Abstract

So far, a young A5V star HR 8799 hosts the only multi-planet system detected using the direct infrared imaging. It consists of four planets of ~10 Jupiter masses in wide orbits between ~25 and ~70 au. A determination of the orbital parameters is a challenging task because the observational windows of ~2 to ~13 years are narrow when compared to the orbital periods between ~50 and ~500 years, and the system is dynamically packed. Many different orbital configurations are possible. A requirement of stable system for the star lifetime makes the problem even more difficult. In a recent paper (Gozdziewski & Migaszewski, 2013), we have proposed a new self-consistent algorithm to constrain the orbital parameters through the planetary migration. Our method relies on the assumption that the planets were formed in wide orbits and then migrated inwards to their current positions. We found basically unique solution in which the planets are involved in a double Laplace mean motion resonance 1:2:4:8. This is the first known rigorously stable configuration of this type in the literature. It is also the first orbital model of the HR 8799 system, which is fully consistent with independent mass estimates from cooling models, debris disk analysis and the star orientation in space. Moreover, the new fitting method makes it possible to predict stable configurations with the fifth, innermost yet undetected planet with ~6 Jupiter masses involved in a triple Laplace resonance 1:2:4:8:16 or in the 1:3:6:12:24 mean motion resonance.

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