Abstract
ABSTRACT The noteworthy four-planet HR 8799 system teeters on the brink of gravitational instability and contains an A-type host star that is characteristic of the progenitors of the majority of known white dwarf planetary system hosts. Goździewski & Migaszewski have demonstrated that the system can retain all four planets for at least 1 Gyr along the main sequence if the planets evolve within an externally unperturbed 8:4:2:1 mean motion resonance configuration. Here, we propagate forward their most stable fit beyond the main sequence, and incorporate external effects from Galactic tides and stellar fly-bys. We find that (i) giant branch mass-loss always breaks the resonance, and usually triggers the ejection of two of the planets, (ii) stellar fly-bys and Galactic tides rarely break the resonance during the main-sequence and giant branch phases, but play a crucial role in determining the final planetary configurations around the eventual white dwarf host star, and (iii) the meanderings of the surviving planets vary significantly, occupying regions from under 1 au to thousands of au. The ubiquitous survival of at least one planet and the presence of the debris discs in the system should allow for dynamical pathways for the white dwarf to be metal-polluted.
Highlights
Over a decade after its discovery (Marois et al 2008, 2010), the HR 8799 planetary system remains a benchmark of exoplanetary science
Galactic tides and stellar flybys always altered the planets’ orbital parameters, these variations were at a small enough level to maintain the resonance
We have modelled the fate of the HR 8799 planetary system under the assumption that all four planets will survive until the end of the main sequence in a resonant configuration
Summary
Over a decade after its discovery (Marois et al 2008, 2010), the HR 8799 planetary system remains a benchmark of exoplanetary science. A host of age indicators for HR 8799, including its galactic space motion (indicating a high likelihood of being a member of the Columba moving group) and the placement of the star in a colourmagnitude diagram suggest a best-estimate age of ≈30 Myr (Zuckerman et al 2011; Malo et al 2013). Based on this age estimate, as well as the measured near-infrared brightnesses of the co-moving companions, Marois et al (2008) assigned mass estimates of 7+−42, 10+−33, and 10+−33 MJup for the HR 8799bcd planets, respectively. Subsequent observations of this system at the more favourable observing wavelength of 3.8 μm (Marois et al 2010) unveiled the direct detection of a fourth planet (“e”) at a significantly smaller projected orbital separation of 14.5±0.4 au, and an estimated mass of 7+−32 MJup, similar to that of the “c” and “d” companions
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