The Chemistry of Extrasolar Planetary Systems J. Bond, D. O’Brien and D. Lauretta

Extrasolar Planets First detected in 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 MgSiO 3 + SiO 2 MgSiO 3 + Mg 2 SiO 4 Mg 2 SiO 4 + MgO

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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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 M Earth at 0.64AU –0.81 M Earth at 1.21AU –0.78 M Earth at 1.69AU

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 M SUN G star 3.30 M J planet at 4.16AU Gl777A – 1.04 M SUN G star 0.06 M J planet at 0.13AU 1.50 M J planet at 3.92AU HD – 1.00 M SUN G star 1.36 M J planet at 1.05AU 1.02 M J planet at 2.68AU

Extrasolar “Earths” [Fe/H]Mg/SiC/O HD Gl HD

HD72659

1.35 M Earth at 0.89AU

HD72659

1.53 M Earth at 0.38AU

HD M Earth 1.35 M Earth 1.53 M Earth 0.38 AU 1.35 M Earth 0.89 AU

Gl777A

1.10 M Earth at 0.89AU 0.27 wt% C

HD108874

0.46 M Earth at 0.38AU 27 wt% C 66 wt% C

HD M Earth at 0.38AU 66 wt% 27 wt%

Two Classes Earth-like & refractory compositions (HD72659) C-rich compositions (Gl777A, HD108874)

Gl777 SiC SiO MgSiO 3 + SiO 2 MgSiO 3 + Mg 2 SiO 4 Mg 2 SiO 4 + 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