Open problems in terrestrial planet formation

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Open problems in terrestrial planet formation

• Sean Raymond
• Laboratoire dAstrophysique de Bordeaux

with audience contributions welcome!
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How did the Solar System form?

• Simulations can roughly reproduce the masses and
  orbits of Earth and Venus (OBrien 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, Sats orbits at early times
• Test relevant physics

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Constraints

• Masses, orbits of terrestrial planets
• Mars small mass is a mystery (Wetherill 1991,
  Chambers 2001)
• Very low eccentricities (OBrien 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
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Gas giants
Earth-sized planets
Cores Embryos
Planete-simals (km)
Dust (µm)
105-7 yrs
104-5 yrs
107-8 yrs
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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
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Key factors for accretion

• 1. Giant Planets (Levison Agnor 2003)
• Formation models predict low eccentricity
• Nice model Jup, Sat closer than 21 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 32 MMR)
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Nice model 2 (J, S in 32 MMR)

• No Mars analogs
• Embryos in asteroid belt
• Inconsistent with observed structure if embryo
  Mars-mass or larger

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Nice model 2 (J, S in 32 MMR)

• No Mars analogs
• Embryos in asteroid belt
• Inconsistent with observed structure if embryo
  Mars-mass or larger

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Eccentric Jup, Sat (e00.1)
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Eccentric Jup, Sat (e00.1)

• 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)

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Influence of giant planets
Raymond, OBrien, Morbidelli, Kaib 2009
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Influence of giant planets
Hard to form low-e, highly concentrated
terrestrial planet systems
Raymond, OBrien, Morbidelli, Kaib 2009
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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
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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
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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)
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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 doesnt solve the
  problem (Morishima et al 2010)
• Hard to reproduce Mars small size
• Strong constraint on Jup, Sats orbits at early
  times
• Was there just a narrow annulus of embryos?
  (Hansen 2009)
• Whats missing?
• Secular resonance sweeping during disk dispersal
  (Nagasawa et al 2005, Thommes et al 2008)
• Something else?

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Recent progress

• Morishima et al 2008, 2010
• Raymond, OBrien, Morbidelli, Kaib 2009
• Hansen 2009
• Thommes, Nagasawa Lin 2008
• OBrien, Morbidelli Levison 2006
• Raymond, Quinn Lunine 2006
• Kenyon Bromley 2006
• Nagasawa, Thommes Lin 2005
• Kominami Ida 2002, 2004
• Chambers 2001
• Agnor, Canup Levison 1999

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

• Current Jup, Sat
• Jup, Sat with e00.1
• e current values after accretion
• Nice Model 1 Jup 5.45 AU, Sat 8.12 AU, e00
• Nice Model 2 Jup, Sat in 32 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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