Open problems in terrestrial planet formation

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

2. 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

3. 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

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

5. Eccentricity Semimajor Axis (AU) Initial conditions for late-stage accretion • 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

6. 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)

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

8. 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

9. 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

10. Eccentric Jup, Sat (e0=0.1)

11. 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)

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

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

14. 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

15. 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

16. 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)

17. 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?

18. 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

19. 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)

22. Current Jup, Sat 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 Cases