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Planet 21

How does the chemical composition of stars influence planet formation?

Principal Investigators

  • Dr. Bertram Bitsch
    Max-Planck-Institut für Astronomie, Heidelberg


Protoplanetary discs, the birth places of planets, consist of mainly of gas and to a small fraction of dust. The dust grains can grow to pebbles (mm-cm in size) through coagulation and condensation of ices. These pebbles can then form planetesimals, which can then grow further by accreting other planetesimals or the small pebbles to form planetary embryos. Once the planetary embryos become big enough (several Earth masses), they can accrete a gaseous envelop to form eventually gas giants. During their growth, the planets migrate through the disc. This whole process is normally modeled in planet population synthesis simulations. However, several big questions remain.

One of the big questions in planet formation is: where do the first planetesimals form? Recent simulations seem to indicate that the water ice line could be the prime location for this, also aided by condensation of water vapor, where usually a 50:50 ratio between water and rock is assumed in these simulations.

In addition, planet formation depends strongly on the amount of dust available to grow to bigger objects. A higher dust-to-gas ratio or metallicity enhances growth and allows more efficient planetesimal and planet formation. The metallicity is determined through the host star abundance, mainly through the iron measurements, [Fe/H].

In planet formation models in the past, a change of [Fe/H] implied an overall change of all elements in the same fashion. However, we know from galactic chemical tracing and evolution that different elements (e.g. C, O, Mg, Si, Fe) are enriched with different factors. As a consequence, different element ratios, e.g. Mg/Fe, Si/Fe, C/O have different slopes as function of [Fe/H].

From chemical models it is clear that oxygen binds preferably with carbon compared to hydrogen. This implies that carbon binds large quantities of oxygen in CO and CO2, and only the remaining oxygen can form water. If the C/O ratio is large, it implies that less oxygen is available to form water. However, if less water is available, water condensation for grain growth and thus planetesimal formation might not work as efficiently. This proposal thus aims to answer the following questions by modeling the growth of planetesimals and subsequent planet formation including planetesimal and pebble accretion as well as planet migration in a single and N-body framework:

1) How do the different volatile abundances (H2O, CO or CO2) influence the formation of pebbles and planetesimals?

2) What influences do the overall abundances of elements have on the chemical composition of planets?

3) Are the observable properties of formed planets different for host stars with different chemical composition?


Jingyi Mah

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