Open problems in terrestrial planet formation Sean Raymond Laboratoire d’Astrophysique de Bordeaux …with audience contributions welcome!

How did the Solar System form? Simulations can roughly reproduce the masses and orbits of Earth and Venus (O’Brien et al 2006; Kenyon & Bromley 2006; Chambers 2001; Agnor et al 1999; Raymond et al 2006) Biggest problem: Mars’ small size (Wetherill 1991) Accretion process strongly dependent on giant planets (Levison & Agnor 2003; Raymond et al 2004) Goal: Reproduce inner solar system Constrain Jup, Sat’s orbits at early times Test relevant physics

Constraints Masses, orbits of terrestrial planets Mars’ small mass is a mystery (Wetherill 1991, Chambers 2001) Very low eccentricities (O’Brien et al 2006) Structure of asteroid belt Separation of S, C types No evidence for remnant embryos (gaps) Accretion timescales from Hf/W, Sm/Nd Earth/Moon: 50-150 Myr (Jacobsen 2005; Touboul et al 2007) Mars: 1-10 Myr (Nimmo & Kleine 2007) Water delivery to Earth Asteroidal source explains D/H (Morbidelli et al 2000) Other models exist (Ikoma & Genda 2007; Muralidharan et al 2008) Stronger Constraints

Late-stage accretion Runaway gas accretion Runaway growth Gas giants Runaway gas accretion Late-stage accretion Earth-sized planets Runaway growth Oligarchic growth Cores Embryos dust sticking Grav. collapse (cm - m) Planete-simals (~km) Dust (µm) 104-5 yrs 105-7 yrs 107-8 yrs

Initial conditions for late-stage accretion Eccentricity Semimajor Axis (AU) Planetary embryos (aka protoplanets) form by runaway and oligarchic growth: ~Moon-Mars sized (~105-6 yrs) (Kokubo & Ida 1998, Leinhardt & Richardson 2005) Late-stage accretion starts when local mass in embryos and planetesimals is comparable (Kenyon & Bromley 2006) (Giant planets must form in few Myr, so they affect late stages) Kokubo & Ida 2002

Key factors for accretion 1. Giant Planets (Levison & Agnor 2003) Formation models predict low eccentricity Nice model: Jup, Sat closer than 2:1 MMR during accretion (Tsiganis et al 2005; Gomes et al 2005) Perhaps in chain of resonances (Morbidelli et al 2007) 2. Disk Properties (Wetherill 1996, Raymond et al 2005) Total mass ~ 5 Earth masses inside 4 AU (Weidenschilling 1977; Hayashi 1981) ∑ ~ r-1.5 (MMSN) or perhaps more complex (Jin et al 2008; Desch 2007)

Nice model 2 (J, S in 3:2 MMR)

Nice model 2 (J, S in 3:2 MMR) No Mars analogs Embryos in asteroid belt Inconsistent with observed structure if embryo Mars-mass or larger

Nice model 2 (J, S in 3:2 MMR) No Mars analogs Embryos in asteroid belt Inconsistent with observed structure if embryo Mars-mass or larger

Eccentric Jup, Sat (e0=0.1)

Eccentric Jup, Sat (e0~0.1) Strong secular resonance (6) at 2.2 AU Mars consistently forms in correct configuration Earth and Venus are dry Inconsistent with Kuiper Belt structure no migration of giant planets possible (Malhotra 1995, Levison & Morbidelli 2003)

Influence of giant planets Raymond, O’Brien, Morbidelli, & Kaib 2009

Influence of giant planets Hard to form low-e, highly concentrated terrestrial planet systems Raymond, O’Brien, Morbidelli, & Kaib 2009

Mars Small Mars forms naturally if inner disk is truncated at 1-1.5 AU (Agnor et al 1999; Hansen 2009) Can reproduce all 4 terrestrial planets if embryos only existed from 0.7-1 AU (Hansen 2009) Hansen 2009

Other effects Gas disk effects: Type 1 migration (McNeil et al 2005; Morishima et al 2010) Secular resonance sweeping (Nagasawa et al 2005; Thommes et al 2008) Collisional fragmentation (Alexander & Agnor 1998; Kokubo, Genda) Morishima et al 2010

Jin et al (2008) disk Assume MRI is effective in inner, outer disk but not in between At boundary between low, high viscosity, get minimum in density Occurs at ~1.5 AU Explanation for Mars’ small mass? Jin et al (2008)

Summary No tested configuration of Jup, Sat reproduces all constraints (Raymond et al 2009) Closest is eccentric Jup, Sat but Earth is dry and JS not consistent with Kuiper Belt Including gas disk effects doesn’t solve the problem (Morishima et al 2010) Hard to reproduce Mars’ small size Strong constraint on Jup, Sat’s orbits at early times Was there just a narrow annulus of embryos? (Hansen 2009) What’s missing? Secular resonance sweeping during disk dispersal (Nagasawa et al 2005, Thommes et al 2008) Something else?

Recent progress Morishima et al 2008, 2010 Raymond, O’Brien, Morbidelli, Kaib 2009 Hansen 2009 Thommes, Nagasawa & Lin 2008 O’Brien, Morbidelli & Levison 2006 Raymond, Quinn & Lunine 2006 Kenyon & Bromley 2006 Nagasawa, Thommes & Lin 2005 Kominami & Ida 2002, 2004 Chambers 2001 Agnor, Canup & Levison 1999

Initial conditions Start of chaotic growth phase (Wetherill 1985; Kenyon & Bromley 2006) Equal mass in 1000-2000 planetesimals and ~100 embryos (5 ME total) Embryos is Mars’ vicinity are 0.1-0.4 Mars masses Integrate for 200 Myr + with Mercury (Chambers 1999)

Mars Low-ecc. Ast. belt Form. time Earth Water Current JS Eccentric JS  Eccentric JS Nice model 1 Nice 1 eccentric Nice model 2 Jin disk

Cases Current Jup, Sat Disk: ∑~r-1 and r-1.5 Jup, Sat with e0~0.1 e ~ current values after accretion Nice Model 1: Jup 5.45 AU, Sat 8.12 AU, e0=0 Nice Model 2: Jup, Sat in 3:2 MMR, low-e Disk: ∑~r-1 and r-1.5 Little difference Disk from Jin et al (2008) Dip in ∑ at ~1.5 AU