The Chemistry of Extrasolar Planetary

Systems

Jade BondPhD 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

-0.50

0.00

0.50

-0.50 0.00 0.50

[ Fe/H ]

[ E

u/H

]

-0.50

0.00

0.50

-0.50 0.00 0.50

[ Fe/H ]

[ Y

/H ]

Host Star EnrichmentMean [Y/H]

Host: -0.05 + 0.03Non-Host: -0.16 + 0.01

Mean [Eu/H]Host: -0.10 + 0.03

Non-Host: -0.16 + 0.02

[Y/H] SlopeHost: 0.87

Non-Host: 0.78

[Eu/H] SlopeHost: 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

0.00

5.00

10.00

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

[ Y/H ]

M s

ini (

M Ju

p)

0.00

1.00

2.00

3.00

4.00

5.00

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

[ Y/H ]

a (A

U)

0.00

0.50

1.00

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

[ Y/H ]

e

0

1000

2000

3000

4000

-0.50 -0.40 -0.30 -0.20 -0.10 0.00 0.10 0.20 0.30

[ Y/H ]

Per

iod

(day

s)

Host Star Enrichment

• No correlation with planetary parameters

• Enrichment is PRIMORDIAL not photospheric pollution

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

• 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

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 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

Assumptions

• 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

• HD19994 1.35 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

Gl777A

Gl777A1.10 MEarth at 0.89AU

HD72659

HD726591.35 MEarth at 0.89AU

HD72659

HD726591.53 MEarth at 0.38AU

Semimajor Axis (AU)

0.0 0.2 0.4 0.6 0.8 1.0

O

Fe

Mg

Si

C

S

Al

Ca

Other

HD72659

1.53 MEarth 1.35 MEarth

HD19994

HD199940.62 MEarth at 0.37AU

7 wt% C

45 wt%

16 wt% 32 wt%

HD4203

HD42030.17 MEarth at 0.28AU

53 wt% 43 wt%

Two Classes

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

• C-rich compositions (HD19994, HD4203)

Mg/Si

0.5 1.0 1.5 2.0

C/O

0.0

0.5

1.0

1.5

MgSiO3 + Mg2SiO4

MgSiO3 +

SiO2 species

SiO

SiC

Mg/Si

0.5 1.0 1.5 2.0

C/O

0.0

0.5

1.0

1.5

Solar

MgSiO3 + Mg2SiO4

MgSiO3 +

SiO2 species

SiO

SiC

Mg/Si

0.5 1.0 1.5 2.0

C/O

0.0

0.5

1.0

1.5

Solar

HD19994

MgSiO3 + Mg2SiO4

HD72659MgSiO3 +

SiO2 species

SiO

SiC

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:1. Earth-like2. C-rich

• Wide variety of planetary implications

Frank Zappa

There is more stupidity than hydrogen in the universe, and it has

a longer shelf life.

Frank Zappa

Questions?

Just in case . . .

Mars fractionation line

Al/Si (weight ratio)

0.00 0.05 0.10 0.15 0.20

Mg

/Si (

wei

gh

t ra

tio

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4Earth fractionation line

Hersant Model

• P gradient– 1/ρ(dP/dz) = -Ω2z – 4πGΣ

• Heat flux gradient– dF/dz = (9/4) ρΩ

• T gradient– dT/dz = -T/

• Surface density gradient– d Σ /dz = ρ