Introduction to ExoStab

Discoveries of Super-Earths raise our hopes to find terrestrial planets moving in the habitable zone (HZ) of Sun-like stars. The proof of habitability of such a planet is an interdisciplinary venture, including astrophysical, biological, geophysical and chemical aspects, to name only a few of an endlessly perpetuateable list.
Yet, one of the most basic requirements for habitability and a possible evolution of a biosphere is the long-term orbital stability of a planet in the HZ. Therefore, we compiled this application called ExoStab (Pilat-Lohinger, Eggl & Winkler, 2008), that allows to verify the long-term stability of an additional low-massed planet moving in a single-star single-planet system.

Intention and Dynamical Model

The application ExoStab allows to check whether a newly discovered planet in a single-star single-planet system moves inside a stable region, using results of the Exocatalogue, which is described in Sandor et al. (2007). In order to avoid confusion, we would like to introduce a definition of the term "stable" as an attribute of a phase space region, denoting that trajectories within stay ordered, whereas "chaotic" means, that trajectories are subject to instabilities due to chaotic diffusion. definition, we will The backbone of ExoStab is an SQL based search engine that picks the requested data out of Exocatalogue-files and transforms them into displayed stability plots via PhP routines. As the Exocatalogue has been compiled using normalized masses and distances, ExoStab performs a conversion to actual system units on the fly, thus aiming for increased user-friendliness.

The dynamical model used for generating stability maps via ExoStab was chosen to be the restricted three body problem (RTBP). This model consists of a star (S) and a known giant planet (GP) moving on Keplerian orbits around their common center of mass plus a newly discovered planet (P) considered to be a massless "test-particle". P moves in the gravitational field given by the other two masses S and GP without influencing their motion. Test-computations (Pilat-Lohinger et al, 2008) have shown, that the RTBP approximation to extra-solar planetary systems is valid if:

mass of P << mass of GP

eccentricity of P and eccentricity of GP are small (< 0.2)

If the real system's deviations from these assumptions stay moderate, the ExoStab-result can be taken as a first approximation to verify the degree of stability of the dynamical region, the new planet is being expected in. Up to now, ExoStab uses data from the Exocatalogue, where the test particle has been set on an initially cirular orbit. Currently both, P and GP, were started at peri-astron position only (mean anomaly of GP and P = 0 deg). Following preliminary results of Exocatalogue, alternative starting positions will have minor effects on the global picture of stability of configurations, except for changes near resonance induced structures.

References

Pilat-Lohinger et al.:ExoStab, in preparation
Sandor et al.:MNRAS, 375, p. 1495 (2007)
Kasting et al.:ICARUS, 101, p. 108 - 128 (1993)
Pilat-Lohinger et al.:Validity of the Restricted Three Body Problem approximation for extra-solar planetary systems, unpublished (2008)