Notices by Prof. Sam Lawler (sundogplanets@mastodon.social), page 3

Vaibhav Chhajed (Michigan State U.) pebble collapse simulations - uses many different size pebbles to mroe realistically simulate structures formed. Older sims use same size pebbles which may artificially increase strength of simulated bodies (like a crystal sort of). Wide ranges of sizes actually results in denser planetesimals due to more efficient packing, and makes more oblate shapes, more realistic with observations. Cool!

#DDA2026

Tommy Chi Ho Lau (U. Chicago) planetesimals form and then migrate inwards in disk, are affected by planets. Has code that does ALL of this, wow. Builds up structure of Kuiper Belt using disk dissipation and planet migration. By eye looks like their sim made WAY too many plutinos... Working on eroding after this initial simulation.

#DDA2026

There is one more talk but I have to do other things now. #DDA2026 was super interesting and I learned a lot (though please note I skipped many talks, including all of the galaxy dynamics talks, due to my own bandwidth this week)

I really appreciate getting to learn fascinating astronomy research that's happening - good motivation to keep fighting satellite companies for access to the sky.

Ryan LoRusso (Indiana U.) Cold Neptunes are common, can they help with redistributing planets in resonant chains? 5 Neptune systems migrate inwards in disk and make nice resonant chains, but are broken by planetesimals.

Neptunes/sub-Neptunes are perfect for secular chaos which destroys resonance without too much ejection. Jupiters eject everything.

Predict peak in debris disk activity at 10-100Myr due to this reshuffling.

#DDA2026

Can we all agree not to use this?

(No, we probably can't... also I've already heard from people in the US that some carriers auto-swap over to Starlink sometimes)

The direct-to-cell satellites are the worst for light pollution (they're huge and on low orbits), likely sat working lifetimes will be even shorter than Starlink's 5 yrs (because they're constantly orbit-raising due to friction with atmosphere), and thus more pollution from chucking them into the atmosphere

https://mastodon.social/@arstechnica/116816625892976050

Robin Canup (SWRI) is giving a prize talk on the formation of the Moon. The Moon was definitely formed by a giant impact, but the details are hard! Mars-size impactor makes most sense, but you have to shed a bunch of angular momentum. Can do this with "evection resonance" which keeps the Moon-Earth-Sun in a specific configuration and messes with the Moon's eccentricity. Big problem: matching isotopic composition. Maybe impactor was the same as Earth? #DDA2026

Between my normal meetings and writing, I'm watching a few talks at the American Astronomical Society's (AAS) Division for Dynamical Astronomy (DDA) annual meeting this week. They have this fantastic option where you pay US$10 and you can watch all the talks at the meeting. I'll try to share summaries of a few highlights using #DDA2026

Peas-in-a-pod exoplanet systems (multiple similar-mass planets closely packed) maybe follow the co-accretion pattern? Simulations with gas migration show a characteristic mass for surviving planets, that doesn't depend strongly on stellar metallicity. Cool!

#DDA2026

She just told a story about being totally obsessed with Saturn as a middle schooler during the Voyager mission. She wrote a letter to JPL and they sent her a packet of Saturn photos and info! Comments that "I bet they had a good outreach budget back then." SIGH.

Saturn has 1 big moon, did smaller moon get Roche-shredded into the rings? Rings appear to be young, so probably not the right explanation.

Can co-accretion and giant impacts work together to explain Uranus/Neptune moons?

#DDA2026

Talks about how tidal dissipation would change as the impact-melted Earth resolidifies.

What about co-accretion? Not for our Moon, but works for jovian planets' large moons. Shows that many generations of moons formed around jovian planets and were eaten by planets during Solar System's planet formation phase. The ones we see today are the last generation before gas disk dispersed.

#DDA2026

Guangyi Zhang (Caltech) Moon-planet tidal system is like a damped harmonic oscillator. 100 bonus points for having a cute animation of a moon on a surfboard "surfing" on the peak "gravito-inertial mode" location as it moves outwards from planet. Applies to Jupiter's and Saturn's moons

#DDA2026

K. Dabroski (U. Idaho) How did Saturn's rings form? Uses only Chrysalis (a.k.a. proto-Hyperion), Titan, and Saturn's J2 as perturbers in REBOUND https://rebound.hanno-rein.de/ Iapetus is important for getting eccentricities high enough for a collision. More sims needed!

#DDA2026

Ian Brunton (Caltech) shows that Io and Europa's 2:1 mean-motion resonance can be primordial, but Ganymede's 4:2:1 mean-motion resonance wouldn't have been stable in the primordial disk and would need to fall into place later

#DDA2026

Grant Weldon (UCLA): oh I like this talk title "Saving Doomed Planets". Hot Jupiters like to fall into their stars. But mass loss is important - by losing mass some of them end up not falling into their stars. High eccentricity migration can be survived, but sometimes hot Jupiters turn into hot Neptunes.

#DDA2026

Yurou Liu (Yale): hot-Jupiter hosting binaries are more eccentric, OR hot Jupiters are preferentially aligned with their binaries. They found this through building a bunch of simulated hot Jupiter systems and letting the Kozai effect change the eccentricities and inclinations and looking at the final distributions

#DDA2026

Wen-Han Zhou (U. Tokyo) why do Saturn A and B rings have such sharp inner rings? Can't be explained by moons. Yarkovsky changes spins through absorbtion and re-radiation of light being in different places (due to rotation). Adding in an eclipse, as for a binary system, changes the average force. This gets REALLY complicated for a ring made of particles all eclipsing each other! Calculate using pkdgrav package, including Saturn radiation. Inner edge is sharp, outer edge leaks outwards

#DDA2026

Konstantin Batygin (Caltech): most common planets are super-Earths on very short orbits. How do they not fall into their star? How do they pick which resonance to lock in to? (Bonus points for joke about a system with a 6:7 resonance for everyone with middle-school-aged kids)

Giant equation in a confetti explosion (this guy likes giving talks). Shows that 6:7 resonance requires planets to form simultaneously at 1-3AU: the "planet factory ring"

#DDA2026

Julia Esposito (Georgia Inst of Tech) looking at planet-planet scattering, uses REBOUND TRACE and Reboundx because need close encounters between planets, long integrations, general relativity, and tides (wow). Cold scattering (distances outside 1AU) is needed to produce hot Jupiters. Made lots of eccentric, aligned, warm Jupiters. Predict warm Jupiters should have nearby companions with >30 degree mutual inclinations

#DDA2026

Sacha Gavino (U. Bologna) millions of sims of 3 equal mass earth planets in extremely compact orbits, mapping out 3 body interactions with orbit spacing. Really complex stability structure, depends on initial longitudes of planets. Holy cow that's a complicated map of "the 3-body resonance network", looking at where resonances overlap and chaos happens, and where resonances push planets into higher stability orbital configurations.

#DDA2026

As part of the CV-rejiggering for academic stuff that I previously complained about, I also need to update my academic website (which is embarrassingly simple, but at least I didn't write it in 1999 and it doesn't have a dancing-linux-penguin-gif like Some Other Academics). Will be trying to do that while listening to the next set of #DDA2026 talks