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