We're going to need a new definition of what is and isn't a planet… Again.

Many people were understandably shaken when the International Astronomical Union rocked the world—err, solar system—by reclassifying Pluto as a dwarf planet in 2006. There were all sorts of opinions about it, even among scientists, but ultimately what the scientific community all agreed on was that our understanding of planets had changed drastically in the nearly eighty years since Pluto had been discovered, and our classifications needed to reflect that.

Some pushed for classifications that would include Pluto as a planet, while others (perhaps tongue-in-cheek) argued that worlds like Earth, Mercury, Venus, and Mars, aren't really planets either. The long and short of it is that any informed opinion recognizes that the concept of a planet is, in the mind of the public at least, very loosely constructed, and largely misinformed.

We now understand that there are multiple types of planet, including rocky inner planets, gas giants, ice giants, and—depending on how you classify things—dwarf planets. Beginning with the most familiar, we have Earth, a rocky, inner planet, and the only place in the universe confirmed to sustain life. Saying that the Earth is a rocky planet really just means that it has a solid surface to land on. Perhaps surprisingly for some, this isn't a fact that is common to all planets: Mercury, Venus, and Mars also have solid surfaces, as do Pluto, Eris, and the other dwarf planets, but the giant planets Jupiter, Saturn, Uranus, and Neptune do not.

In fact, the closest you could come to landing on any of the giant planets would be to land on one of their planet-sized moons. Jupiter's moon Ganymede is larger than the planet Mercury, as is Saturn's moon Titan, which has clouds, rivers, seas, sand dunes, and a fully formed atmosphere. All said, there are over a dozen moons in the solar system—including our own—that are large enough to be either planets or dwarf planets, and all of them have solid surfaces. (Two of them are larger than Mercury, a total of seven are larger than Pluto, and in all sixteen are larger than the dwarf planet Ceres.)

Planet-sized moons* in the Solar System:

Earth	1	“Luna”¹
Jupiter	4	Europa¹, Ganymede², Io¹, Callisto¹
Saturn	5	Tethys, Dione, Rhea, Titan², Iapetus
Uranus	4	Ariel, Umbriel, Titania, Oberon
Neptune	1	Triton¹
Pluto	1	Charon
Total	16

* All listed moons are larger than the dwarf planet Ceres
¹ Larger than dwarf planet Pluto
² Larger than planet Mercury

In many ways, these planet-sized moons fit the common idea of what a planet is better than the giant planets do. They are all large enough to be round. They have solid surfaces that we can land on. They have enough gravity to keep us from flying off into space, but not enough to kill us. A few of them even have vast oceans of liquid water, which would allow us to build underwater colonies, and perhaps even find life outside of our home world.

For a bit of contrast, Jupiter is made almost entirely of thick clouds of poisonous gas. At some point, the gas may transition to liquid, but the pressure and gravitational forces of the giant planet would crush any spacecraft long before it got that far. On top of that, it produces extreme levels of radiation, so much so that Jupiter actually puts out more heat than it gets from the Sun, scorching the faces of its nearby moons. Jupiter's gravitational pull is so strong that from nearly half a billion miles away (about 780 million kilometers), it pulls on the Sun itself, causing the gravitational center of the solar system to actually be an empty point in space somewhere between the Sun and Mercury. On top of this, Jupiter has 67 known moons orbiting around it—almost like its own mini solar system—complete with four that are planet-sized. The stories for the other giant planets are similar, with the key take-home message being that these are places to orbit around, not places to visit and land on. In this way, although they are clearly not stars, they are in some ways much more like stars than like the concept of planets that most people have.

In the coming decades and centuries, as we venture out further into the solar system and begin more in-depth exploration—and even colonization—of the planet-sized moons, these contrasts between the giant planets and the rest of the worlds out there will become even more apparent. As such, I propose that in the coming centuries, we will likely again adopt a new definition of a planet: A round object with a solid surface that humans can land on.

This new definition would bring the number of (known) planets up to around seventy, including four rocky planets³; one asteroid planet⁴; eighteen satellite planets⁵; and at least 50 icy planets out past Neptune⁶. Part of the debate at the time of Pluto's reclassification centered on the idea that allowing dwarf planets to be called planets would lead to an unwieldy number of planets—too many for school children to learn. Frankly, this argument is absurd. (Continued below.)

³ In order from the Sun: Mercury, Venus, Earth, and Mars

⁴ Ceres, currently classified as a dwarf planet

⁵ Earth's moon “Luna”; Jupiter's moons Europa, Ganymede, Io, and Callisto; Saturn's moons Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus; Uranus's moons Miranda, Ariel, Umbriel, Titania, and Oberon; Neptune's moon Triton; and Pluto's orbital companion Charon

⁶ This includes the currently official dwarf planets Pluto, Haumea, Makemake, and Eris, along with OR₁₀, Quaoar, Sedna, and many others.

With over seventy planets, the difficulty of learning them all could indeed become an issue, but as we come to explore and colonize these worlds, they will each develop their own unique histories and cultures, much like the countries of our own world, of which there are many more than seventy. Just as children learn a few facts about major countries and regions of the world today in their geography classes, they would learn the principle facts about the major planets and groupings of planets in their classes on solar system “geography”. Planetary populations—such as the planets orbiting Jupiter, or those orbiting Saturn or those orbiting within the Kuiper belt—would become the new continents, and interplanetary and other regional alliances would be studied and understood just as well as other countries, regions, and continents are understood today.

The principle “continental” regions of the solar system:

• Inner system
  • Mercury
  • Venus
  • Earth & Luna
  • Mars
  • Near-Earth objects
• Asteroid belt
• Ceres
• Vesta, Pallas, and the other asteroids
• Jupiter system
  • Europa
  • Ganymede
  • Io
  • Callisto
  • minor moons and trojans of Jupiter
• Saturn system
  • Mimas
  • Enceladus
  • Tethys
  • Dione
  • Rhea
  • Titan
  • Iapetus
  • rings and minor moons of Saturn
• Uranus system
  • Miranda
  • Ariel
  • Umbriel
  • Titania
  • Oberon
  • rings, minor moons, and trojans of Uranus
• Neptune system
  • Triton
  • minor moons and trojans of Neptune
• Outer system
  • Pluto & Charon
  • Haumea
  • Quaoar
  • Makemake
  • OR₁₀
  • Eris & Dysnomia
  • Sedna
  • many others

To be perfectly clear, I'm not proposing a new definition of planet for right now, or for a decade or even a century from now. I'm simply making the observation that as we step out into the solar system, our current idea of what is and isn't a planet will likely naturally shift as we gain specific experience with the various worlds of the solar system. As this happens, it will become more and more clear that the giant planets are more things to orbit around than things to land on, and as such, it is likely that we will begin to think of them as essentially different from the solid bodies of the solar system.

The Hebrew Alphabet is a Mess

For the past week or so, I've been learning modern Hebrew on Duolingo, just for fun. Hebrew is a fascinating language, and an amazing example of a nearly-dead language taking on new life to become a flourishing part of everyday life for millions of people. But as my title already says, the Hebrew alphabet is a mess.

The Hebrew alphabet—or alef‑bet—officially has 22 letters, but five of these letters have separate forms at the end of a word. If that were all, it might be OK. But that's not all—not by a long shot. First of all, vowels aren't written in Hebrew, so when you see a word for the first time, even if you know the consonants, there's no way to know what the vowels are, which can completely change the meaning of a word. And knowing the consonants isn't a trivial matter either!

Letters with more than one sound

There are six consonant letters that can each make more than one sound. So bet sometimes makes a b sound and sometimes a v sound; he is sometimes pronounced like an English h and is sometimes not pronounced at all; vav is sometimes pronounced as a v, sometimes as an o, and sometimes as u; yod can either be the y sound or the i sound; kaf can either make a k sound or a kh sound (like Scottish ch in ‘loch’); pe makes either a p or f sound; and shin/sin is either pronounced sh or s. Of course, it's possible to use special dots on the letters to show the exact pronunciation, but this is only common in dictionaries and very basic children's books. This means that as an adult learning Hebrew, when you see a new word, you really have to guess how to pronounce it. For example, seeing the word שכב (shin/sin-kaf-bet) for the first time, there's absolutely no way to know whether it's pronounced sakab, sakav, sakhav, sakhab, shakab, shakav, shakhav, or shakhab. (It's shakhav.) And once again, that's assuming you know the vowels as well. There are some patterns—like that pe, bet, and kaf are more likely to be pronounced as p, b, and k at the start of a word and as f, v, and kh everywhere else—but there are no guarantees!

Letter	Sound(s)
א	(silent)
ב	b or v
ג	g
ד	d
ה	h
ו	v, o, or u
ז	z
ח	kh
ט	t
י	y or i
כ, ך	k or kh
ל	l
מ, ם	m
נ, ן	n
ס	s
ע	(silent)
פ, ף	p or f
צ, ץ	ts
ק	k
ר	r
ש	sh or s
ת	t

Sounds with more than one letter

On top of all this, when you hear a new word for the first time, there's often no way to tell how it should be spelled. It seems like with twenty-two letters, six of which have multiple pronunciations, there should be around twenty-eight different consonant sounds in Hebrew, right? Well, actually there are only twenty consonant sounds. This is because seven of the consonant sounds of Hebrew can be written in multiple ways, so even with multiple pronunciations for some letters, there are still fewer sounds than letters. The v sound can either be written with bet or with vav; the kh sound is either kaf or khet; the t sound is either tet or tav; the k sound is either kaf or kof; the s sound is either samekh or sin; and to top it all off, in spoken Israeli Hebrew, there are two letters (alef and ayn) that are always silent and one (he) that is usually silent at the end of a word, and sometimes silent at the start of the a word. This means that if someone tells you the word for cake in Hebrew is ooga, there's no way to know whether it's אוגה, אוגא, אוגע, עוגה, עוגא, or עוגע. (It's עוגה.) Even for a seemingly straightforward word like katav (“he wrote”), there are eight possible spellings (כתב, כתו, כטב, כטו, קתב, קתו, קטב,  or קטו—the right one is כתב)!

Sound	Letter(s)
(silent)	א, ע, (ה)
b	ב
v	ב, ו
g	ג
d	ד
h	ה
z	ז
kh	ח, כך
t	ט, ת
y	י
k	כך, ק
l	ל
m	מם
n	נן
s	ס, ש
p	פף
f	פף
ts	צץ
r	ר
sh	ש

The vowels are even worse

Of course, these lists don't even include the vowels. They're rarely written, but when they are, they have the same problem as the consonants. By most accounts, there are only five vowel sounds in modern Israeli Hebrew: a, e, i, o, and u, which are pronounced the same as in most European languages. That seems fairly straightforward, except that for those five vowels, there are fourteen different sets of niqqud (the little dots used in children's books and dictionaries). So when you hear the a sound, it could be אַ or אָ or אֲ or אֳ (all of these examples are given with alef as the base letter); when you hear an e sound, it could be אֵ or אֶ or אֱ or אְ; and the i, o, and u sounds can all be written either with or without a helping consonant (yod for i and vav for o and u). In older dialects of Hebrew, all these different symbols made different sounds—some were long and others were short, one a was pronounced further back than the other, etc.—but in modern Israeli Hebrew, there's really no way to tell which of the four a sounds someone is actually pronouncing, or whether or not to write a vav for an o sound. Because of this, hearing a word like zot for the first time, there's no way to know whether the o is just written as a dot (i.e., usually not written at all) or written with a vav. (In this case, it's actually not written at all, and there's a silent alef where the 'vav' would be: זאת [zayin-alef-tav].) A word with multiple vowel sounds ends up having multiple possible spellings, like lehitraot, which has sixty-four (!) different potential spellings, depending on the different versions of e, i, a, and o used. (And that's assuming you already know the right consonants to use!)

64 possible ways to write lehitra'ot:
לֵהִיתרָאוֹת, לֵהִיתרָאֹת, לֵהִיתרַואֹת, לֵהִיתרַאֹת, לֵהִיתרֲאוֹת, לֵהִיתרֲאֹת, לֵהִיתרֳאוֹת, לֵהִיתרֳאֹת, לֵהִתרָאוֹת, לֵהִתרָאֹת, לֵהִתרַאוֹת, לֵהִתרַאֹת, לֵהִתרֲאוֹת, לֵהִתרֲאֹת, לֵהִתרֳאוֹת, לֵהִתרֳאֹת, לֶהִיתרָאוֹת, לֶהִיתרָאֹת, לֶהִיתרַאוֹת, לֶהִיתרַאֹת, לֶהִיתרֲאוֹת, לֶהִיתרֲאֹת, לֶהִיתרֳאוֹת, לֶהִיתרֳאֹת, לֶהִתרָאוֹת, לֶהִתרָאֹת, לֶהִתרַאוֹת, לֶהִתרַאֹת, לֶהִתרֲאוֹת, לֶהִתרֲאֹת, לֶהִתרֳאוֹת, לֶהִתרֳאֹת, ל`הִיתרָאוֹת, ל`הִיתרָאֹת, ל`הִיתרַאוֹת, ל`הִיתרַאֹת, ל`הִיתרֲאוֹת, ל`הִיתרֲאֹת, ל`הִיתרֳאוֹת, ל`הִיתרֳאֹת, ל`הִתרָאוֹת, ל`הִתרָאֹת, ל`הִתרַאוֹת, ל`הִתרַאֹת, ל`הִתרֲאוֹת, ל`הִתרֲאֹת, ל`הִתרֳאוֹת, ל`הִתרֳאֹת, לֱהִיתרָאוֹת, לֱהִיתרָאֹת, לֱהִיתרַאוֹת, לֱהִיתרַאֹת, לֱהִיתרֲאוֹת, לֱהִיתרֲאֹת, לֱהִיתרֳאוֹת, לֱהִיתרֳאֹת, לֱהִתרָאוֹת, לֱהִתרָאֹת, לֱהִתרַאוֹת, לֱהִתרַאֹת, לֱהִתרֲאוֹת, לֱהִתרֲאֹת, לֱהִתרֳאוֹת, לֱהִתרֳאֹת

Vowel	niqqud, etc.
a	אַ, אָ, אֲ, אֳ
e	אֵ, אֶ, אְ, אֱ
i	אִ, יִ
o	אֹ, וֹ
u	אֻ, וּ

Time for a reform?

When you add the vowels' ambiguity to the consonants' ambiguity, you end up with even more possible spellings for any new word you hear. The reformist in me wishes we could just pare it down to one sound per letter, and one letter per sound. So, for example, instead of bet sometimes being pronounced as b and sometimes as v, and vav also sometimes being pronounced v, bet would always be pronounced b and vav would always be pronounced v. As long as we're thinking about changing things, there's really no reason for five letters to have different word-final forms while the other letters don't, so we could get rid of the word-final forms, too. This would give us exactly twenty consonant sounds and twenty letters:

Sound	Letter(s)
(silent)	א
b	ב
v	ו
g	ג
d	ד
h	ה
z	ז
kh	ח
t	ט
y	י
k	ק
l	ל
m	מ
n	נ
s	ס
p	פ
f	ף
ts	צ
r	ר
sh	ש

If we did this, we could keep the word-final form of pe for the f sound. This is the only alternate sound that can't be written with a separate letter, so keeping the final form would be a reasonable thing to do. Also worth noting is that kaf, ayn, and tav would be removed completely, since their sounds are covered by other letters (kof, alef and tet).

This all seems well and good. If we did this, a word like shakhav could be unambiguously written as שחו (shin-khet-vav), and a person who's never seen the word before would know exactly how to pronounce it. (Well, the consonants, anyway.) But our spelling reform would have some unfortunate unintended consequences. Just like English, Hebrew's weird spelling system actually has one benefit: it makes it easier to see the relationships between words. A good example of this in English is the word electric and the related word electricity. If we got rid of the letter C and just used k or s instead, we could spell these words elektrik and elektrisity, making the pronunciation more closely tied to the spelling, but also making it ever so slightly less obvious that these words are related. This is the case even more in Hebrew, where words like pa'al and nif'al are less obviously related when the p and the f are completely separate letters.

A compromise

For this reason, along with a host of others, Hebrew speakers haven't implemented any major alphabetic reforms. But they have introduced a few conventions that make it ever so slightly less impossible to figure out the correlations between spelling and pronunciation. One of these conventions—using yod for the I sound and vav for the o and u sounds—actually dates back centuries. Without it, a word like ooga (which means ‘cake’, from a few paragraphs back) would simply be written עגה (ayn-gimel-he), and we would have absolutely no idea what the vowels were. With the vav, we can at least narrow the first vowel down to one of two possibilities. Along with this, when vav does make a v sound instead of a vowel sound, it's often written as a double vav, as in the word barvaz (ברווז) ‘duck’, which, without the double vav could be confused for barooz or baroz. Similarly, when yod makes an ay or ey sound, it's often written as a double yod, as in tsaharaym (צהריים) 'noon'.

While these conventions don't solve the problem—it still feels very much like the Hebrew alphabet was designed for a completely different language—they help a little. And short of convincing every Hebrew speaker everywhere to change the way they do things, I don't think there's much I can do other than just view reading and writing in Hebrew as a special sort of challenge. After all, if I can figure this out, that's saying something, right?

Meters, Feet, and the Speed of Light

A first proposal

The speed of light is currently defined as exactly 299,792,458 meters per second. A better way of saying this is that we define the length of a meter by how far light travels in one second: One meter is defined as being exactly 1/299,792,458 the distance light travels in one second. The idea of defining our units of measure based on actual physical properties of the universe is a good one, and clearly much better than using variable-sized objects like body parts or other things. What bothers me though, is that as long as they were officially redefining what a meter is, why didn't they just use the much more round number of 300,000,000? This would be infinitely easier to remember, doing so would make a lot of sense, and not much would actually have to change.

The new meter (exactly 1/300,000,000th the speed of light) would be almost exactly the same length as the current meter—0.99930819333 current meters, in fact. For daily use, almost nothing would have to change. A standard meter stick used today would still be accurate enough for all basic measurements, as the difference of less than one current millimeter would be imperceptible for most individuals. Larger scales at which the small difference would actually matter would be calculated by computers, as they already are today, and individual people really wouldn't be affected by the change at all in their daily lives. With a new kilometer equal to exactly 1/300,000 the speed of light, driving 100 km/h in new kilometers would be the same as driving 99.930819333 kilometers in the current system, meaning that for the most part, current speedometers and road signs would not need to be changed. Although most maps and atlases would need to be changed, the use of digital maps by a growing number of individuals and businesses would make the transition mostly painless.

Another option

Another option is to use a unit of measure that's even easier to remember. Why use a unit equal to 1/300,000,000 the speed of light when we could create a new unit equal to exactly 1 billionth the speed of light? This would be exactly 0.299792458 current meters, or about 0.3 meters. If you're anything like me, you'll realize right away that there already is a unit of measure that's close to 0.3 meters—it's a foot, currently defined as exactly 0.3048 meters. In fact, one foot is almost exactly equal to the distance light travels in one nanosecond. Why not redefine the foot so that it is exactly equal to the distance light travels in one nanosecond? A new foot—which we could call a “light-foot” and abbreviate as 𝓁𝒻—would equal exactly 0.98387105643 current feet.

Of course, the main objection most people have to the foot as a unit of measure has nothing to do with the foot itself, and everything to do with other related units of measure, such as the inch (¹⁄₁₂ of a foot) and the mile (5,280 feet). In comparison with the metric system, in which a meter is subdivided into centimeters (¹⁄₁₀₀ of a meter) and multiplied into kilometers (1,000 meters), the use of feet, inches, and miles seems ridiculous. For this reason, the “light foot” could be defined as a decimal unit of measure, meaning that it would be subdivided by a unit equal to exactly ¹⁄₁₀ of a light-foot: the light-inch. The current inch is equal to 2.54 centimeters. The light-inch would be only slightly larger at 2.99883898 cm. In metric units, I am 187 cm tall. In current inches and feet, I am 6 feet, 1⅝ inches tall—I usually just say I'm 6′2″. In light-feet and inches, I would be exactly 62.3574660884 light-inches tall, or around 6 light-feet, 2.35 light-inches—I could still tell people I'm 6′2″. Because a light-foot is slightly smaller than a current foot, but ¹⁄₁₀ of a foot is slightly larger than ¹⁄₁₂ of a foot, once again, most daily measurements wouldn't change noticeably. A standard 10-inch light-foot ruler would be almost the same length as a current 12-inch ruler. But this is as far as the seemingly good idea goes.

Along with changing from duodecimal to decimal inches as a subdivision of the light-foot, it wouldn't make sense not to change larger measurements as well. These changes would be more drastic and more difficult to implement. Instead of using a mile as a larger unit of measure, we would probably want to use a unit equal to 1000 𝓁𝒻 (one kilolightfoot). There are currently no common comparable units of measure. A kilolightfoot would be equal to 0.18633921523 current miles (not quite ⅕ of a mile) or 299.792458 current meters (not quite ⅓ of a kilometer). Using 10,000 𝓁𝒻 instead (one myrialightfoot?) would give a unit equal to 1.86339215233 current miles, or almost exactly 3 current km (2.99883897999937 km—a difference of 1.16 meters). While these distances seem more reasonable, either multiple of a light-foot would require changes to all existing road signs and vehicle speedometers, as well as all maps and atlases. Because of this, although the coincidence that light travels approximately one foot in one nanosecond is interesting, the idea of using a decimal light-foot as a basic unit of measure would cause many more problems than it would solve.

The Astronomical Unit as a unit of measure

A third possibility is that we could define the meter using some other natural property of the universe, such as the astronomical unit (AU—the average distance from the sun to the Earth), which is currently defined as exactly 149,597,871 km. If we instead defined one kilometer as equal to exactly 1/150,000,000 the average distance from the sun to the Earth, a new meter would once again be nearly the same size as a current meter—0.99731814 current meters, to be exact. Once again, a standard meter stick would have only a negligible difference—less than three millimeters—meaning that for most purposes, the old meter stick could still be used. Differences at larger scales would be more pronounced, so driving 100 new km/h would be equivalent to driving 99.731814 current km/h, but even these differences wouldn't be enough to necessarily warrant a change in speedometers or road signs.

The AU-mile?

Of course, we could go the other way, and define the mile using the AU, which is currently equivalent to 9,295,581 miles. Adjusting the length of the mile so that 1 AU would equal 10,000,000 miles instead would make each mile about seven percent shorter than it currently is, but this would also face the problem of deciding what to do with feet and inches. If the AU-mile were used as the standard unit of measure, a subdivision equal to 1/1000th of an AU-mile could be used as a smaller measure, and would be equal to 4.90818624014 current feet or 1.63606208005 current yards. A unit equal to 1/10,000th of an AU-mile would be equal to about half of a current foot, 5.88982348817 current inches, or about 15 current centimeters (14.9601516599). In any case, these new measurements would necessitate changes in all aspects of measurement, since even mile markings on road signs and speedometers would need to be changed. Once again, using miles and feet does not seem like a good idea.

What about light-years?

While it might seem tempting to use the light-year as a basis for units of measure, it doesn't lend itself as easily to nice round numbers. One light-year is about 9.5 trillion kilometers (9,460,730,474,334). Redefining the kilometer so that one light-year would equal exactly 10 trillion kilometers would result in a new meter equal to about 0.946 current meters. This difference, though still not enormous, is large enough that all current measurements would have to be changed, including meter sticks, speedometers, road signs, maps, and atlases. The disparity, even for everyday use, would be noticeable to even individuals in non-technical fields.

Redefining miles based on light-years would make even less sense. In current measurements, one light-year equals 5,878,625,374,273 miles, which doesn't adjust nicely to any multiple of ten. Adjusting the definition of a mile so that one light-year would equal exactly 5 trillion miles would cause similar problems to a light-year-based meter, resulting in a large enough disparity that all existing measurements would have to be changed.

My final proposal

Ruling out the two proposals for light- and AU-based imperial units, we're left with the two proposals for metric units. While either would be a reasonable change requiring minimal difference in everyday life, the light-based system is demonstrably superior. To begin with, the AU is currently defined as the average distance from the sun to the Earth. Because the Earth's orbit is not a perfect circle, the actual distance varies from around 0.983 AU to 1.016 AU—a total potential difference of nearly 5 million current km (4,936,729.743 ) between the two extremes. This means that although we have set a defined length for the AU, it isn't easily verifiable, and it's almost never the actual distance between the sun and the Earth. In addition to this, the possibility that humankind will expand beyond Earth in the coming centuries makes any geocentric measurement system non-ideal. In contrast, the speed of light is a universal constant, measurable from anywhere in the universe. Because of this, I propose the light-based meter, defined as being exactly equal to 1/300,000,000 the speed of light, as a new international standard of measure.

TL;DR
One meter is currently defined as exactly 1/299,792,458 the distance light travels in one second. Changing this to exactly 1/300,000,000 instead would make a lot of sense and wouldn't even require us to change our current meter sticks, speedometers, or road signs. We should do it.

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.