The Diversity of Extrasolar Terrestrial Planets

1. The Diversity of Extrasolar Terrestrial Planets J. Bond, D. Lauretta & D. O’Brien USyd Colloquium 14th July 2008

2. 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 • Combine dynamical models of terrestrial planet formation with chemical equilibrium models of the condensation of solids in the protoplanetary nebulae

3. Two Big Questions • Are terrestrial planets likely to exist in known extrasolar planetary systems? • What would they be like?

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9. 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) • Incorporate dynamical friction • Start with 25 Mars mass embryos and ~1000 planetesimals from 0.3 AU to innermost giant planet • Neglects mass loss

10. 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

11. Equilibrium thermodynamics predict bulk compositions of planetesimals

12. “Ground Truthing” • Consider the CJS1 system: • 1.15 MEarth at 0.64AU • 0.81 MEarth at 1.21AU • 0.78 MEarth at 1.69AU

13. Results

14. Results • Reasonable agreement with planetary abundances • Values are within 1 wt%, except for Mg, O and S and Si (EJS only) • Deviations: • Mg ~ 5 wt% • O & S ~ 4 wt% • Si ~ 2 wt% (EJS only) • Mg/Si ratio less than planetary (0.47-0.76), implying there is some other way to fractionate one or both of these elements in the early Solar System

15. 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 • Varied positions and masses of known giants and stellar mass • Assumed closed systems

16. Extrasolar “Earths” • Terrestrial planets formed in ALL systems studied • Most <1 Earth-mass within 2AU of the host star • Often multiple terrestrial planets formed

17. 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 • HD75732 (55Cnc) - 1.03 MSUN G star, [Fe/H] = 0.33 • 0.05 MJ at 0.04AU • 0.78 MJ at 0.12AU • 0.22 MJ at 0.24AU • 3.92 MJ at 5.26AU • HD4203 – 1.06 MSUN G star, [Fe/H] = 0.22 • 2.10 MJ planet at 1.09AU

18. Gl777A

19. Gl 777A • 1.10 MEarth at 0.89AU

20. HD72659

21. HD72659 • 1.03 MEarth at 0.95AU

22. HD75732 (55Cnc)

23. HD75732 (55Cnc)

24. HD75732 (55Cnc)

25. HD75732 (55Cnc) • 0.99 MEarth at 1.25AU 7 wt% C

26. HD4203

27. HD4203 • 0.17 MEarth at 0.28AU 53 wt% C

28. HD4203 • 0.17 MEarth at 0.28AU

29. Two Classes • Earth-like compositions (Gl777A, HD72659) • C-rich compositions (55 Cnc, HD4203)

34. 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 . . .