When Stars Play Planetary Pinball


Many of us remember playing pinball at the local arcade while growing up; it turns out that some stars like it as well. Binary stars can play tug-of-war with an unfortunate planet, flinging it into a wide orbit that allows it to be captured by first one star and then the other, in effect “bouncing” it between them before it is eventually flung out into deep space.

The new paper, by Nick Moeckel and Dimitri Veras of the University of Cambridge, will be published in a future issue of Monthly Notices of the Royal Astronomical Society.

The gravitational pull of large gas giant planets can affect the orbits of smaller planets; that scenario is thought to have occurred in our own solar system. In some cases, the smaller planet may be flung into a much wider orbit, perhaps even 100 times wider than Pluto’s. In the case of single stars, that’s normally how it ends. In a binary star system, however, the two stars may play a game of “cosmic pinball” with the poor planet first.

Moeckel and Dimitri conducted simulations of binary star systems, with two sun-like stars orbiting each other at distances between 250 and 1,000 times the distance of the Earth from the Sun. Each star had its own set of planets. The planetary systems would often become unstable, resulting in one of the planets being flung out, where it could be subsequently captured by the other star’s gravity. Since the new orbit around the second star would also tend to be quite wide, the planet would be vulnerable to recapture again by the first star. This could continue for a long time, and the simulations indicated that more than half of all planets initially ejected would get caught in this game of “cosmic pinball.”

In the end, some planets would settle back into an orbit around one of the stars, but the majority would escape both stars altogether, finally being flung out into deep space forever.

According to Moeckel, “Once a planet starts transitioning back and forth, it’s almost certainly at the beginning of a trip that will end in deep space.”

We are fortunate to live in a solar system where our planet is in a nice, stable orbit. For others out there who may not be so lucky, it would be like living through a disaster movie played out over eons.

The paper is available here.

5 Replies to “When Stars Play Planetary Pinball”

  1. As long as no one tilts.

    We *are* fortunate but not terribly tempting luck to find ourselves on a planet and a system with nice orbits. If there was a steady small risk for large orbital deviations it would be very unlikely to find us here.

    Similarly many systems have been found with stable orbits, it is harder to find those that has a disturbed dynamics because it is temporary.

    Having said that, the flip (flipper?) side is that those pinball systems would be easier to find. Now we know to look for them. So thanks for the heads up!

    1. these orbital gyrations would likely occur over long timescales compared to human lifetimes. they would also impart complicated and low amplitude radial velocity signatures for any observer to reconstruct, let alone wait for them to happen. it reminds of the theoretical ‘linear orbits’ that have been postulated to be possible for a planet in equilibrium between two stars of equal mass. likely to be very temporary and rarer still.

      that said, there are probably a lot of orphans flinging around between the stars.

  2. The paper by Moeckel and Veras is very interesting. The Jacobi constant of motion is a constraint, but it is not in general integrable in closed form. If the system is integrable the path of the planet in this system would be a closed curve, which will only happen for a narrow set of initial conditions.

    These dynamics for three or more bodies are not integrable. They approximate a solution in closed form if masses are sufficiently small or the orbital distances large. If you have two closely orbiting large masses and a third satellite which orbits close to them the motion will be very complex and chaotic. The unpredictable nature of these orbits persists even if the motion approximates a closed integrable solution. This occurs in the solar system as well. There are nudging forces on the orbits of the terrestrial planets by Jupiter and Saturn. The highest order effect is to change the perihelion of the orbits, and a higher order (less influcential) is to adjust the orbital radius or semi-major axis upwards.


    1. i wonder if a planet could become trapped in the volume of space at the center of mass between two stars in a binary system? bathed in continuous daylight it would spiral down to the stable point of equilibrium.

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