Long-term stability of the HR 8799 planetary system without resonant lock

Abstract

HR 8799 is a star accompanied by four massive planets on wide orbits. The observed planetary configuration has been shown to be unstable on a timescale much shorter than the estimated age of the system (~ 30 Myr) unless the planets are locked into mean motion resonances. This condition is characterised by small-amplitude libration of one or more resonant angles that stabilise the system by preventing close encounters. We simulate planetary systems similar to the HR 8799 planetary system, exploring the parameter space in separation between the orbits, planetary masses and distance from the Sun to the star. We find systems that look like HR 8799 and remain stable for longer than the estimated age of HR 8799. None of our systems are forced into resonances. We find, with nominal masses and in a narrow range of orbit separations, that 5 of 100 systems match the observations and lifetime. Considering a broad range of orbit separations, we find 12 of 900 similar systems. The systems survive significantly longer because of their slightly increased initial orbit separations compared to assuming circular orbits from the observed positions. A small increase in separation leads to a significant increase in survival time. The low eccentricity the orbits develop from gravitational interaction is enough for the planets to match the observations. With lower masses, but still comfortably within the estimated planet mass uncertainty, we find 18 of 100 matching and long-lived systems in a narrow orbital separation range. In the broad separation range, we find 82 of 900 matching systems. Our results imply that the planets in the HR 8799 system do not have to be in strong mean motion resonances.

Accepted for publication in A&A

References in corpus (8)

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• Dynamical Outcomes of Planet-Planet Scattering
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• The Gemini Planet Imager: First Light
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• Debris Distribution in HD 95086 - A Young Analog of HR 8799
• Near-Infrared Detection and Characterization of the Exoplanet HD 95086 b with the Gemini Planet Imager
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Cited by in corpus (9)

• VLT/SPHERE robust astrometry of the HR8799 planets at milliarcsecond-level accuracy Orbital architecture analysis with PyAstrOFit
• The First Dynamical Mass Measurement in the HR 8799 System
• The Pan-Pacific Planet Search VII: The most eccentric planet orbiting a giant star
• The Young Substellar Companion ROXs 12 B: Near-Infrared Spectrum, System Architecture, and Spin-Orbit Misalignment
• Shaping HR8799's outer dust belt with an unseen planet
• In-depth direct imaging and spectroscopic characterization of the young Solar System analog HD 95086
• The post-main-sequence fate of the HR 8799 planetary system
• A study of the frequency and characteristics of stellar companions and Jupiter-like planets in nearby open clusters
• Enrichment of the HR 8799 planets by minor bodies and dust