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?

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

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?

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

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