The Diversity of Extrasolar Terrestrial

Planets

J. Bond, D. Lauretta & D. O’Brien

USyd Colloquium14th July 2008

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

Two Big Questions

1. Are terrestrial planets likely to exist in known extrasolar planetary

systems?

2. What would they be like?

?

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

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

Equilibrium thermodynamics predict bulk compositions of

planetesimals

“Ground Truthing”

• Consider the CJS1 system:– 1.15 MEarth at 0.64AU

– 0.81 MEarth at 1.21AU

– 0.78 MEarth at 1.69AU

Results

Increasing Volatility

Al Ti Ca Mg Si O Ni Fe Cr P Na S H

En

rich

men

t F

acto

r

0

2

4

6

8

10

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

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

Extrasolar “Earths”• Terrestrial planets formed in ALL systems studied

• Most <1 Earth-mass within 2AU of the host star

• Often multiple terrestrial planets formed

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

Gl777A

Gl 777A• 1.10 MEarth at 0.89AU

HD72659

HD72659• 1.03 MEarth at 0.95AU

HD75732 (55Cnc)

HD75732 (55Cnc)

HD75732 (55Cnc)

HD75732 (55Cnc)• 0.99 MEarth at 1.25AU

7 wt% C

HD4203

HD4203• 0.17 MEarth at 0.28AU

53 wt% C

HD4203• 0.17 MEarth at 0.28AU

Two Classes

• Earth-like compositions (Gl777A, HD72659)

• C-rich compositions (55 Cnc, HD4203)

Mg/Si

0.5 1.0 1.5 2.0 2.5

C/O

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Mg2SiO4 + MgSiO3

SiO

SiC

MgSiO3 + other

SiO2 speciesMg2SiO4 + other

Mg species

Mg/Si

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

C/O

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Mg2SiO4 + MgSiO3

SiO

SiC

MgSiO3 + other

SiO2 species

Solar

Mg/Si

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

C/O

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Mg2SiO4 + MgSiO3

SiO

SiC

MgSiO3 + other

SiO2 species HD72659

55Cnc

HD4203

Solar

Mg/Si

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

C/O

0.0

0.5

1.0

1.5

2.0

2.5

Mg2SiO4 + MgSiO3

SiO

SiC

MgSiO3 + other

SiO2 species

Solar

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 . . .