Planets that revolve around two suns may surprisingly survive the violent late stages of the stars' lives, according to new research out of the NASA Goddard Space Flight Centre and York University. The finding is surprising because planets orbiting close to a single sun, like Mercury and Venus in our solar system, would be destroyed when the aging star swells into a red giant.
Led by Veselin Kostov at the NASA Goddard Space Flight Centre, in collaboration with York University master's student Keavin Moore and Professor Ray Jayawardhana, the study found that planets orbiting two (binary) stars - also referred to as circumbinary planets or "Tatooine worlds" after the iconic planetary home of Luke Skywalker in Star Wars - often escape death and destruction by moving out to wider orbits.
On the Fate of Unstable Circumbinary Planets: Tatooine's Close Encounters with a Death Star
Authors:
Sutherland et al
Abstract:
Circumbinary planets whose orbits become unstable may be ejected, accreted, or even captured by one of the stars. We quantify the relative rates of these channels, for a binary of secondary star's mass fraction 0.1 with an orbit of 1AU. The most common outcome is ejection, which happens ~80% of the time. If binary systems form circumbinary planets readily and sloppily, this process may fill the Milky Way with free-floating planets. A significant fraction of the time, ~20%, the unstable planet strikes the primary or secondary. We tracked whether a Jupiter-like planet would undergo tidal stripping events during close passages, and find that these events are not strong enough to change the trajectory of the planet, though this may be observable from a changed structured for free-floating planets that are produced by this process.
Planet formation around binary stars: Tatooine made easy
Authors:
Bromley et al
Abstract:
We examine characteristics of circumbinary orbits in the context of current planet formation scenarios. Analytical perturbation theory predicts the existence of nested circumbinary orbits that are generalizations of circular orbits in a Keplerian potential. They contain forced epicyclic motion aligned with the binary as well as higher frequency oscillations, yet they do not cross, even in the presence of massive disks and perturbations from large planets. For this reason, dissipative gas and planetesimals can settle onto these "most circular" orbits, facilitating the growth of protoplanets. Outside a region close to the binary where orbits are generally unstable, circumbinary planets form in much the same way as their cousins around a single star. Here, we review the theory and confirm its predictions with a suite of representative simulations. We then consider the circumbinary planets discovered with NASA's Kepler satellite. These Neptune- and Jupiter-size planets, or their planetesimal precursors, may have migrated inward to reach their observed orbits, since their current positions are outside of unstable zones caused by overlapping resonances. In situ formation without migration seems less likely, only because the surface density of the protoplanetary disks must be implausibly high. Otherwise, the circumbinary environment is friendly to planet formation, and we expect that many Earth-like "Tatooines" will join the growing census of circumbinary planets.
