside of the sun, somewhere between the Sun and Mercury. How can this be? Well, the reality is that Jupiter is so massive that its gravity actually pulls on the Sun, making its own rotation wobble back and forth, similar to the way Pluto and its moon Charon orbit each other. Although the Sun is unquestionably the center of the solar system, the pull of Jupiter makes it so Jupiter and everything beyond it actually orbit an empty point in space, somewhere above the surface of the Sun (referred to as the solar system's barycenter).

In other solar systems there are sometimes two stars that orbit each other, with planets orbiting around both of them together or around each of them individually. When it comes down to it, our solar system is actually more like this than we might think, with the majority of objects orbiting the sun, but a number of objects ranging from tiny asteroids to planet-sized moons orbiting Jupiter and the other giant planets as well.

The asteroid belt is a sparse, lonely place

In just about every space-themed TV show or movie, at some point, the characters end up flying into an asteroid field, where they have to carefully dodge and sometimes even blow up the hundreds of asteroids that are all just rolling around crashing together in space. The reality is that most of the asteroids in the asteroid belt are actually so far apart that if you were standing on one, you wouldn't even be able to see any others without a telescope. Even asteroids whose orbits are relatively close together are hundreds of thousands of kilometers apart, and much of the time, they're at opposite ends of their orbits, so they're even further than that!

On top of all that, asteroids just aren't that big. Asteroids fly by Earth every single day (there's even an official online newspaper with daily asteroid headlines) but most of them are so small that we can only see them with professional telescopes. The biggest asteroid, Ceres, is less than one-tenth the size of the Earth, and just over a fourth the size of the moon. Even if you smashed the entire asteroid belt together in one place, it would still only make up a mass 4% that of the moon; it would actually still be smaller than Pluto.

Over 97% of the objects in the solar system were discovered after the year 2000

When I was born, there were around 10,000 known objects (planets, moons, asteroids, etc.) in the solar system. Because our technology gets better and better every year, we discover more and more solar-system objects every year. By the end of December, 1999, we had more than doubled the number of known objects, with around 20,700 objects discovered at the end of 1999. As of the time of writing, there are over 700,000 known objects in the solar system. (717,768 to be exact. You can actually check the exact current number in the right-side menu on this page; look under Minor Planets Discovered, All Time.) This means that literally, over ninety-seven percent of everything we know about in the solar system was discovered after the year 2000

The take-home

I could say a lot more, but I'll stop there. With new discoveries being made every year, the theories and understanding that were held by the best scientists just a few years ago are already largely obsolete. Just this last year, compelling evidence was presented indicating that there's another full sized planet out beyond Pluto, and possibly even an Earth-like planet orbiting the nearest star. There are undoubtedly more things out there that we haven't even begun to imagine yet, both in our own solar system and beyond.

What space-related facts surprised you? Share in the comments!

The name of the blog

To start off, I'll explain a bit about the name Gwistix. It would be cool if there were some sort of mystical, historical, or scientific meaning to Gwistix, but there's not. When I was an undergrad studying linguistics, I noticed that whenever people abbreviate the word linguistics, they use the first syllable of the word, ling. The classes I took were all listed in the catalog at LING classes, and even the department website used ling rather than the full word. I thought it would be fun to use the other half of the word instead, so I started labeling all of my linguistics-related web-apps with the IPA pronunciation [ˈɡwɪs.tɪks]. I set up a blog on blogger (this one, which I've had on the books for a few years but have never posted on), a Google Sites page (which I've never done anything with) and even ended up buying the domain name gwistix.com, which, for complicated reasons, I still own but no longer have control of. (Maybe I'll explain why some other time in another post.) Since then, I've started using @gwistix as my seldom-used Twitter handle, as well as for StackExchange, Steam, and who knows how many other places, and I've set up a Neocities page at gwistix.neocities.org (which I've put a little bit of time into, but not much).

So there you have it. Gwistix started off as shorthand for linguistics, and started off being about just linguistics. My interests have broadened since those days. After a BA and an MA in linguistics, I certainly have things to say about the scientific study of Language, but that's not all I have to say. I'll probably also frequently post about space, science, technology, and any number of other nerdy things. I also really like data, demographics, and comparing things, which I'll probably do a lot of. I guess we'll see.

First post

So, I've been meaning to start a blog for a while now. I wanted to make it big and grand, with a really cool theme and a bunch of different sections. I wanted to have it all planned out piece by piece and post by post. I even started writing posts in preparation for some day when I would finally make my musings public. And after years of planning and plotting, I've finally decided to just do it. For now, I'm just using a standard template. I don't have a bunch of well-organized categories or sub-blogs. But I do have some ideas of things I want to talk about. And that's really what this blog is about. It's just stuff that I want to talk about; ideas I want to flesh out; things that I've been thinking about. And with that comes a caveat...

This blog is mostly for me. I hope that at least some other people stumble upon it and find a post here or there interesting, but it's really just for me to have a place to put my thoughts down. Hopefully it will make sense, and it might even be interesting or informative, or helpful to someone else, but ultimately, it's for me. It might be for you, too, but that's for you to decide.