Open problems in terrestrial planet formation Sean Raymond
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 • Accretion process strongly dependent on giant planets (Wetherill 1991) (Levison & Agnor 2003; Raymond et al 2004) • Goal: Reproduce inner solar system – Constrain Jup, Sat’s orbits at early times – Test relevant physics
Constraints – 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 • Masses, orbits of terrestrial planets
Gas giants Earthsized planets Cores Embryos Planete -simals (~km) Late-stage accretion Runaway gas accretion Runaway growth dust sticking Oligarchic growth Grav. collapse (cm - m) Dust (µm) 104 -5 yrs 105 -7 yrs 107 -8 yrs
Initial conditions for late-stage accretion 1998, Leinhardt & Richardson 2005) • Late-stage accretion starts when local mass in embryos and planetesimals is comparable Eccentricity 1. Planetary embryos (aka protoplanets) form by runaway and oligarchic growth: ~Moon. Mars sized (~105 -6 yrs) (Kokubo & Ida (Kenyon & Bromley 2006) (Giant planets must form in few Myr, so they affect late stages) Semimajor Axis (AU) 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 (e 0=0. 1)
Eccentric Jup, Sat (e 0~0. 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 (Mc. Neil 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)
Current JS Eccentric JS Nice model 1 Mars Lowecc. Ast. belt Nice 1 eccentric Form. time Earth Water Nice model 2 Jin disk
Cases • Current Jup, Sat • Jup, Sat with e 0~0. 1 – e ~ current values after accretion • Nice Model 1: Jup 5. 45 AU, Sat 8. 12 AU, e 0=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
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