Will the Moon Become a Ploonet?

You may have heard that the Moon is slowly moving away from the Earth.  Following up on last week’s episode of Sciency Words, does this mean the Moon will one day become a ploonet: a moon that’s escaped its original orbit and become a planet in its own right?

Currently, the Moon is receding from the Earth at a rate of approximately 4 centimeters per year.  Simultaneously, and not by coincidence, Earth’s rotation rate is slowing down.  The exact reasons for this are, I admit, too math-heavy for my artistic/writerly brain to comprehend, but it has something to do with tidal forces and the exchange of angular momentum.

As explained in this article from Universe Today:

The same tidal forces that cause tides on Earth are slowing down Earth’s rotation bit by bit.  And the Moon is continuing to drift away a few centimeters a year to compensate.

And as further explained in this article from Futurism:

As is true of many rocky relationships, the Earth and Moon only need a bit of time and space to work things out.  Ultimately, we just need to be patient.  In about 50 billion years, the Moon will stop moving away from us and settle into a nice, stable orbit.

So in the very, very, very distant future, assuming the expansion of the Sun doesn’t destroy us first, Earth and the Moon will achieve a new balance.  Earth’s day will be considerably longer, and the Moon will be considerably farther away.  Also, just as the same side of the Moon always faces the Earth, the same side of Earth will always face the Moon.

But the Moon will still be a moon.  It will not become a ploonet.

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?