The chemistry of extrasolar planetary systems
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The Chemistry of Extrasolar Planetary Systems. Jade Bond PhD Defense 31 st October 2008. 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). Neutron Capture Elements.

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The Chemistry of Extrasolar Planetary Systems

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The Chemistry of Extrasolar Planetary Systems

Jade Bond

PhD Defense

31st October 2008

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)

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

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

Host Star Enrichment

Mean [Y/H]

Host: -0.05 + 0.03

Non-Host: -0.16 + 0.01

[Y/H] Slope

Host: 0.87

Non-Host: 0.78

Mean [Eu/H]

Host: -0.10 + 0.03

Non-Host: -0.16 + 0.02

[Eu/H] Slope

Host: 0.56

Non-Host: 0.48

Host Star Enrichment

Host stars enriched over non-host stars

Elemental abundances are in keeping with galactic evolutionary trends

Host Star Enrichment

Host Star Enrichment

No correlation with planetary parameters

Enrichment is PRIMORDIAL not photospheric pollution

Two Big Questions

  • Are terrestrial planets likely to exist in known extrasolar planetary systems?

  • 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

  • Astrobiologically significant

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

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

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



  • 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

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


In-situ formation (dynamics)

Inner region formation (dynamics)

Snapshot approach (chemistry)

Sensitive to the timing of condensation and equilibration (chemistry)

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”

  • 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

    • HD199941.35 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



1.10 MEarth at 0.89AU



1.35 MEarth at 0.89AU



1.53 MEarth at 0.38AU


1.53 MEarth

1.35 MEarth



0.62 MEarth at 0.37AU

7 wt% C

16 wt%

32 wt%

45 wt%



0.17 MEarth at 0.28AU

53 wt%

43 wt%

Two Classes

  • Earth-like & refractory compositions (Gl777A, HD72659)

  • C-rich compositions (HD19994, HD4203)

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 . . .

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

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:



Wide variety of planetary implications

There is more stupidity than hydrogen in the universe, and it has a longer shelf life.

Frank Zappa

Frank Zappa


Just in case . . .

Hersant Model

P gradient

1/ρ(dP/dz) = -Ω2z – 4πGΣ

Heat flux gradient

dF/dz = (9/4) ρΩ

T gradient

dT/dz = -T/

Surface density gradient

d Σ /dz = ρ

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