Outward Migration of Terrestrial Embryos in Binary Systems

We consider the formation and migration of protoplanetary embryos in disks around the stars in tight binary systems (separations ~ 20 AU. In such systems, the initial stages of runaway embryo formation are expected to only take place within some critical disk radius a_{crit}, due to the perturbing effect of the binary companions (Thebault et al. 2009). We perform n-body simulations of the evolution of such a population of inner-disk embryos surrounded by an outer-disk of smaller planetesimals. Taking Alpha Centauri-B as our fiducial reference example in which a_{crit} ~ 0.7 AU, and using a Minimum Mass Nebular Model with $\Sigma \propto a^{-3/2}$, we find that within 10^6 yrs (10^7 yrs), systems will on average contain embryos which have migrated out to 0.9 AU (1.2 AU), with the average outer-most body having a mass of 0.2 M_{earth} 0.4 M_{earth}. Changes to increase the surface density of solids or to use a flatter profile both produce increased embryo migration and growth. At a given time, the relative change in semi-major axis of the outer-most embryo in these simulations is found to be essentially independent of a_{crit}, and we note that little further embryo migration takes place beyond 10^7 years. We conclude that the suppression of runaway growth outside a_{crit} does not mean that the habitable zones in such tight binary systems will be devoid of detectable, terrestrial mass planets, even if a_{crit} lies significantly interior to the inner edge of the habitable zone.

💡 Research Summary

The paper investigates how terrestrial embryos can form and subsequently migrate outward in tight binary star systems, using Alpha Centauri‑B as a fiducial case. In such systems the gravitational perturbations from the companion star suppress runaway growth of planetesimals beyond a critical radius a₍crit₎ (≈ 0.7 AU for Alpha Cen‑B). The authors set up N‑body simulations that place a population of already‑grown embryos inside a₍crit₎ and a surrounding disk of smaller planetesimals extending outward to ~2.5 AU. The disk follows a Minimum Mass Nebular Model with surface density Σ∝a⁻³ᐟ², but additional runs explore higher solid surface densities and flatter profiles.

Key findings are: (1) Within 10⁶ years the outermost embryo typically migrates to ~0.9 AU; by 10⁷ years it can reach ~1.2 AU, well inside the habitable zone of Alpha Cen‑B. (2) During migration embryos accrete planetesimals, growing from an initial mass of a few 10⁻² M⊕ to an average of 0.2 M⊕ after 1 Myr and 0.4 M⊕ after 10 Myr. (3) Increasing the solid surface density or adopting a flatter Σ profile amplifies both the radial displacement and the mass growth, confirming that a more massive or less steep disk makes outward scattering more efficient. (4) The relative change in semi‑major axis (Δa/a₍crit₎) is essentially independent of the absolute value of a₍crit₎, indicating that the migration efficiency is set by the mass ratio between the inner embryo swarm and the outer planetesimal reservoir rather than the exact location of the runaway‑growth cutoff. (5) After ~10⁷ years the system reaches a dynamical steady state; further migration is negligible.

These results overturn the simplistic view that tight binaries are barren of habitable‑zone terrestrial planets because runaway growth is suppressed beyond a₍crit₎. Even when a₍crit₎ lies well inside the inner edge of the habitable zone, embryos formed interior to it can be scattered outward, accrete additional material, and end up as detectable, Earth‑mass (or super‑Earth) bodies within the habitable zone. The study therefore suggests that observational campaigns targeting close binaries should remain optimistic about finding terrestrial planets, and that planet‑formation models for such systems must incorporate the dynamical coupling between inner embryos and an outer planetesimal disk.