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
[ Y/H ]
0.50
Mean [Y/H]
Host: -0.05 + 0.03
Non-Host: -0.16 + 0.01
0.00
-0.50
-0.50
0.00
[Y/H] Slope
Host: 0.87
Non-Host: 0.78
0.50
[ Fe/H ]
[ Eu/H ]
0.50
Mean [Eu/H]
Host: -0.10 + 0.03
Non-Host: -0.16 + 0.02
0.00
-0.50
-0.50
0.00
[ Fe/H ]
0.50
[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
10.00
1.00
5.00
0.50
e
M sini (MJup)
Host Star Enrichment
0.00
-0.50
0.00
-0.40
-0.30
-0.20
-0.10
0.00
0.10
0.20
0.30
-0.50
-0.40
-0.30
-0.20
[ Y/H ]
0.10
0.20
0.30
0.00
0.10
0.20
0.30
4000
Period (days)
4.00
a (AU)
0.00
[ Y/H ]
5.00
3.00
2.00
3000
2000
1000
1.00
0
0.00
-0.50
-0.10
-0.40
-0.30
-0.20
-0.10
[ Y/H ]
0.00
0.10
0.20
0.30
-0.50
-0.40
-0.30
-0.20
-0.10
[ Y/H ]
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
Enrichment Factor
6
4
2
0
Al
Ti
Ca Mg
Si
O
Ni
Fe
Cr
P
Increasing Volatility
Na
S
H
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
Gl777A
1.10 MEarth at 0.89AU
Enrichment Factor
6
4
2
0
Al
Ti Ca Mg Si
O
Ni Fe Cr
P
Increasing Volatility
Na
S
HD72659
HD72659
1.35 MEarth at 0.89AU
Enrichment Factor
6
4
2
0
Al
Ti Ca Mg Si
O
Ni Fe Cr
Increasing Volatility
P Na S
HD72659
HD72659
1.53 MEarth at 0.38AU
Enrichment Factor
30
20
10
0
Al
Ti Ca Mg Si
O
Ni Fe Cr
Increasing Volatility
P Na S
HD72659
1.53 MEarth
0.0
0.2
0.4
1.35 MEarth
0.6
Semimajor Axis (AU)
0.8
1.0
O
Fe
Mg
Si
C
S
Al
Ca
Other
HD19994
HD19994
0.62 MEarth at 0.37AU
7 wt% C
16 wt%
45 wt%
32 wt%
Fe
Si
C
Other
HD4203
HD4203
0.17 MEarth at 0.28AU
53 wt%
43 wt%
Fe
Si
C
Other
Two Classes
• Earth-like & refractory
compositions (Gl777A,
HD72659)
• C-rich compositions
(HD19994, HD4203)
1.5
C/O
1.0
SiC
SiO
0.5
MgSiO3 +
0.0
MgSiO3 + Mg2SiO4
SiO2 species
0.5
1.0
1.5
Mg/Si
2.0
1.5
C/O
1.0
SiC
SiO
Solar
0.5
0.0
MgSiO3 +
SiO2 species
0.5
MgSiO3 + Mg2SiO4
1.0
1.5
Mg/Si
2.0
HD4203
1.5
HD19994
C/O
1.0
SiC
SiO
Solar
0.5
0.0
HD72659
MgSiO3 +
SiO2 species
0.5
MgSiO3 + Mg2SiO4
1.0
1.5
Mg/Si
2.0
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-like
2. C-rich
•
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
Questions?
Just in case . . .
1.4
Earth fractionation line
Mg/Si (weight ratio)
1.2
1.0
0.8
0.6
0.4
Mars fractionation line
0.2
0.0
0.00
0.05
0.10
Al/Si (weight ratio)
0.15
0.20
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