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Open problems in terrestrial planet formation
Sean RaymondLaboratoire 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) Str
onge
r C
onst
rain
ts
Dust (µm)
Planete-simals (~km)
Cores Embryos
Earth-sized
planets
104-5 yrs 105-7 yrs 107-8 yrs
dust sticking
Grav. collapse (cm - m)
Runaway growth
Oligarchic growth
Gas giants
Late-stage accretion
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Runaway gas accretion
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)
Kokubo & Ida 2002
Ecc
entr
icit
y
Semimajor Axis (AU)
(Giant planets must form in few Myr, so they affect late stages)
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)
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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)
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• 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)
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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 dryQuickTime™ and a
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Inconsistent with Kuiper Belt structure
–no migration of giant planets possible (Malhotra 1995, Levison & Morbidelli 2003)
Influence of giant planets
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Raymond, O’Brien, Morbidelli, & Kaib 2009
Influence of giant planets
Raymond, O’Brien, Morbidelli, & Kaib 2009
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Hard to form low-e, highly concentrated terrestrial planet systems
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)
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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?
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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)
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MarsLow-ecc.
Ast. belt
Form. time
Earth Water
Current JS
Eccentric JS
Nice model 1
Nice 1 eccentric
Nice model 2
Jin disk
Cases
• 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