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: Submoon

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:


After my recent post about exomoons and trickster moons, a reader commented asking about moons with moons.  Honestly, I couldn’t think of any reason why that wouldn’t be possible, but I felt like it must be an extremely rare thing. Otherwise we probably would’ve found something like that in our own Solar System by now.

And according to this paper entitled “Can Moons Have Moons?” the answer is yes.  Theoretically, under certain circumstances, a moon could have a very, very tiny moon of its own.

It’s important to note, however, that for an object to truly be considered a moon, its orbit must be stable.  For example, there are multiple objects that are in temporary orbit around Jupiter, but since those objects are not expected to stick around for more than a few years, or maybe a few decades at the most, they are not included in the official count of Jupiter’s moons.

In most cases, a small object caught in orbit around a moon will have a very difficult time maintaining that orbit.  The gravitational attraction of the nearby planet will just keep tugging and tugging, stretching the orbital path into a wider and wider ellipse.  It won’t take long before the moon’s gravity can no longer hold the small object it captured.

But according to that “Can Moons Have Moons?” paper, if a moon is relatively large (like our own Moon) and orbits relatively far away from its host planet (also like our own Moon), and if there aren’t a whole lot of other moons around to make gravitational interactions complicated, then yes: that moon could have a moon in a stable orbit.  A very, very tiny moon.  Something asteroid sized.

The research paper I’m citing proposes calling the moon of a moon a submoon, but that’s not an official scientific term.  Not yet.  It probably won’t be until an actual submoon is discovered somewhere out there.  Until then, other terms have been proposed, like meta-moon, nested moon, grandmoon, and moonmoon.  Moonmoon seems to be the most popular choice on the Internet, probably because of the Internet meme.  Which means when the time comes the I.A.U. will almost certainly not pick that one.  More likely, the I.A.U. will go with “dwarf moon” and insist that no further discussion of the matter shall be permitted.

For right now, I think submoon is the term with the most scientific legitimacy.  For the purposes of Sciency Words and other sciency writings, I think that’s the term to go with.  But what do you think?  What would you call the moon of a moon?

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!