Sciency Words: Moon

September 29, 2017

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


There are three things I want to cover with today’s post. Firstly, for anyone who may not already know, Earth’s moon is officially called the Moon (with a capital M). Unless you don’t speak English, in which case it’s called whatever it’s called in your language, provided that you treat the word as a proper noun. This according to the International Astronomy Union (I.A.U.), the one and only organization with the authority to name and classify astronomical objects.

Phases of the Moon.

Of course the Moon is not the only moon out there, so I also want to talk a little about the official I.A.U. sanctioned definition of the word moon. Unfortunately there isn’t one, which seems odd given how the I.A.U. are such stickers about their official definition of the word planet.

A common unofficial definition is that a moon is any naturally occurring object orbiting a planet, dwarf planet, or other kind of minor planet (such as an asteroid or comet). Except this definition creates some problems:

Saturn has like a bazillion moons!

Since there’s no lower limit on size or mass, you could consider each and every fleck of ice in Saturn’s rings to be a moon.

The Moon isn’t a moon!

In a very technical sense, the Moon does not orbit the Earth. The Earth and Moon both orbit their combined center of mass, a point called a barycenter. In the case of the Earth-Moon system, the barycenter happens to lie deep inside the Earth, so this distinction may not seem important, but…

Pluto is Charon’s moon, and Charon is Pluto’s!

The barycenter of the Pluto-Charon system is a point in empty space between the two objects. Pluto is the larger of the pair, so we generally consider Charon to be Pluto’s moon; however, you could argue that Pluto and Charon are moons of each other. You could even write a love song about their relationship.

Of course I’m not seriously arguing that Saturn has billions upon billions of moons, nor am I arguing that our own Moon is not really a moon. There does seem to be some ambiguity about Charon’s status (is Charon a moon, or are Pluto and Charon binary dwarf planets?), but I’m not sure if this ambiguity has caused any real confusion in scientific discourse.

Still, as we learn more about moons in our own Solar System and also moons in other star systems, I think the I.A.U. will eventually have to come up with an official definition. And that brings me to the third and final thing I wanted to cover today: exomoons.

An exomoon would be defined as a moon (whatever that is) orbiting a planet or other planetary body outside our Solar System. Finding exoplanets is hard enough, so as you can imagine, searching for exomoons really stretches the limits of current telescope technology. But astronomers are trying, and next month (October, 2017) the Hubble Space Telescope will be making special observations of a planet named Kepler-1625b in an attempt to confirm a possible exomoon detection.

Sciency Words: Xena (An A to Z Challenge Post)

April 28, 2017

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, X is for:


In January of 2005, astronomers at the Palomar Observatory in California discovered a new “planet.” Except this planet had a highly inclined (tilted) and wildly eccentric (non-circular) orbit. Pluto’s modestly eccentric, Neptune-crossing orbit was weird enough, but this? Planets aren’t supposed to have orbits like this, are they?

The Palomar Observatory astronomers decided to name their discovery Xena.

Personally, I think that name fits: a convention defying name for what was, at the time, a convention defying planet. But Xena was only intended to serve as a placeholder until the International Astronomy Union (I.A.U.) could assign an official name, and they chose the name Eris.

In Greek mythology, Eris was the goddess of discord. This name also seems fitting, given the amount of discord that would soon follow, because Eris was officially classified not as a planet but as a dwarf planet, along with Pluto.

There is now a proposal to reclassify Pluto, Eris, and about a hundred other Solar System objects as planets. It’s a proposal I like, for reasons I tried to lay out in a previous post, but it’s not something I expect to go anywhere. Most professional astronomers seem to be against it.

Anyway, the story of Xena/Eris is an example of something that seems to happen a lot in the field of astronomy. New discoveries get temporary names (pop culture references aren’t uncommon here) until the I.A.U. can review the discovery and assign a name officially.

As another example, the team behind NASA’s New Horizons mission came up with a ton of names for geological features on Pluto and its moon, Charon. Many of these names came from Star Trek, Star Wars, Doctor Who, Battlestar Galactica, The Lord of the Rings… apparently there are a ton of nerds at NASA. You can expect the I.A.U. to change most of those names—but perhaps not all of them. Sometimes a pop culture reference gets the I.A.U.’s okay (especially Lord of the Rings references, I’ve noticed).

In the case of Eris, Eris’s moon (originally named Gabrielle) was officially renamed Dysnomia. Dysnomia was the ancient goddess of lawlessness, and Lucy Lawless was the actress who played Xena on T.V. That was apparently an intentional, though rather convoluted, way to honor what could have been Xena: Warrior Dwarf Planet.

Next time on Sciency Words: A to Z, in the beginning there was the Big Bang. Then there was ylem. A whole lot of ylem.

Sciency Words: Planet (An A to Z Challenge Post)

April 19, 2017

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, P is for:


In 2006, the International Astronomy Union approved a new, official definition of planet, and Pluto didn’t make the cut. Word has it Pluto took the news well.

The I.A.U.’s concern at the time was that more and more small, Pluto-like objects were being discovered, making Pluto seem less like the ninth planet and more like the first of some new class of thing.

To be fair, the I.A.U. did try to come up with a planet definition that would include Pluto while excluding the dozens or perhaps hundreds of other objects potentially out there. But it just didn’t work out.

So to meet the official, I.A.U. sanctioned definition, an astronomical body must meet three requirements:

  • It must orbit the Sun.
  • It must be spherical, due to the pull of its own gravity.
  • It must have cleared its orbital path of debris (this is the part of the planet test that Pluto failed).

Of course, if a definition can be changed once, it can be changed again. Recently, a group of six NASA scientists—specifically, six scientists from NASA’s New Horizons mission to Pluto—put forward a new proposal, which reads:

  • A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

In other words, if it’s round, and it’s not a star or wasn’t a star at some point in the past, then it’s a planet. Under this new definition, Pluto’s back in the planet club! And so is the Moon, weirdly enough, along with many other moons elsewhere in the Solar System. In fact, the new definition would reclassify over one hundred Solar System objects as planets—possibly more than that.

The next I.A.U. general assembly meeting will be held in August, 2018. If they’re going to change the definition of planet again, that’s when they’ll do it. But I very much doubt it’ll happen.

Even though this is probably a lost cause, I want to say something in defense of the New Horizons team’s proposal. The strongest objection seems to be that moons should not be planets. I get that, but in my mind any world that I can picture myself standing on or walking on… I don’t know, that just feels planet-y to me.

I frequently catch myself calling Titan and Europa planets, even though they’re moons. Same for Pluto, Eris, Ceres, and all the other objects currently in the dwarf planet category. And I can’t help myself, but I keep calling Endor from Star Wars a planet, even though it’s specifically referred to multiple times in dialogue as a “forest moon.” All of these places—even fictional moons like Endor—feel planet-y to me.

And yes, even the Moon—the most quintessential moon of them all—has a certain planet-esque quality to it when I imagine myself living there, walking around, going about my daily business. I could get used to the Moon being a planet.

Next time on Sciency Words: A to Z, we’ll shrink from planet-scale to the scale of subatomic particles, and we’ll find out what’s so quantum about quantum mechanics.

Sciency Words: I.A.U. (An A to Z Challenge Post)

April 11, 2017

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, I is for:


Yesterday I mentioned the International Commission on Stratigraphy (I.C.S.), the organization that assigns names to the geological strata of our planet. When you really get into this sciency words stuff, it seems like just about every single field of scientific research has its own international commission or union or organization like the I.C.S.

Which brings us to the International Astronomy Union or I.A.U. I’m willing to bet you already know about this organization, even if you didn’t know its name. They did something that made them very famous. Or perhaps I should call them infamous. And what did the I.A.U. do to become so infamous?

Yup. They’re the people who decided Pluto isn’t a planet.

Also, if you’ve ever wanted to name a star after your girlfriend or boyfriend or most beloved pet, the I.A.U. would like you to know that you’re not allowed to do that. Sorry. (They’re not actually sorry.)

Okay, it’s easy (and fun) to get mad at the I.A.U. over Pluto, and I know it’s disappointing to find out your thoughtful star name gift isn’t valid. I’ve known people to get pretty upset about that star name thing.

But according to the I.A.U.’s website, their goal is to establish “unambiguous astronomical nomenclature” for use in scientific literature. That means assigning official names to astronomical objects and writing official definitions for terms astronomers use, so as to avoid confusion or miscommunications in scientific discourse.

While I’m not exactly a big fan of the I.A.U., I do get where they’re coming from. Having dozens of stars named Jessica or Mary or Bobby would create a lot of confusion. And as for that matter with Pluto… we’ll come back to that later this month.

Next time on Sciency Words: A to Z, we’ll say hello to NASA’s Juno mission.

Sciency Words: Frost Line

December 23, 2016

Welcome to a very special holiday edition of Sciency Words! Today’s science or science-related term is:


When a new star is forming, it’s typically surrounded by a swirling cloud of dust and gas called an accretion disk. Heat radiating from the baby star plus heat trapped in the disk itself vaporizes water and other volatile chemicals, which are then swept off into space by the solar wind.

But as you move farther away from the star, the temperature of the accretion disk tends to drop. Eventually, you reach a point where it’s cold enough for water to remain in its solid ice form. This is known as the frost line (or snow line, or ice line, or frost boundary).

Of course not all volatiles freeze or vaporize at the same temperature. When necessary, science writers will specify which frost line (or lines) they’re talking about. For example, a distinction might be made between the water frost line versus the nitrogen frost line versus the methane frost line, etc. But in general, if you see the term frost line by itself without any specifiers, I think you can safely assume it’s the water frost line.

Even though our Sun’s accretion disk is long gone, the frost line still loosely marks the boundary between the warmth of the inner Solar System and the coldness of the outer Solar System. The line is smack-dab in the middle of the asteroid belt, and it’s been observed that main belt asteroids tend to be rockier or icier depending on which side of the line they’re on.

It was easier for giant planets like Jupiter and Saturn to form beyond the frost line, since they had so much more solid matter to work with. And icy objects like Europa, Titan, and Pluto—places so cold that water is basically a kind of rock—only exist as they do because they formed beyond the frost line. This has led to the old saying:


Okay, maybe that’s not an old saying, but I really wanted this to be a holiday-themed post.