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The Chemistry of Extrasolar Planetary
Systems
Jade BondPhD Defense
31st October 2008
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Extrasolar Planets• First detected in 1995
• 313 known planets inc. 5 “super-Earths”
• Host stars appear metal-rich, esp. Fe
• Similar trends in Mg, Si, Al
Santos et al. (2003)
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Neutron Capture Elements
• Look beyond the “Iron peak” and consider r- and s-process elements
• Specific formation environments
• r-process: supernovae
• s-process: AGB stars, He burning
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Neutron Capture Elements
• 118 F and G type stars (28 hosts) from the Anglo-Australian Planet Search
• Y, Zr, Ba (s-process) Eu (r-process) and Nd (mix)
• Mg, O, Cr to complement previous work
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-0.50
0.00
0.50
-0.50 0.00 0.50
[ Fe/H ]
[ E
u/H
]
-0.50
0.00
0.50
-0.50 0.00 0.50
[ Fe/H ]
[ Y
/H ]
Host Star EnrichmentMean [Y/H]
Host: -0.05 + 0.03Non-Host: -0.16 + 0.01
Mean [Eu/H]Host: -0.10 + 0.03
Non-Host: -0.16 + 0.02
[Y/H] SlopeHost: 0.87
Non-Host: 0.78
[Eu/H] SlopeHost: 0.56
Non-Host: 0.48
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Host Star Enrichment
• Host stars enriched over non-host stars
• Elemental abundances are in keeping with galactic evolutionary trends
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Host Star Enrichment
0.00
5.00
10.00
-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
[ Y/H ]
M s
ini (
M Ju
p)
0.00
1.00
2.00
3.00
4.00
5.00
-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
[ Y/H ]
a (A
U)
0.00
0.50
1.00
-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
[ Y/H ]
e
0
1000
2000
3000
4000
-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30
[ Y/H ]
Per
iod
(day
s)
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Host Star Enrichment
• No correlation with planetary parameters
• Enrichment is PRIMORDIAL not photospheric pollution
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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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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
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Chemistry meets Dynamics
• Combine dynamical models of terrestrial planet formation with chemical equilibrium models of the condensation of solids in the protoplanetary nebulae
• Determine if terrestrial planets are likely to form and their bulk elemental abundances
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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)
• Start with 25 Mars mass embryos and ~1000 planetesimals from 0.3 AU to 4 AU
• Incorporate dynamical friction
• Neglects mass loss
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Equilibrium thermodynamics predict bulk compositions of
planetesimals
Davis (2006)
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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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“Ground Truthing”
• Consider a Solar System simulation:– 1.15 MEarth at 0.64AU
– 0.81 MEarth at 1.21AU
– 0.78 MEarth at 1.69AU
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Results
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Results
• Reasonable agreement with planetary abundances– Values are within 1 wt%, except for Mg, O, Fe and S
• Normalized deviations:– Na (up to 4x)– S (up to 3.5x)
• Water rich (CJS)
• Geochemical ratios between Earth and Mars
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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
• Embryos from 0.3 AU to innermost giant planet
• No planetesimals
• Assumed closed systems
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Assumptions
• In-situ formation (dynamics)
• Inner region formation (dynamics)
• Snapshot approach (chemistry)
• Sensitive to the timing of condensation and equilibration (chemistry)
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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
• Low degrees of radial mixing
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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
• HD19994 1.35 MSUN F star, [Fe/H] = 0.23• 1.69 MJ at 1.43AU
• 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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Gl777A1.10 MEarth at 0.89AU
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HD72659
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HD726591.35 MEarth at 0.89AU
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HD72659
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HD726591.53 MEarth at 0.38AU
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Semimajor Axis (AU)
0.0 0.2 0.4 0.6 0.8 1.0
O
Fe
Mg
Si
C
S
Al
Ca
Other
HD72659
1.53 MEarth 1.35 MEarth
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HD19994
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HD199940.62 MEarth at 0.37AU
7 wt% C
45 wt%
16 wt% 32 wt%
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HD4203
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HD42030.17 MEarth at 0.28AU
53 wt% 43 wt%
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Two Classes
• Earth-like & refractory compositions (Gl777A, HD72659)
• C-rich compositions (HD19994, HD4203)
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Mg/Si
0.5 1.0 1.5 2.0
C/O
0.0
0.5
1.0
1.5
MgSiO3 + Mg2SiO4
MgSiO3 +
SiO2 species
SiO
SiC
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Mg/Si
0.5 1.0 1.5 2.0
C/O
0.0
0.5
1.0
1.5
Solar
MgSiO3 + Mg2SiO4
MgSiO3 +
SiO2 species
SiO
SiC
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Mg/Si
0.5 1.0 1.5 2.0
C/O
0.0
0.5
1.0
1.5
Solar
HD19994
MgSiO3 + Mg2SiO4
HD72659MgSiO3 +
SiO2 species
SiO
SiC
HD4203
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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, planetary detection . . .
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Water and Habitability
• All planets form “dry”• Exogenous delivery and adsorption
limited in C-rich systems – Hydrous species– Water vapor restricted
• 6 Earth-like planets produced in habitable zone
• Ideal targets for future surveys
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Take-Home Message
• Extrasolar planetary systems are enriched but with normal evolutions
• Dynamical models predict that terrestrial planets are common
• Two main types of planets:1. Earth-like2. C-rich
• Wide variety of planetary implications
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Frank Zappa
There is more stupidity than hydrogen in the universe, and it has
a longer shelf life.
Frank Zappa
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Questions?
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Just in case . . .
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Mars fractionation line
Al/Si (weight ratio)
0.00 0.05 0.10 0.15 0.20
Mg
/Si (
wei
gh
t ra
tio
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4Earth fractionation line
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Hersant Model
• P gradient– 1/ρ(dP/dz) = -Ω2z – 4πGΣ
• Heat flux gradient– dF/dz = (9/4) ρΩ
• T gradient– dT/dz = -T/
• Surface density gradient– d Σ /dz = ρ