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The Chemistry of Extrasolar Planetary Systems
J. Bond,
D. O’Brien and
D. Lauretta
Extrasolar Planets
• First detected in 1995
• 374 known planets
• Host stars appear metal-rich, esp. Fe
• Similar trends in Mg, Si, C, O, Ti, Al, Na, Mn,Co, Ni, Sc, V, Cu, Zr and Nd
Santos et al. (2003)
Host Star Enrichment
• Elemental abundances are in keeping with galactic evolutionary trends
• No correlation with planetary parameters
• Enrichment is PRIMORDIAL not photospheric pollution
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
Two Big Questions
1. Are terrestrial planets likely to exist in known extrasolar planetary systems?
2. What would they be like?
?
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
• Combine dynamical models of extrasolar terrestrial planet formation with chemical equilibrium models of the condensation of solids in the protoplanetary nebulae
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 innermost giant planet
• Incorporate dynamical friction
• Neglects mass loss
Equilibrium thermodynamics predict bulk compositions of planetesimals
Davis (2006)
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
“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
Results
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 (Al/Si and Mg/Si) between Earth and Mars
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)
• No planetesimals
• Assumed closed systems
Assumptions
• In-situ formation (dynamics)
• Inner region formation (dynamics)
• Snapshot approach; sensitive to the timing of condensation (chemistry)
• PRELIMINARY SIMULATIONS!
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
Extrasolar “Earths”
• HD72659 – 0.95 MSUN G star• 3.30 MJ planet at 4.16AU
• Gl777A – 1.04 MSUN G star• 0.06 MJ planet at 0.13AU• 1.50 MJ planet at 3.92AU
• HD108874 – 1.00 MSUN G star• 1.36 MJ planet at 1.05AU• 1.02 MJ planet at 2.68AU
Extrasolar “Earths”
[Fe/H] Mg/Si C/O
HD72659 -0.14 1.23 0.40
Gl777 0.24 1.32 0.78
HD108874 0.14 1.45 1.35
HD72659
HD726591.35 MEarth at 0.89AU
HD72659
HD726591.53 MEarth at 0.38AU
HD72659
1.53 MEarth 1.35 MEarth1.53 M Earth
0.38 AU1.35 M Earth
0.89 AU
Gl777A
Gl 777A1.10 MEarth at 0.89AU
0.27 wt% C
HD108874
HD1088740.46 MEarth at 0.38AU
27 wt% C66 wt% C
HD1088740.46 MEarth at 0.38AU
66 wt%
27 wt%
Two Classes
• Earth-like & refractory compositions (HD72659)
• C-rich compositions (Gl777A, HD108874)
Gl777SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
HD72659
HD108874
Implications
• Plate tectonics
• Atmospheric composition
• Biology
• Detectability
Habitability
• 10 Earth-like and 3 C-enriched planets produced in habitable zone
• Ideal targets for future surveys; Kepler
Water Worlds?
• All planets form “dry”• Giant planet migration is likely to increase
water content
• Exogenous delivery and adsorption limited in C-rich systems – Hydrous species– Water vapor restricted
Mass Distribution
• Carbide phases are refractory in nature
• Alternative mass distribution may be needed with high C systems
Mass Distribution
Where to next?
• Migration simulations– Hypothetical giant planet systems
• M-dwarfs– Difficult to obtain stellar abundances
• Alternative mass distributions– Require detailed disk models
• Planetary structures and processes– Equations of state for unusual compositions
Take-Home Message
• Extrasolar planetary systems are enriched but with normal evolutions
• Two main types of planets:1. Earth-like
2. C-rich
• Wide variety of planetary and astrobiological implications
Frank Zappa
There is more stupidity than hydrogen in the universe, and it has a
longer shelf life.
Frank Zappa
