Pebble Accretion by Planetary Embryos on Eccentric Orbits

Steve Desch Alan Jackson, Chuhong Mai, Jessica Noviello School of Earth and Space Exploration Arizona State University

Exoplanets in our Backyard February 5, 2020 Astronomers have discovered that accretion of planetary embryos is fast!

ALMA observations of young protoplanetary disks like HL Tau (~105 yr old) show gaps quite likely due to Jupiter- sized planets at 30 and 70 AU (Yen et al. 2016). 50 AU

Inner hole 5 AU

HL Tau (ALMA)

TW Hya, ~8 Myr old (ALMA) Meteoriticists / planetary scientists already knew that planetary embryos (Mars) grew fast

Hf-W dating of martian meteorites*: Mars (0.1 ME) was a planetary embryo that accreted by pebble accretion ~ 1-3 Myr after CAIs (Dauphas and Pourmand 2011).

* 182Hf decays to 182W

(t1/2 = 9 Myr).

Hf stays in mantle, W goes into core.

The earlier the core forms, the bigger 182W/180W is in the mantle.

Dauphas & Pourmand (2011) Planetary embryos on eccentric orbits? Chondrules probably formed by passing through bow shocks in front of large (> 0.1 ME) planetary embryos on eccentric (e ~ 0.4) orbits at 1.5 - 3 Myr (Morris et al. 2012; Boley et al. 2013; Mann et al. 2016).

(Boley et al. 2013) Boley et al. (2013) Eccentricities damp over 105 yr (combination of gas drag and dynamical friction) Boley et al. (2013) Jupiter’s core formed in < 1 Myr

Jupiter’s 20-30 ME core formed in ~ 0.4-0.9 Myr to separate solar system into two isotopic reservoirs (Trinquier et al. 2009; Warren 2011; Kruijer et al. 2017). Distribution of CAIs in the solar nebula requires Jupiter’s core formed at ~ 3 AU, at ~0.6 Myr (Desch et al. 2018).

ApJS 238, 11. arXiv: 1710.03089v3.pdf

constrained by by constrained

– Time of formation formation of Time radiometric dating & thermal thermal models & dating radiometric (Desch et al. 2018) Earth/Theia = 2 embryos Proto-Earth, Theia each reached > 0.4 ME in < 3 Myr, to ingas solar nebula H2 (Wu et 1. Initial Conditions: Planetary Embryos in the Solar Nebula al. 2018; Desch and Robinson 2019). Theia Proto-Earth 0.40 ME 0.62 M Just two or (three) large embryos. E Mg/Si = 0.89 Mg/Si = 1.20 30 δ Si =-0.57 δ30Si = -0.30 δD = -130‰ δD= +25‰ DIW = -5 DIW = -2

2. Planet Evolution: Nebular ingassing lowers planetary D/H. Si partitioning into cores raises mantle δ30Si and FeO

Nebular Theia Nebular H Mantle mass 0.27 M 2 Proto-Earth H2 E Mg/Si = 1.00 Mantle mass 0.42 ME δ30Si =-0.29 Mg/Si = 1.26 30 Mantle FeO = 15 wt.% δ Si =-0.27 Mantle FeO = 5 wt.% Mantle H2O = 220 ppm Si Mantle δD = Si Total H2O = 1800 ppm -740‰ to -130‰ Mantle δD = Core = 5.2wt% Si -220‰ to +25‰ Core = 1.4 wt.% Si Hallis et al. (2015) 3. Giant Impact Desch & Robinson Merger (Canup, 2012): cores merge, mantles mix (2019)

4. Final Conditions: Earth and Moon

Earth Moon

1.00 ME 0.012 ME Mantle mass 0.68 ME Mantle mass 0.011 ME Mg/Si ≈ 1.2 Mg/Si = 1.1 δ30Si = -0.27 δ30Si = -0.28 Mantle FeO = 7.7 wt.% Mantle FeO = 10.6 wt.%

Bulk mantle H2O = 1200 ppm Bulk mantle H2O = 850 ppm Bulk mantle δD ≈ -20‰ Bulk mantle δD = -70‰ Core = 3.1 wt.% Si Core = 5.2 wt.% Si? Robinson et al. (2016) Exoplanets inform planet accretion models

Most Kepler exoplanets > 1.6 RE must have H2/He atmospheres [Weiss & Marcy 2014; Rogers 2015] e.g., Kepler 11f (M= 2.3 +2.2/-1.2 ME) has H2/He atmosphere > 0.4% planet’s mass (enough to supply > 300 oceans) [Lopez et al. 2012]

Lifetimes of disks typically < 3 Myr (Haisch et al. 2001)

9 Exoplanets inform planet accretion models Rocky exoplanets in multi-planet California- Kepler survey systems: • Large (easily ~5 Earth masses) • Similar in size • Evenly spaced orbits

Weiss et al. (2018) 10 Pebble Accretion inspired by both solar system and exoplanet studies Pebble Accretion (Lambrechts & Johansen 2012; Lambrechts et al. 2014) allows large planetesimals to rapidly grow into planetary embryos.

But this accretion may be limited (Kuwahara & Kurokawa 2019) Pebble Accretion + eccentric orbits? Masses grow faster while orbit is eccentric

Embryos sweep out larger areas, at faster relative velocities

Eccentricities are damped faster when planet is massive Net result: planetary embryos on eccentric orbits tend to reach similar masses regardless of starting mass Synergies between Exoplanet and Solar System Studies

Solar System Studies inform Exoplanet Studies:

• Growth by pebble accretion while embryo are on eccentric orbits could be even faster than standard pebble accretion models predict.

• Planetary embryos may accrete most their mass while on eccentric orbits

• Explains “peas-in-a-pod” result?

Exoplanet studies inform solar system studies: Existence of super-Earths makes plausible:

• Embryos ingassed hydrogen into their magma oceans

• Proto-Earth + Theia was just 2 planetary embryos.

• Venus was a single embryo? 15