Ruth Murray-Clay University of California, Santa Barbara

A Diversity of Worlds: Toward a Theoretical Framework for the Structures of Planetary Systems Ruth Murray-Clay University of California, Santa Barbara

Strange New Worlds. Slide credit: Scott Gaudi ~1500 Confirmed Planets ~3300 Planet Candidates

Slide credit: Scott Gaudi (Ida & Lin)!

Many important physical processes have highly uncertain parameters. Population comparisons are more robust than absolute comparisons to synthesis models, since there are likely fewer parameters controlling the differences across planetary systems than there are theoretical knobs. Ex: Stellar mass, total solid mass in the disk (metallicity), stellar X-ray flux available to ionize the disk (activity)

~ 20-30 AU ice giants ~ 1 AU rocky planets ~ 5-10 AU gas giants

Solids grow through collisions in the disk

Growth timescale is set by: A: cross-section for collisions v: relative velocities of colliding bodies n: density of bodies available to accrete A v t Volume = Avt number density = n Sun v H ~ v torb Fast growth: high density, large cross-section, short orbital period # collisions in time t ~ n Avt ~ (nh) A (t/torb)

Pre-exoplanets: ~ 20-30 AU ice giants More Material ~ 1 AU rocky planets Longer Orbital Time Less Material ~ 5-10 AU gas giants

Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

Major uncertainties Initial conditions: the MMSN isn t good enough anymore accretion physics (non-ideal MHD) Is early disk structure (set by fast and episodic accretion, varying stellar luminosity, etc.) preserved and imprinted on the planet population? Outer disk scale: cloud infall, early binarity, dynamical interactions in a cluster? Do stellar interactions perturb the gas disk? Can the disk approach gravitational fragmentation? Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

Henning & Semenov 2013

Xuening Bai

Major uncertainties Initial conditions: the MMSN isn t good enough anymore accretion physics (non-ideal MHD) Is early disk structure (set by fast and episodic accretion, varying stellar luminosity, etc.) preserved and imprinted on the planet population? Outer disk scale: cloud infall or do early binarity & dynamical interactions in a cluster matter? Do stellar interactions perturb the gas disk? Can the disk approach gravitational fragmentation? Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

ALMA data will help, but not necessarily through direct measurements of the surface density distribution HL Tau ALMA Partnership 2015 planets? non-ideal MHD zonal flows? accumulation of dust in pressure maxima?

Major uncertainties Initial conditions: the MMSN isn t good enough anymore accretion physics (non-ideal MHD) Is early disk structure (set by fast and episodic accretion, varying stellar luminosity, etc.) preserved and imprinted on the planet population? Outer disk scale: cloud infall or do early binarity & dynamical interactions in a cluster matter? Do stellar interactions perturb the gas disk? Can the disk approach gravitational fragmentation? Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

HR 8799 Neptune s orbital distance big planets or small stars? Marois et al. 2010

Gravitational instability Collapse must occur at the end of infall or the fragment will grow into a binary star Kratter, Murray-Clay, & Youdin (2010)

Test case HR 8799: Brown Dwarfs or Planets? A population comparison M p /M * Planetary companions 10 0 10 1 10 2 10 3 10 4 Kratter, Murray-Clay, & Youdin, ApJ (2010) Data: Zuckerman & Song 2009; exoplanet.eu 10 0 10 2 10 4 r p (AU) Brown dwarf companions HR 8799 Jupiter Saturn ~stars ~brown dwarfs ~planets Larger than most protoplanetary disks

Major uncertainties Planetesimal growth: meter size barrier---overcome from the top down or the bottom up? When do planetesimals grow and how many grow large? Implications for drift, solid and compositional redistribution, interpretation of dust observations. Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

Gas Alters the Orbits of Planetesimals planetesimal wants to orbit The resulting star at v Kep drag acceleration F D m is small large radial pressure gradient but gas orbits more slowly v orb <v Kep v 2 orb r = GM r 2 + 1 dp dr

Planetesimals can drift well past the nominal snow line before desorbing Piso, Oberg, Birnstiel, & Murray-Clay (submitted)

Major uncertainties Why do some planets become giants while others stay small? Timescales? Ice line enhancements? Inability to accrete disk gas? Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

Capture by gas drag allows fast enough growth to nucleate a massive atmosphere core Sun increased cross-section planetesimal Murray-Clay et al., in prep Ormel & Klahr 2010, Perets & Murray-Clay 2011, Lambrechts & Johansen 2012

Xu, Bai, & Murray-Clay in prep

Michael Rosenthal - Gas Assisted Growth of Planetesimals Gas drag dramatically changes the timescale predicted by typical core accretion models (e.g. Lambrechts & Johansen, 2012) We use an order of magnitude model to calculate these timescales, as well as to take into account additional effects, such as turbulence.

If pebble accretion allows fast core growth, then why aren t Uranus and Neptune gas giants?

Renata Frelikh Gas accretion onset: Late enough to avoid runaway, but before disk fully dissipates Both planets have a short timeframe to finish core growth and accrete ~10% by mass atmospheres Full gas disk Gas disk depleted to ~0.025 of original value Gas disk fully dissipated (~3 Myr timescale for gas to fully dissipate) Uranus and Neptune Formation: Fine Tuned?

Major uncertainties Why do some planets become giants while others stay small? Timescales? Ice line enhancements? Inability to accrete disk gas? I will come back to this question. Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

Major uncertainties Dynamical redistribution: Among easily observable giants, it is clearly important More generally, it is probably important: Type II (gap opening) disk migration--understood if disk interactions are well modeled with an effective viscosity Type I: something is wrong! planet-planet interactions (scattering, secular chaos, Kozai) planet-stellar companion Kozai Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

P M V E M J S U N Wright et al. 2009

Another population comparison: Metal-rich stars host more hot Jupiters and highly eccentric planets: A signature of planet-planet interactions hot Jupiters highly eccentric planets ~ in situ formation? Dawson & Murray-Clay 2013

Solar system? Kuiper belt Neptune Asteroid belt

5:2 resonance population is large UCSB undergraduate Mathew Yu is characterising the population that could be transiently stuck from the population currently scattered by Neptune Volk, Murray-Clay, Gladman, OSSOS team, submitted

Major uncertainties Atmospheric bulk compositions: bulk solid composition --> outgassing bulk accreted gas composition atmospheric escape chemistry & geophysics we don t know what life will do Infall and disk accretion sets the initial conditions for growth of planetesimals and planets which then migrate as they interact with residual gas and planetesimals and then evolve dynamically over long timescales all the while developing atmospheres that depend on this history.

Density [g/cc] Neutral Fraction Temperature [K] Line-of-sight Velocity [cm/s] 0.00 s 3.00e4 s 6.00e4 s 1.85e5 s 1.39e6 s Tripathi, Kratter, Murray-Clay, and Krumholz, 2015

A timely question: Where do the Kepler super-earths come from? What determines whether a core accretes a giant atmosphere?

gas giant Disk Mass rock ice Solar System lower planet mass relative to disk mass last gasp gas migration drift Murray-Clay & Rogers in prep Distance from Star planetesimal-driven migration

Where are the solar system analogs? Guess: orbiting low metallicity stars