The Diversity of Extrasolar Terrestrial Planets
J. Carter-Bond, D. O’Brien & C. Tinney
RSAA Colloquium
12 April 2012
Stars → building blocks → terrestrial planets
• In terms of extrasolar terrestrial planets, we generally consider other “Earth-like” planets– Metallic core, silicate mantle and crust– Atmosphere– Active on the planetary scale
• BUT extrasolar planetary systems are chemically and dynamically unusual!
• Since these different compositions may produce different planetary building blocks, what types of terrestrial planets may exist?
How common is Earth?
How common is Earth?
How common is Earth?
How common is Earth?
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
WARNING:SOMEWHAT
CONTROVERSIAL
SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
Chemistry meets Dynamics
• Most dynamical studies of planetesimal formation have neglected chemical constraints
• Most chemical studies of planetesimal formation have neglected specific dynamical studies
Combine dynamical models of terrestrial planet formation with chemical equilibrium models of the
condensation of solids in the protoplanetary nebulae
Dynamical simulations “produce” 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
• Use spectroscopic photospheric abundances of 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 radial P-T profiles
• Assume no volatile loss during accretion, homogeneity and equilibrium is maintained
Equilibrium thermodynamics predict bulk compositions of planetesimals
Extrasolar “Earths”
• Terrestrial planets formed in ALL systems studied
• Most <1 Earth-mass
• Often multiple terrestrial planets formed
Extrasolar “Earths”• Examine three ESP systems
• HD72659 – 0.95 MSUN G star, [Fe/H] = -0.14• 3.30 MJ planet at 4.16AU
• Gl777A – 1.04 MSUN G star, [Fe/H] = 0.24• 0.06 MJ planet at 0.13AU• 1.50 MJ planet at 3.92AU
• HD108874 – 1.00 MSUN G star, [Fe/H] = 0.23• 1.30 MJ planet at 1.05AU• 1.07 MJ planet at 2.75AU
Gl777SiC
SiO
MgSiO3 + SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 + MgO
HD72659
HD108874
HD72659
OFe MgSi CSAlCaOther
HD72659
1.03 M Earth
0.95AUC/O = 0.40
Gl777
OFe MgSi CSAlCaOther
Gl777
1.10 M Earth
0.89AUC/O = 0.78
HD108874
OFe MgSi CSAlCaOther
HD108874
0.46 M Earth
0.38AUC/O = 1.35
OFe MgSi CSAlCaOther
HD72659 Gl777 HD108874
1.03 M Earth
0.95AUC/O = 0.40
1.10 M Earth
0.89AUC/O = 0.78
0.46 M Earth
0.38AUC/O = 1.35
What About Migration?
Giant planet migration is believed to have occurred in many extrasolar planetary systems.
Migration can move a large amount material both inwards and outwards → possibly change final planet composition.
Consider Three Scenarios
1. In-Situ formation (Jupiter-mass body doesn’t migrate from 1AU)
2.Jupiter-mass body migrates from 5AU to 1AU
3.Jupiter-mass body migrates from 5AU to 0.25AU
Consider Three Compositions
1.Solar
2.Gl777 (C/O = 0.78)
3.HD4203 (C/O = 1.85)
Solar Composition:
Gl777 Composition:
HD4203 Composition:
Water?
• Water is primarily present as water ice and serpentine
• Significantly less water in high C/O systems– Less water ice– Restricted water vapor distribution
Water?
• Migration can deliver massive amounts of water to the “dry” inner regions of the planetary systems.
• In-situ – no water delivered
• Migration – water and hydrous phases delivered to inner regions, refractories delivered to outer regions
Water?
Water?
Example: 3.05 MEarth at 0.91AU
Atmospheres
• Atmospheric formation is a complex process combining capture, outgassing, loss, etc.
• For close in planets with Teff > 1000K (within about 0.1AU), we can approximate an atmospheric composition by assuming equilibrium between the crust and atmosphere
Atmospheres
Teff ~ 950K Teff ~ 270K
AtmospheresSolar Composition:
79% H2
20% H2O
1% H2S
CO and CO2 with ices
HD4203:
53% CO
42% H2
21% CO2
17%H2O
0.5%CH4
Atmospheres
Taken from Lissauer et al., 2011.
Biological Elements
• Lacking biological elements – N & P missing on all– H missing on in-situ
• Possible alternative delivery through migration and volatiles?
Biological Elements
• Condensates, Clathrates & Hydrates:
CH3OH NH3 CO2
H2S PH3 CH4
CO N2
STAY TUNED!
Terrestrial Planets are likely to be relatively common
• Large range of stellar compositions suggests a wide range of terrestrial planets
• Final planet compositions depend somewhat on giant planet migration history
• Wide variety of biological, chemical and planetary implications
Frank Zappa
There is more stupidity than hydrogen in the universe, and it has a longer shelf life.
Frank Zappa