Thread by @sraymond_astro

New paper in Nature Astronomy: “An upper limit on late accretion and water delivery in the Trappist-1 exoplanet system”

A thread to explain why this is new and interesting...

nature.com/articles/s4155…

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Impacts on planets after they form can:
-Deliver volatiles
-Erode/modify atmospheres
-Cause extinctions
-Make ridiculous movie plots (Deep Impact is my personal favorite asteroid-about-to-kill-everyone movie)

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In the Solar System, impact rates are measured mainly using 1) crater counts (e.g. for the Moon), or 2) highly-siderophile elements (for Earth)

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Analyses of highly-siderophile elements find that ~0.5% of Earth’s mass came from planetesimals after the Moon-forming impact.

This is called “late accretion” – it’s one place where cosmochemists and modelers meet (image from @sethajacobson+ 2014 -- ui.adsabs.harvard.edu/abs/2014Natur.…)

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It would seem hopeless to try to constrain late accretion on exoplanets. No craters. No highly-siderophile elements.

But wait, there’s more…

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Our idea: focus on multi-resonant systems.

These resonances are “fragile”. Their survival puts an upper limit on the perturbations they’ve felt over their system lifetimes.

(I labeled the resonances in this image)

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We performed a simple numerical experiment:

Take the Trappist-1 system and determine the minimum perturbation needed to break it.

Perturbations are from ‘rogue’ bodies leftover from planet formation (as for late accretion on Earth).

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Result 1: a rogue planetary embryo more massive than the Moon disrupts Trappist-1’s resonances.

Implication: ~Moon-mass is the most massive impactor on any planet since formation (most likely targets: planets f, g, h)

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Result 2: a swarm of rogue planetesimals more massive than 0.05 Earth masses disrupts Trappist-1’s resonances.

The resonances are narrow so it doesn’t take a big “kick” to move the planets out.

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Using the statistics of impacts in our simulations we put upper limits on late accretion on each planet.

Punchline: the upper limits are tiny! They are less than Earth for 4 inner planets, similar to Earth for outer 3.

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Since the Trappist-1 planets underwent very little late accretion, this means they formed fast, and were done growing by the end of the gaseous disk phase (consistent with migration-driven models).

Blog post on "breaking the chains" model: planetplanet.net/2017/04/18/bre…

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Time zero for late accretion for Trappist-1: end of gas disk phase

Time zero for late accretion on Earth: Moon-forming impact

This means our Trappist-1 limits are – in terms of the relevant time interval – stricter than for Earth!

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Water delivery from late accretion on Trappist-1 is minimal. Upper limits are a few oceans for outer planets, much less for inner ones.

(1 “ocean” = Earth’s total surface water)

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But do the Trappist-1 planets even have any water? We ran 5 sets of new interior+water models constrained by full data from Agol et al (2021).

None of our models finds water at >2-sigma for any of the planets. Planet g is the best candidate (see papers by @Lena__Noack+)

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Punchline: *If* any of the Trappist-1 planets has water, it must have been incorporated during formation (not from late accretion)

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Philosophical point: our paper shows how much we can learn by focusing on what could NOT have happened rather than on what did.

media.giphy.com/media/l2JIk0sW…

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IAU rule B5: a planet must
1.Orbit the Sun
2.Be massive enough to be round
3.Have cleared its orbit

Let’s accept 1 and 2. Our new paper demonstrates 3. So we can officially call Trappist-1 “planets”!

iau.org/static/resolut…

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Naturally, our method is generalizable to all resonant chain systems (in progress)

Kepler-223 and TOI-178 are ideal candidates.

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en.wikipedia.org/wiki/Kepler-223

It was a great collaboration with wonderful co-authors: @izidoro_astro, Emeline Bolmont, Caroline Dorn, @FranckSelsis, @Nonomamades, @AgolEric, @ExoplanetMaster, @Exotides, @rdasgupta_earth, Michael Gillon, and Simon Grimm

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I also wrote a poem about the paper: planetplanet.net/2021/11/25/bom…

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Finally, this paper is dedicated to the memory of our friend and colleague Franck Hersant, who used to come visit @FranckSelsis and me in our office pretty much every day. I called him “Monsieur H” and he called me “Monsieur Dude”.

@AgolEric gets full credit for realizing this!

Full readable link to paper: rdcu.be/cB2VH

I should also mention that this whole project started with an email from @Exotides and @ExoplanetMaster last summer asking if there were any constraints on the bombardment history of the Trappist-1 planets...