Mercury A to Z: Density

Hello, friends!  For this year’s A to Z Challenge, my theme is the planet Mercury.  Mercury may not be the most exciting planet in the Solar System, but he’s interesting in his own way, and I think he deserves a little more love and attention than he usually gets.  In today’s post, D is for:


In recent years, astronomers have discovered literally thousands of exoplanets (planets orbiting stars other than our Sun).  Every once in a while, one of these newly discovered exoplanets will be described as “Mercury-like.”  Now what do you think makes a planet “Mercury-like” in the minds of exoplanet hunters?  Are Mercury-like exoplanets small?  No, not necessarily.  Are they very close to their suns?  Again, not necessarily.  The most Mercury-like quality of a Mercury-like exoplanet is its density.  Mercury is an abnormally dense planet, due to the fact that Mercury has an abnormally large core.

Mercury’s core takes up roughly 85% of the planet’s internal volume.  For the sake of comparison, Earth’s core constitutes only 17% of Earth’s total volume.  For this reason, I sometimes like to call Mercury the avocado planet, because much like the seed inside an avocado, the core of Mercury is shockingly large.

The most likely explanation is that Mercury started out as a much larger planet, perhaps even an Earth-sized planet.  But then, in the very early days of the Solar System, young Mercury collided with another planetary body (in case anyone’s wondering, this would have happened long before the collision that created Caloris Basin).  Most of Mercury was destroyed.  Most of the debris from the collision probably fell into the Sun.  All that’s left today is the planet’s original iron core, buried under a relatively thin skin of rocky material.

So modern day Mercury is almost entirely made of iron, an extremely dense metal—which explains why Mercury is such an extremely dense planet.  The second densest planet in the Solar System, after Earth.

Now I have to level with you: I thought this was going to be one of the easier blog posts to write for this A to Z series, because I thought I already knew basically everything I needed to know about this topic.  But apparently there’s been some new research since the last time I read up about Mercury’s density and internal structure.

Decades ago, scientists assumed that Mercury’s core would be solid.  A planet as small as Mercury surely would have lost all his internal heat by now.  However, Mercury does have a magnetic field.  Planetary magnetic fields are usually caused by liquid metal sloshing around in a planet’s interior; ergo, Mercury must have a liquid core after all.  Right?

But apparently a few years ago (and this is the part I only learned about a few days ago), scientists were looking over gravity data from NASA’s MESSENGER Mission and realized that Mercury’s core cannot be entirely liquid.  Mercury’s core must be part liquid, to explain the magnetic field, but also part solid to explain MESSENGER’s gravity measurements.  So scientists now believe Mercury has a solid inner core surrounded by a liquid outer core.

So that’s a new thing that I have learned, and now it is a thing that you have learned, too.


I’m going to recommend this article from, explaining (in layperson’s terms) how scientists determined that Mercury must have this part liquid/part solid core.

And for anyone interested in the original research, here’s a link to the original research paper about Mercury’s liquid/solid core (I haven’t had a chance to read that paper yet, but I’m looking forward to doing so soon).

I also want to mention this article from, which briefly discusses one of those Mercury-like exoplanets I was talking about in the beginning of this post.  In fact, the exoplanet K2-229b is so Mercury-like that scientists have nicknamed it “Freddy” (get it?—because of the singer Freddy Mercury!).

Sciency Words: Plasma Torus

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


Astronomers have discovered thousands of planets out there.  Exoplanet hunting techniques have gotten so good that astronmers are now moving on to the next great challenge: finding exomoons.  And one possible method for detecting exomoons involves something called a plasma torus.

Ever since the 1960’s, we’ve known something weird was happening with Io, one of the moons of Jupiter.  In 1964, an astronomer by the name of E.K. Bigg determined that Io had some strange power over Jupiter’s magnetosphere.  Subsequent research identified clouds of ionized sulfur and sodium in the vicinity of Io’s orbit.  Then in 1979, NASA’s Voyager 1 space probe photographed Io up close, catching Io in the act of spewing a mix of sulfur compounds and other noxious chemicals into space.

We now know that Io is the most volcanically active object in the Solar System and that Io’s volcanic activity directly affects Jupiter’s magnetic field.  As you can see in this totally legit Hubble image, Io has created a nasty mess around Jupiter.

All those nasty chemicals get swept up in Jupiter’s powerful magnetic field, which acts like a supersized particle accelerator, turning those chemicals into a high-energy plasma.

I can’t be sure who coined the term plasma torus, but a multitude of papers from the 1960’s and 70’s (like this one, or this one, or this one) attempt to model the plasma clouds surrounding Jupiter as a torus—torus being the fancy mathematical term for “donut-shape.”

The nifty thing about Io’s plasma torus is that you can detect it even from a great distance.  Even if you’re too far away to observe Io directly, you can still infer that she’s there based on all those ionized chemicals swirling around Jupiter and the effect those chemicals have on Jupiter’s magnetic field.

So could we find volcanically active exomoons by looking for plasma tori?  According to this paper from The Astrophysical Journal, we sure can—and maybe we already have!  The paper identifies the signatures of possible plasma tori encircling several large exoplanets.

One thing I’m not sure about: when we find a plasma torus, can we be 100% certain it’s caused by an exomoon?  Are there any other natural (or unnatural) phenomena that might cause a plasma torus to form?  I don’t know.

P.S.: Safety warning to any space adventurers who might be reading this.  A plasma torus is a high radiation environment.  Keep your distance!

Sciency Words: Ploonets

Sciency Words: (proper noun) a special series here on Planet Pailly focusing on the definitions and etymologies of science or science-related terms.  Today’s Sciency Word is:


If you’ve ever played Super Planet Crash (cool game, highly recommended, click here), then you know how difficult it is to maintain a stable orbit.  The planets just keep pulling each other this way and that.  It’s gravitational chaos!  Fortunately, Super Planet Crasher doesn’t include moons.  I imagine the game would be way harder if it did.

Recent research (click here) gives us a better idea of what happens to moons that get yanked out of their proper, moonly orbits.  According to computer simulations, many destabilized moons will crash into their planets.  A few will crash into the sun or be hurled out of the solar system entirely.  But a surprisingly large number—almost half of them—will settle into new orbits around their suns, becoming planets in their own right.

The scientists behind this research have proposed a new term for these runaway moons.  They want to call them “ploonets.”  And furthermore, they describe four different kinds of ploonet we might find out there.

  • Outer ploonet: a ploonet orbiting beyond the orbit of its original planet.
  • Inner ploonet: a ploonet orbiting inside the orbit of its original planet.
  • Crossing ploonet: a ploonet that crossed the orbit of its original planet.
  • Nearby ploonet: a ploonet that shares almost the same orbital path as its original planet.

We may even be able to confirm the existence of ploonets in the near future.  All we have to do it look toward so-called “hot Juipters”—Jupiter-like planets that have migrated dangerously close to their suns.  If those computer simulations are correct, hot Jupiters should have shed small, icy ploonets all over the place during their migratory journeys.

I think we can all agree ploonet is an adorable word, but is this actually a useful term for astronomers and astrophysicists?  I’m not sure.  I guess it depends.  How important is it, do you think, to make a distinction between planets that were always planets and planets that used to be moons?

Sciency Words A to Z: Quijote

Welcome to a special A to Z Challenge edition of Sciency Words!  Sciency Words is an ongoing series here on Planet Pailly about the definitions and etymologies of science or science-related terms.  In today’s post, Q is for:


The International Astronomy Union (I.A.U.) still seems to think they were right about the whole Pluto thing.  However, they also seem to realize that they made a mistake in being so very dismissive of public opinion on the matter, and they’ve been trying to do a better job with public outreach since then.

To that end, in 2014 the I.A.U. announced a partnership with Zooniverse, and they enlisted the general public in the process of assigning official names to exoplanets.  As stated in this I.A.U. press release:

For the first time, in response to the public’s increased interest in being part of discoveries in astronomy, the International Astronomy Union (IAU) is organizing a worldwide contest to give popular names to selected exoplanets along with their host stars.

Now the I.A.U. already had a system in place for naming exoplanets, but that system produced “names” like HD 219134g, or KOI-4427b, or PSR 1257+12c.  There are astronomers who can rattle off this alphanumeric gobbledygook with ease, but I have a tough time with it.  As Doctor Who once said about planets: “I’m terribly old-fashioned. I prefer names.”

But of course letting the general public decide these sorts of things doesn’t always go well.  The I.A.U. did not want something like the Boaty McBoatface scenario to happen to some poor planet.

So the official process was that astronomy clubs and non-profit astronomy organizations (i.e.: people who would take this seriously) got to submit names, and then an I.A.U. committee picked the best options and put those up for a vote.

Quijote—as in Don Quijote (or Don Quixote, as it’s spelled in English) of the famous Spanish novel—was one of the winners.  According to Wikipedia, Quijote was initially thought to have a highly eccentric orbit, but after we learned more about the planet, it turned out its orbit was not as eccentric as it first seemed.  I’m not super familiar with the Don Quijote story, but from what I’ve heard the name seems fitting.

In that same I.A.U. naming contest, Quijote’s star got the name Cervantes, in honor of the author of Don Quijote, and all the other known planets in the system were named after other characters from the book.  As for astrobiological interest in Quijote, the planet does lie within Cervantes’ Goldilocks zone; however, Quijote is a gas giant, so it’s E.S.I. score must be quite low.

Still, it’s conceivable that Quijote might have Earth-like moons. So as we continue our quixotic search for alien life, Quijote might not be a bad place to check.

Next time on Sciency Words A to Z, could it be that we really are alone in the universe?

P.S.: Scattered disk object (225088) 2007 OR10 is currently the largest unnamed object in the Solar System.  If you’d like to vote on what the I.A.U. should name it, click here.

P.P.S.: I cast my vote for “Holle,” the only female name on the ballot, because I think we need more female representation in the cosmos.

Exomoons and Trickster Moons

I’ve been looking forward to this for many years now: we’ve discovered thousands of exoplanets out there, and now we may have discovered our very first exomoon!

There are a handful of moons in our own Solar System that may be home to alien life, so if we can start observing and studying exomoons, in addition to exoplanets, that greatly expands the number of places we can search for alien life and greatly increases the chance that we might find something.

However, exomoons may also pose a serious problem for astrobiologists.  You see, one of the things astrobiologists are looking for are planets with atmospheres in a state of “chemical disequilibrium.”  For example, chemicals like oxygen and methane should react with each other and thus remove each other from the atmosphere.  The only way those two chemicals can coexist long term is if some ongoing process (like biological activity) is constantly replenishing them.

But imagine an exoplanet with an oxygen-rich atmosphere and an exomoon with a methane-rich atmosphere.  From here on Earth, that planet-moon system could easily be mistaken for a single exoplanet, with the two separate atmospheres appearing to be one atmosphere in that much coveted state of disequilibrium.

In this paper—a paper which describes its results as “inconvenient, yet unavoidable”—this is referred to as the exomoon false-positive scenario, but I’m calling it the trickster moon problem, because someday some undetected exomoon might trick us into thinking we’ve discovered alien life when we haven’t.

The good news is that we may have already detected one exomoon, so in time we should get better at detecting others.  But as that “inconvenient yet unavoidable” paper warns, it may be decades (at least) before we can reliably tell which exoplanets do or do not have moons.  Until then, fellow space explorers, beware of those trickster moons!

The Girliest Planet

Venus may be the only planet named after a woman, but it is no longer the girliest planet in the known universe.  Scientists recently announced the discovery of the planet GJ 504b.  This is one of the few planets outside our Solar System that astronomers have been able to photograph, and they report that the planet is pink.  Really pink.  Like Barbie Dreamhouse pink.

Scientists have gathered a lot interesting data about the pink planet aside from it’s color, such as it’s distance from its parent star and its temperature.  What hasn’t been widely reported is that the planet Venus hasn’t taken the news well and is super jealous.

Venus and the Pink Planet
The surface temperature of Venus is between 800 and 900 degrees Fahrenheit, whereas GJ 504b’s is estimated to be roughly 460.

For more information on the pink planet, click here.