Bombardment of 7 orbs

This poem is based on my new paper just published in Nature Astronomy (paper here).

Look up at the Moon. Do you see those round cracks?
They’re craters resulting from massive impacts.
An object from space sped in and crashed down
Exploded and dug up a hole in the ground!

Just like the Moon, space junk hits Earth too.
Yup, Earth has some craters but only a few.
The reason: our craters are covered up fast
Tectonics erase the impacts from our past.

Photograph of the Moltke crater taken by Apollo 10. From the Apollo archive via Wikimedia commons.

We love exoplanets, we’d love to compare
What kind of collisions on planets out there?
The challenge is huge, it couldn’t be greater
We have zero data – we can’t see a crater!

Now don’t give up hope. We found a neat trick
A workaround method that’s awfully slick
It only applies to just the right case
But where should we look? I know just the place.

Remember those planets that made such a splash
All over the world you could hear the plates crash?
The subject of poems and stories galore
The Trappist-1 system! Well guess what? There’s more!

Yes, this is the system – our method’s best chance.
The planets are trapped in a resonant dance.
Neighboring orbits are all in alignment
(Like dancers, each given a special assignment)

Take any two planets, let’s say d and e
When e completes two orbits, d has done three.
They meet up again at the very same phase
This is the resonance (that is the phrase).

Artist’s illustration of the planets in the Trappist-1 system (credit: NASA/JPL-Caltech). Resonances between each pair of planets are labeled.

When planets are hit by stuff floating in space
It crashes down, knocks them a bit out of place
If too many rocks hit a planet by chance
It breaks up the system’s whole resonant dance!

This means that the resonance, just its existence
Can tell us the planets have heaps of persistence
They haven’t received a strong enough push
To break up their dance. They’re still in the smush.

What impacts can make the resonance break?
Exactly how big of a kick does it take?
To figure this out, we used simulations
Each planet starts out in its present location
I threw some rogue leftover stuff in there too
Their gravity kicks can cause much ado

The Trappist-1 orbits are really not agile
The resonant structure is really quite fragile
A single rogue object the size of the Moon
Will break up the system and pop that balloon

Don't Blame Asteroids for the Late Heavy Bombardment!
Artist’s impression of bombardment on the early Earth. Credit: Chris Butler/SPL.

The maximum number of planet collisions
Is really quite small. We can say with precision
That none of the planets was strongly bombarded
With very few impacts that might well have scarred it.

In total it’s less than the stuff that hit Earth
Since our Moon was formed, the time of its birth
The Trappist-1 system has been nice and calm
A peaceful existence like snuggles from mom.

The system is 40-odd light years away
But thanks to its orbital dance, its ballet
Our bombardment limits are just as precise
As we have for Earth. That’s really quite nice!

This means that the Trappist-1 planets formed fast
The resonant chain built way back in the past
Was formed within just a few millions of years
That’s ten whole times faster our Earth appeared!
(Well, really, Earth’s growth was just a lot slower
The rate of emergence was quite a bit lower.)

Snapshot from a hydrodynamical simulation of five planets migrating in a gaseous planet-forming disk. This is the likely origin of Trappist-1’s resonant chain. Credit: Arnaud Pierens.

There’s something else that we can rightly deduce
From knowing these planets didn’t take much abuse
Those very few impacts could not have provided
Very much water down where they collided
If the planets have water it must have come first
They must have formed wet and quenched their own thirst.

To wrap up this poem, let’s get philosophical
An issue with Pluto that’s really quite topical:
How shall we choose what things to call “planet”?
Must they be massive, or made out of granite?

The IAU’s plan is based on three rules
To teach kids what “planet” means when they’re in school.
The first is the object must orbit a star
No, not a planet, a moon (or a car)
The second: the object must have a round shape
Not like a potato. But more like a grape.
It must be quite massive. It can’t be too small
To overcome friction and form a round ball
Third, the orbit must be all cleared out
No leftover objects left traipsing about

The first two requirements, easy to crack
With telescope measurements, we’re on the right track
The third one is where our study comes in
We showed that there simply can never have been
Leftover objects – we couldn’t have missed ‘em
The planets have cleared them all out of the system.

So Trappist-1’s not just a system of “things”
It’s official, they’re “planets” with all that it brings.

Looking ahead, our method is ready
We know of a few other systems already
With resonant orbits our technique could use
To figure which planets were battered and bruised.

This project united a really great team
Working with all of them — really a dream
They helped figure out all the parts of this system
Please look down below. That’s where I will list ‘em

Thank you for reading way down to the bottom
Science and rhymes – you want ‘em, we got ‘em.
For all of the details, please lend me your ear
Or, rather, your eyes. Our paper’s right here.

The Team:

Other Resources:


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