the diversity of extrasolar terrestrial planets
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The Diversity of Extrasolar Terrestrial Planets. J. Bond, D. Lauretta & D. O’Brien USyd Colloquium 14 th July 2008. Chemistry meets Dynamics. Most dynamical studies of planetesimal formation have neglected chemical constraints - PowerPoint PPT PresentationTRANSCRIPT
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The Diversity of Extrasolar Terrestrial
Planets
J. Bond, D. Lauretta & D. O’Brien
USyd Colloquium14th July 2008
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Chemistry meets Dynamics
• Most dynamical studies of planetesimal formation have neglected chemical constraints
• Most chemical studies of planetesimal formation have neglected specific dynamical studies
• This issue has become more pronounced with studies of extrasolar planetary systems which are both dynamically and chemically unusual
• Astrobiologically significant
• Combine dynamical models of terrestrial planet formation with chemical equilibrium models of the condensation of solids in the protoplanetary nebulae
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Two Big Questions
1. Are terrestrial planets likely to exist in known extrasolar planetary
systems?
2. What would they be like?
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?
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Dynamical simulations reproduce the terrestrial
planets• Use very high resolution n-body accretion
simulations of terrestrial planet accretion (e.g. O’Brien et al. 2006)
• Incorporate dynamical friction
• Start with 25 Mars mass embryos and ~1000 planetesimals from 0.3 AU to innermost giant planet
• Neglects mass loss
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Equilibrium thermodynamics predict bulk compositions of
planetesimals• Consider 16 elements: H, He, C, N, O, Na, Mg, Al, Si,
P, S, Ca, Ti, Cr, Fe, Ni
• Assign each embryo and planetesimal a composition based on formation region
• Adopt the P-T profiles of Hersant et al (2001) at 7 time steps (0.25 – 3 Myr)
• Assume no volatile loss during accretion, homogeneity and equilibrium is maintained
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Equilibrium thermodynamics predict bulk compositions of
planetesimals
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“Ground Truthing”
• Consider the CJS1 system:– 1.15 MEarth at 0.64AU
– 0.81 MEarth at 1.21AU
– 0.78 MEarth at 1.69AU
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Results
Increasing Volatility
Al Ti Ca Mg Si O Ni Fe Cr P Na S H
En
rich
men
t F
acto
r
0
2
4
6
8
10
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Results
• Reasonable agreement with planetary abundances– Values are within 1 wt%, except for Mg, O and S and Si
(EJS only)
• Deviations:– Mg ~ 5 wt%– O & S ~ 4 wt%– Si ~ 2 wt% (EJS only)
• Mg/Si ratio less than planetary (0.47-0.76), implying there is some other way to fractionate one or both of these elements in the early Solar System
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Extrasolar “Earths”• Apply same methodology to extrasolar systems
• Use spectroscopic photospheric abundances (H, He, C, N, O, Na, Mg, Al, Si, P, S, Ca, Ti, Cr, Fe, Ni)
• Compositions determined by equilibrium
• Varied positions and masses of known giants and stellar mass
• Assumed closed systems
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Extrasolar “Earths”• Terrestrial planets formed in ALL systems studied
• Most <1 Earth-mass within 2AU of the host star
• Often multiple terrestrial planets formed
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Extrasolar “Earths”• Examine four ESP systems
• Gl777A – 1.04 MSUN G star, [Fe/H] = 0.24• 0.06 MJ planet at 0.13AU• 1.50 MJ planet at 3.92AU
• HD72659 – 0.95 MSUN G star, [Fe/H] = -0.14• 3.30 MJ planet at 4.16AU
• HD75732 (55Cnc) - 1.03 MSUN G star, [Fe/H] = 0.33• 0.05 MJ at 0.04AU• 0.78 MJ at 0.12AU• 0.22 MJ at 0.24AU• 3.92 MJ at 5.26AU
• HD4203 – 1.06 MSUN G star, [Fe/H] = 0.22• 2.10 MJ planet at 1.09AU
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Gl777A
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Gl 777A• 1.10 MEarth at 0.89AU
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HD72659
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HD72659• 1.03 MEarth at 0.95AU
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HD75732 (55Cnc)
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HD75732 (55Cnc)
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HD75732 (55Cnc)
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HD75732 (55Cnc)• 0.99 MEarth at 1.25AU
7 wt% C
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HD4203
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HD4203• 0.17 MEarth at 0.28AU
53 wt% C
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HD4203• 0.17 MEarth at 0.28AU
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Two Classes
• Earth-like compositions (Gl777A, HD72659)
• C-rich compositions (55 Cnc, HD4203)
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Mg/Si
0.5 1.0 1.5 2.0 2.5
C/O
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Mg2SiO4 + MgSiO3
SiO
SiC
MgSiO3 + other
SiO2 speciesMg2SiO4 + other
Mg species
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Mg/Si
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
C/O
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Mg2SiO4 + MgSiO3
SiO
SiC
MgSiO3 + other
SiO2 species
Solar
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Mg/Si
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
C/O
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Mg2SiO4 + MgSiO3
SiO
SiC
MgSiO3 + other
SiO2 species HD72659
55Cnc
HD4203
Solar
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Mg/Si
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
C/O
0.0
0.5
1.0
1.5
2.0
2.5
Mg2SiO4 + MgSiO3
SiO
SiC
MgSiO3 + other
SiO2 species
Solar
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Terrestrial Planets are likely in most ESP systems
• Terrestrial planets are common• Geology of these planets may be unlike
anything we see in the Solar System– Earth-like planets– Carbon as major rock-forming mineral
• Implications for plate tectonics, interior structure, surface features, atmospheric compositions . . .