Planetesimal Compositions in Exoplanet...

Planetesimal Compositions in Exoplanet SystemsTorrence V. Johnson1, J. I. Lunine2, and O. Mousis3

(1)Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA; (2) Dipartimento di Fisica, Università degli Studi di Roma "Tor Vergata", 00133 Rome, Italy

(3) Institut UTINAM, CNRS-UMR 6213, Observatoire de Besançon, Besançon, France.

4. Summary

Acknowledgements. TVJ’s work has been conducted at the Jet PropulsionLaboratory, California Institute of Technology, under a contract with NASA.Government sponsorship acknowledged.

• Exoplanet systems around stars with different compositions from theSun may have planetesimals formed beyond the ‘’snow line’’ whichhave a greater range of rock and metal, carbon and ice proportionsthan solar system planetesimals.• The fraction of rock plus metal in extrasolar planetesimals is moststrongly dependent on the C/O ratio in their stellar nebula rather thanmetallicity, [Fe/H].• These characteristics may be investigated for extrasolar systems inthe future through their impact on the refractory and volatile content ofextrasolar planetesimal belts and the amount of heavy elementenrichment of extrasolar giant planets (e.g. Mousis et al. 2009, [9]).

[1] Wong, M. H., et al., in Oxygen in the Solar System, G. J. MacPherson, Ed. (Mineralogical Society of America, Chan-tilly, VA, 2008), vol. Reviews in Mineralogy and Geochemi-stry Vol. 68, pp. 241-246. [2] Prinn, R. G. and B. Fegley (1981) Astrophys. J. 249, 308-317. [3] Gonzales, G., et al. (2001) Astron. J. 121, 432-452. [4] Takeda, Y., et al. (2001) Publications of the Astronomi-cal Society of Japan 53, 1211-1221. [5] Bond, J. C., et al. (2010) Astrophys. J. 715, 1050-1070. [6] Fossati, L., et al. (2010) Astrophys. J. 720, 872-886. [7] Johnson, T. V. and J. I. Lunine (2005) Nature 435, 69-71. [8] Asplund, M., et al. (2009) Annual Review of Astronomy and Astrophysics 47, 481-522. [9] Mousis, O., et al. (2009) Astron. Astrophys. 507, 1671-1674. [10] Mousis, O., et al. (2011) Astrophys. J. 727:77 (7pp)[11] Mousis, O., et al. (2009) Astrophys. J. 696, 1348-1354.

(a)

(b)

Figure 1 Rock-metal fraction f(r-m) vs C/O. [C/O] is expressed as logarithmicunits, dex, relative to the Sun = 0, [C/O] = log (C/O)star – log (C/O)Sun. (a)Oxidizing: All carbon in gaseous phase as CO (b) Reducing: All carbon ingaseous phase as CH4. Solar values are from Asplund et al. (2009) [8].

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1. The Solar SystemIn our solar system, the composition of planetesimals formed beyondthe ‘snow line’ is believed to depend strongly on the abundances of Cand O in the solar nebula, the redox state of C (i.e. CO rich vs. CH4rich regions), and the amount of carbon in the form of solid organics.These factors largely determine the fraction of rock plus metal, f(r–m),versus volatile ice in condensates, and therefore the material(uncompressed) density of the condensates [1].

For “warm” nebula models where mid-plane temperatures are >~50K,CO and CH4 remain in the gaseous state and water ice (and somehydrates) is the primary condensed volatile ice. In the outer solarnebula, CO is expected to be the dominant C-bearing gas, resulting inless O available to form water ice, and therefore higher values of f(r-m). In warmer circumplanetary environments with higher density,kinetic considerations are expected to produce more reducingconditions, with CH4 being the primary carbon species [2], resulting inmore water ice and lower values of f(r-m). In “cool” nebula models,with lower mid-plane temperatures, more volatile ices and clathrates,including CO, CH4 and, H2S, are added to the water ice and f(r-m) islower.

2. Exoplanet Systems

Exoplanet host stars have observed metallicities that cluster aroundthe solar value or higher. However, the detailed compositions for ma-jor planet forming elements are observed to differ significantly.Assuming this reflects equivalent differences in the protoplanetarynebulae from which planets formed in these systems, we can estimatethe range of possible f(r-m) in systems for which stellar compositiondata are available (the most important solid forming elements being C,O, Si, S, Fe and Ni for these purposes). Stellar data are from twosurveys [3, 4], with a total of 25 exoplanet host stars, and ten starscompiled for a study of diversity in extrasolar terrestrial planets [5].Also, an example of a host star of a transiting planet, WASP-12 wasincluded [6]. We calculated f(r-m) for these compositions, assumingwater is the only condensed volatile, following the solar compositionscheme developed in [1,7]. As expected, the condensate rock plusmetal fraction is generally correlated with metallicity, [Fe/H]. However,it is even more strongly influenced by the C/O ratio. Figure 1 shows f(r-m) vs. [C/O], in dex with Sun=0, for both the oxidizing and reducingnebula cases. Note that for C/O > ~0.8 both cases become ‘oxygen’starved, with little or no water ice for the most carbon-rich stars.

The total range of f(r-m) possible in these systems isconsiderably greater than the solar case (~ 0.47-0.76).

3. Composition of Planetesimals vs Stellar C/OTo explore the range of planetesimal composition as function of theC/O ratio of the host star, we combined the f(r-m) calculation from [1]with a more complete thermodynamic treatment of the volatile ices[9,10,11]. We used the set of stars compiled by Bond et al. [5],covering a wide range of C/O stellar values and calculated the massfraction of each condensed species for both “warm” and “cool”nebular cases. Silicate plus metal fractions for the “warm” case aresimilar to the values in Fig. 1, reduced by the addition of smallamounts of ices containing CO2 (oxidizing) and NH3 hydrates(reducing). For the “cool” cases, larger amounts of CO and CH4 icesreduce f(r-m) still further . Fig. 2 illustrates the relative proportions ofthe condensed phases for oxidizing conditions over a range of stellarC/O. Note that for the extreme carbon-rich star HD 4203, lack of Oproduces a shift to reducing conditions.

References

Fig. 2 Planetesimal Composition Warm Nebula - Oxidizing Cool Nebula - Oxidizing

Warm Nebula - Reducing Cool Nebula - Reducing

Silicate+Metal39%

H2O35%

CH40%

CH3OH1%

CO15%

CO25%

N23%

NH31% H2S

1%PH30%

HD19994 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal39%

H2O17%

CH40%

CH3OH2%

CO25%

CO211%

N23%

NH31%

H2S2%

PH30%

Sun  

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal41%

H2O0%

CH43%CH3OH

3%

CO34%

CO216%

N22%

NH30%

H2S1%

PH30%

HD108874 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal30%

H2O42%

CH40%

CH3OH1%

CO18%

CO27%

N21%

NH30%

H2S1%

PH30%

HD177830 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal48%

H2O11%

CH40%

CH3OH2%

CO25%

CO211%

N22%

NH30%

H2S1%

PH30%

HD213240 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal40%

H2O5%

CH40%CH3OH

3%

CO33%

CO215%

N22%

NH30%

H2S2%

PH30%

Gl777A 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal35%

H2O4%CH4

0%

CH3OH3%

CO38%

CO217%

N22%

NH30% H2S

1%PH30%

HD72659 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal53%

H2O38%

CH40%

CH3OH1%

CO0%

CO26%

N20%

NH31% H2S

1%

PH30%

HD19994 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

Silicate+Metal65%

H2O18%

CH40%

CH3OH2% CO

0%CO212%

N20%

NH31% H2S

2% PH30%

Sun  

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal77%

H2O0%

CH40%

CH3OH3%

CO0%

CO218%

N20%

NH30%

H2S2% PH3

0%

HD108874 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

PH3

Silicate+Metal42%

H2O48%

CH40%

CH3OH1%

CO0% CO2

8%N20%

NH30% H2S

1% PH30%

HD177830 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

Silicate+Metal75%

H2O11%

CH40%

CH3OH2%

CO0%

CO211%

N20%

NH30%

H2S1%

PH30%

HD213240 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

Silicate+Metal73%

H2O5%

CH40%

CH3OH3%

CO0%

CO217%

N20%

NH30%

H2S2% PH3

0%

Gl777A 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

Silicate+Metal70%H2O

5%

CH40%

CH3OH4%

CO0%

CO220%

N20%

NH30% H2S

1% PH30%

HD72659 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

Silicate+Metal66%

H2O11%

CH419%

CH3OH0%

CO0%

CO20%

N20%

NH33% H2S

1%

PH30%

HD4203 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

Silicate+Metal66%

H2O11%

CH419%

CH3OH0%

CO0%

CO20%

N20%

NH33% H2S

1%

PH30%

HD4203 

Silicate+Metal

H2O

CH4

CH3OH

CO

CO2

N2

NH3

H2S

STAR C/O

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STAR C/O

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STAR C/O

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STAR C/O

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STAR C/O

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STAR C/O

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