PLANETPLANET

Note: here is a video of me reciting this poem.

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Can moons orbit moons? wondered Juna and I.
Some planets have moons, you know, up in the sky
But none of those moons has its own moon around it.
When Juna’s son learned this he just was astounded!
We wanted to figure it out, solve the mystery:
Where did those moons of moons go? What’s their history?

And one thing that gave this another dimension
The exomoon candidate got our attention.
When Teachey and Kipping found evidence for it
Those submoons called up and we couldn’t ignore it.

“Submoons” or “moonmoons” — now what’s in a name?
A name tends to stick so it shouldn’t be lame.
What should we call them? There’s oodles of choices
And plus, thanks to Twitter, there’s millions of voices.
There’s lots of opinions, there’s: moonmoons, mooncitos,
There’s moonlets and lunettes and planet burritos.

It’s only a name, there’s no science or glory
I’m sticking with submoons. Now, back to our story…

Around every planet there’s sort of a zone
In which a moon’s stable if left all alone
It orbits in peace ’round the planet in charge
And up in its sky, well, that planet looms large.
And ’round every moon there’s a similar space,
A submoon in there should just orbit in place.

Where things can get messy and fall off the table:
It’s tides, it turns out, that can make things unstable
The planet’s large gravity tugs on the moon
And stretches it out like a poodle balloon

When stretched out, its gravity changes a nick
The submoon can feel this and gets a small kick
The kicks push the submoon first to and then fro
Its orbit can either get smaller or grow.
The submoon can crash down upon the moon’s lawn
Or else can be pushed out until it’s just gone

The very best spot for a submoon to thrive
Is ’round a big moon. And to help it survive
The moon also needs to be far from its planet
And that applies whether it’s icy or granite.

There’s three or four moons in our system that work
Around which a nice stable submoon could lurk
There’s our Moon, Callisto and also Iapetus
(a weird moon of Saturn’s — now let’s check my abacus).

If submoons are stable, then where could they be?
Those moons don’t have submoons. No, none of the three.

For Earth’s Moon, we think at the time of its birth
Its orbit was much much much closer to Earth
So even though submoons are stable there now
They never could form. They just didn’t know how.

Callisto is one of the moons Galilean,
There’s four around Jupiter with room to play in.
The gravity kicks from the moons all add up;
The safe zone for submoons just shrivels right up.

Iapetus is kinda weird. Just a smidge.
Along its equator it’s got a long ridge
We think that a submoon did form up around.
The ridge was produced when the submoon crashed down.

The exomoon candidate’s really quite big
With plenty of space for a sweet submoon rig.
The bad thing is submoons are real hard to find
And, to exo-submoons we’re totally blind.

Moons might be friendly to life, up in space.
So what about submoons? Are they a good place?

To have a big submoon that might have tectonics
And don’t forget water (and, yes, gin and tonics)
The host moon must be pretty big and quite far.
It also should orbit a pretty big star.

For big stars the hab zone is farther away
And planets out there give moons more space to play
Tides are much weaker, so submoons can thrive
Even a submoon like Earth might survive!

Let’s not hold back. Let’s see this thing through.
Subsubmoons: could they exist out there too?
The answer is yes but they’d have to be wimpy
‘Cuz tides get so strong that the stable zone’s shrimpy.

And now a last thought: tell me, what should humanity
Do just in case we succumb to insanity?
Where can we stash all the best things we’ve done:
Inventions, discoveries, art by the ton?

A human-made submoon that orbits the Moon
Could hold all that stuff in a giant cocoon.
For billions of years it could tell our last fable
(Although we should make sure its orbit is stable).

And now we are done. So I’ll head off to bed
With visions of submoons afloat in my head….

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Additional resources and information

• Our paper, now expanded and published (official link here).
• My original blog post, Can moons have moons?
• My favorite article written about submoons, written by Phil Plaitt.
• A pioneering paper by Barnes and O’Brien on the stability of moons around planets that are close to their stars, whose calculations we adopted.
• Evidence for a large exomoon orbiting Kepler-1625b, by Alex Teachey and David Kipping
• Follow me on twitter: @sraymond_astro
• Wikipedia now has an entry for subsatellites!
• Here I am reciting the poem: