Planets around binary stars fascinate me, doubtless because of Alpha Centauri’s proximity and the question of whether there are planets there. About ten percent of the planets we’ve found around main sequence stars are found in binary systems, and most of these binaries have wide separations, in the range of 100 to 300 AU. But, like Alpha Centauri, close binaries remain promising targets. I’m looking at a new paper by Andras Zsom, Zsolt Sándor and Kees Dullemond (Max-Planck-Institute für Astronomie) dealing with early stage planet formation in binaries, and they’re quick to note that planets in close binary systems put constraints on planet formation theories.
After all, if we find planets in these systems, our planet forming theories have to produce satisfactory explanations for their existence. Does core accretion, then, work in these environments? We can look to close binaries with planets, systems like Gamma Cephei (separation 18.5 AU), GL 86 (18.4 AU), HD 41004 (23 AU) and HD 196885 (17 AU). What Zsom et al. are interested in is the question of the growth and fragmentation of dust in binary systems, the gas and dust disk around the primary star being perturbed by the secondary, and their paper compares their model — with and without eccentricity — to the dust population in disks around single stars.
Core Accretion and Its Problems
Think about how planets form under the core accretion model. Tiny dust grains eventually form planetesimals, which then form protoplanetary cores that over time gain a gaseous envelope (or not, depending on the presence of gas), and undergo a chaotic impact phase until their orbits are finally stable. It’s a complicated picture that binary stars make even more treacherous, and in several ways. For one thing, contemplate what a stellar companion does to the dust disk:
The tidal torques of the companion generate strong spiral arms in the disk around the primary. Angular momentum is transferred to the binary orbit which will truncate and restructure the disk… This dynamical effect of the secondary has several consequences which might influence planet formation: it decreases the lifetime of the disk, increases the temperature of the disk and modifies the stable orbits around the primary.
But it’s not just the early disk that’s affected. Here the authors go into what happens to the evolving planetesimals, and how their fate may shape planet formation:
The evolution of planetesimals is also influenced in a binary system. The perturbation of the secondary increases the relative velocity of the planetesimals and/or creates unstable regions where the planetary building blocks cannot maintain a stable orbit as shown by e.g., Heppenheimer (1978), Whitmire et al. (1998), Thébault et al. (2004). The increased relative velocities can then lead to the disruption of the planetesimals. However, Marzari & Scholl (2000) showed that the combined effects of the gravitational perturbation and the gas drag may increase the efficiency of their accretion by reducing their relative velocity and produce, later on, terrestrial planets. Marzari et al. (2009) calculated the relative velocity of planetesimals in highly inclined systems and concluded that planet formation appears possible for inclinations as high as 10◦ , if the separation between the stars is larger than 70 AU. The region where planetesimals can accumulate into protoplanets shrinks consistently for lower binary separations.
Of course, it’s the closer binaries that we’re interested in today. This paper homes in on the earliest stages of core accretion, the key issue being: can planetesimals form in the first place, or is the disk too thoroughly disrupted to allow the process? Some theories suggest that a ‘pressure bump’ forming around the snow-line could concentrate particles so that planetesimals could form in the inner system, but the authors’ simulations show that this effect quickly disappears in infant systems, so quickly that the needed planetesimals never emerge.
Various other models are out there that could concentrate particles as needed for the early stages of planet formation — the authors run through the possibilities — but they’re still under investigation and may be rendered unworkable by the presence of the secondary star. As the authors note, “Particle concentration mechanisms in general need to cope with the continuous gravitational stirring and perturbations of the secondary.” And we’re by no means sure that core accretion can function within these models when a secondary star is present.
A Different Formation Model?
If that’s the case, what about the planets we’ve already found around close binaries? Are they the result not of core accretion but gravitational instability, the alternate model of planet formation? In the latter, unstable regions in the protoplanetary disk form clumps of gas and dust which eventually coagulate into a single core. In this model, knots of matter collapse rapidly, to form planets in a much shorter time frame.
The gas giants we’ve found around close binaries thus far could be the result of gravitational instability. But here we have to fall back on the limits of our observations, for as some readers have already been discussing in the comments to a previous post, close binaries are tricky targets. We have yet to find low-mass planets around close binaries, but this may simply be because of the limits of our current techniques, and because of an observational bias that has kept such systems out of many early surveys. But if further work rules out core accretion in these environments, then the alternate model — gravitational instability — becomes the focus of more targeted study.
How dust turns into planetesimals, and thence to planets, is the subject of intense ongoing work. And if the Zsom paper has you discouraged about the prospects around Alpha Centauri, we’ll look tomorrow at another new paper that tackles early disk issues and comes to a different conclusion about what may be possible there. For today, though, the paper is Zsom et al., “The First Stages of Planet Formation in Binary Systems: How Far Can Dust Coagulation Proceed?” accepted at Astronomy & Astrophysics and available as a preprint. You may want to check the comments to my recent article on the SIM mission, where this paper first came up (and thanks to Centauri Dreams regulars andy and spaceman for the discussion).