With the high number of extrasolar planets discovered by now, it becomes possible to constrain theoretical formation models in a statistical sense. This paper is the first in a series in which we carry out a large number of planet population synthesis calculations. We begin the series with a paper mainly dedicated to the presentation of our approach, but also the discussion of a representative synthetic planetary population of solar like stars. Based as tightly as possible on observational data, we have derived probability distributions for the most important initial conditions for the planetary formation process. We then draw sets of initial conditions from these distributions and obtain the corresponding synthetic planets with our formation model. Although the main purpose of this paper is the description of our methods, we present some key results: We find that the variation of the initial conditions in the limits occurring in nature leads to the formation of planets of large diversity. This formation process is best visualized in planetary formation tracks, where different phases of concurrent growth and migration can be identified. These phases lead to the emergence of sub-populations of planets distinguishable in a mass-semimajor axis diagram. The most important ones are the "failed cores", a vast group of core-dominated low mass planets, the "horizontal branch", a sub-population of Neptune mass planets extending out to 6 AU, and the "main clump", a concentration of giant gaseous giants planets at around 0.3-2 AU.
Deep Dive into Extrasolar planet population synthesis I: Method, formation tracks and mass-distance distribution.
With the high number of extrasolar planets discovered by now, it becomes possible to constrain theoretical formation models in a statistical sense. This paper is the first in a series in which we carry out a large number of planet population synthesis calculations. We begin the series with a paper mainly dedicated to the presentation of our approach, but also the discussion of a representative synthetic planetary population of solar like stars. Based as tightly as possible on observational data, we have derived probability distributions for the most important initial conditions for the planetary formation process. We then draw sets of initial conditions from these distributions and obtain the corresponding synthetic planets with our formation model. Although the main purpose of this paper is the description of our methods, we present some key results: We find that the variation of the initial conditions in the limits occurring in nature leads to the formation of planets of large divers
As of spring 2009, more than 300 extrasolar planets have been discovered (J. Schneider's Extrasolar Planet Encyclopedia at http://exoplanet.eu). The richness and diversity of the characteristics of these exoplanets like their mass or semimajor axis is impressive, and was not necessarily expected from the single example -our own solar system -that was available to study before the discovery of 51 Peg b (HD 217014b) by Mayor & Queloz (1995).
Since then, the observational field of extrasolar planet search has seen a rapid evolution leading to numerous additional discoveries of planets orbiting other stars. These discoveries have also triggered numerous theoretical studies about the formation and evolution of these planets. Key physical processes in planet formation and evolution could be identified whose importance was not fully realized in previous works based on the solar system alone. Some of these discovered planets, and multiple planetary systems, are sufficiently interesting by themselves to warrant individual theoretical studies. Examples are the extrasolar planetary system with three Neptune-mass planets around HD 69830 (Lovis et al. 2006;Alibert et al. 2006), or the transiting Neptune mass planet GJ 436b (Butler et al. 2004;Gillon et al. 2007;Figueira et al. 2008). Of course, the giant planets in our own solar system provide a much larger and detailed set of constraints than any known extrasolar planet. Therefore, each formation model applied to discuss extrasolar planet formation should also be put to the test to reproduce the characteristics of our own giant planets (Pollack et al. 1996, Alibert et al. 2005b;Hubickyj et al. 2005;Benvenuto & Brunini 2005).
The modeling of the formation of such single systems while a necessary condition to validate formation models is not satisfactory by itself. Indeed, the number of model parameters is generally large while the number of constraints deriving from a single system is small, and not strong enough to completely constrain any formation model.
Thanks to the rapid growth of the number of known extrasolar planets, the situation has however dramatically changed:
Instead of having only a single object or a single system to study, we now begin to be able to describe an entire population of extrasolar planets orbiting FGK stars in the solar neighborhood. While this population is still smaller than one would ideally like, it nevertheless already allows to extract statistically a wealth of information (e.g. Udry & Santos 2007;Cumming et al. 2008) to constrain formation models that exceeds by far what one extrasolar planet can do. This is especially true since most of the extrasolar planets have been discovered by radial velocity measurements so that only a few orbital elements and a minimum mass are known for one individual object. For the growing number of transiting planets more physical properties can be derived and compared with internal structure models (Baraffe et al. 2008;Figueira et al. 2008). Unfortunately, transiting planets known so far are all in close proximity to their host star. Hence it is sometimes unclear to what extend their characteristics are still related to their formation or rather to subsequent evolution (e.g. evaporation).
Parallel to the discovery of more and more end-products of the planetary formation process i.e. planets, large observational progress (e.g. Meyer et al. 2006) has also been made in characterizing the initial conditions for this process, i.e. the protoplanetary disks. Thanks to these observations, we begin to be able to determine the probability of occurrence of any particular initial condition for planetary formation, like disk metallicity, mass or lifetime.
With these two sets of observational data at hand, a new interesting class of theoretical planet formation studies has become possible, where a theoretical model serves as the link between these two groups of observations: The synthesis of populations of planets by Monte Carlo methods. In this approach the observed distributions of disk properties are used as varying initial conditions for the model. The final characteristics of the synthetic planets that form in the model can then be compared statistically to those of the actual observed populations. This addresses the question if the observed diversity of extrasolar planets is simply the consequence of the diversity of disk properties.
As we shall show, such studies have proven to be very fruitful, as they not only allow to reproduce observations but also show the links and correlations between the different initial conditions and the characteristics of the resulting planets. Thereby they provide great insights into the formation mechanism. Last but not least, such an approach by predicting the actually existing planet population as opposed to the actually detected one, allows to optimize future searches and instruments when coupled to a synthetic detection bias for a particular detection method.
Compared
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