Download Mount Stromlo Colloquium

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
WARNING:
SOMEWHAT
CONTROVERSIAL
SiC
SiO
MgSiO3 +
SiO2
MgSiO3 + Mg2SiO4
Mg2SiO4 +
MgO
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
HD108874
SiC
Gl777
SiO
MgSiO3 +
SiO2
HD72659
MgSiO3 + Mg2SiO4
Mg2SiO4 +
MgO
HD72659
HD72659
O
Fe
Mg
Si
C
S
Al
Ca
Other
1.03 M Earth
0.95AU
C/O = 0.40
Gl777
Gl777
O
Fe
Mg
Si
C
S
Al
Ca
Other
1.10 M Earth
0.89AU
C/O = 0.78
HD108874
HD108874
O
Fe
Mg
Si
C
S
Al
Ca
Other
0.46 M Earth
0.38AU
C/O = 1.35
HD72659
Gl777
HD108874
O
Fe
Mg
Si
C
S
Al
Ca
Other
1.03 M Earth
0.95AU
C/O = 0.40
1.10 M Earth
0.89AU
C/O = 0.78
0.46 M Earth
0.38AU
C/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
Atmospheres
Solar 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
H2S
CO
NH3
PH3
N2
CO2
CH4
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
There is more stupidity than hydrogen in the
universe, and it has a longer shelf life.
Frank
Frank Zappa
Zappa