Statistics of Magrathea exoplanets beyond the main sequence

Abstract

Context. Notwithstanding the tremendous growth of the field of exoplanets in the last decade, limited attention has been paid to the planets around binary stars, which represent a small fraction of the total discoveries to date. Circumbinary planets (CBPs) have been discovered primarily with transit and eclipse timing variation methods, mainly around main sequence (MS) stars. No exoplanet has been found orbiting double white dwarf (DWD) binaries yet. Aims. In the interest of expanding our understanding of the final fate of CBPs, we modelled their long-term evolution, throughout the life stages of their hosts, from the MS to WD. Our goal is to provide the community with theoretical constraints on the evolution of CBPs beyond the MS and with the occurrence rates of planet survival throughout the ageing of the systems. Methods. We further developed the publicly available Triple Evolution Simulation (TRES) code, to adapt it to the mass range of sub-stellar objects (SSOs). We did so by implementing a variety of physical processes that affect giant planets and brown dwarfs. We used TRES to simulate the evolution, up to one Hubble time, of two synthetic populations of circumbinary giant planets. Each population was generated using different priors for the planetary orbital parameters. Results. In our simulated populations we identified several evolutionary categories, such as survived, merged, and destabilised systems. Our primary interest is those systems in which the planet survived the WD formation of both stars in the binary. We named these planets Magrathea. We found that a significant fraction of simulated CBPs survive the entire system evolution and become Magratheas, regardless of their mass. In the absence of multi-planet migration mechanisms, this category of CBPs is characterised by long orbital periods. Conclusions. Magrathea planets are a natural outcome of triple-system evolution, and our study indicates that they should be relatively common in the Galaxy. These gas giants can survive the death of their binary hosts if they orbit far enough away to avoid engulfment and instabilities. Our results can ultimately be a reference to orient future observations of this uncharted class of planets and to compare different theoretical models.