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Planetary Problems Around Wide Binaries

The news about possible surface water on today’s Mars points out how far we are from characterizing life’s possibilities even in our own Solar System, much less around other stars. It may take boots on the ground on Mars to solve the question once and for all, but life in underground aquifers certainly is a plausible proposition, and the sooner we have proof (and samples to study), the better for astrobiology in general.

Meanwhile, we push on with the very early wave of exoplanet studies, remembering that it’s just over a decade since 51 Peg gave us the first confirmed detection around a main sequence star. I can’t imagine a more fruitful field for a young astronomer to head for, with so many possibilities for study that you begin to wonder whether we’ll have the human resources to keep up with the vast data inflow that’s coming.

Some of the more intriguing recent work concerns binaries and the planets around them. If we’re getting fairly sanguine about the possibility of planets around close binaries like Centauri A and B, the question of what happens when the stars are farther apart stands open. A new paper by Daniel Malmberg (Lund Observatory, Sweden) and colleagues probes this question for two kinds of binary systems. Co-planar systems are those in which the two stars evolved as a binary from the start, drawing on the same primordial materials. Planets around these stars are likely in the same orbital plane. But the paper’s real focus is on stars that acquire a binary partner later in their evolution in a young stellar cluster.

When that happens, the result is a planetary system that’s randomly oriented with respect to the companion star. The authors go to work on such systems using the Kozai mechanism, first applied to asteroids in inclined orbits in our own Solar System as they are affected by Jupiter. The Kozai mechanism causes planets in wide binaries (the authors study 1000 AU separations, for example, as opposed to the 23 AU mean separation of Centauri A and B) to vary their orbital eccentricity. This is one way to explain the high eccentricity of the planet around 16 Cygni B.

Interesting things follow from all this. If you start with our Solar System and introduce a second, widely separated star, Neptune and Uranus will cross in their orbits if the inclination between companion star and the plane of the planets is greater than 43 degrees. Planetary ejections can follow. The Kozai numbers indicate this scenario will be the case in 73 percent of the binary systems formed by stellar encounters. And it could be that some of the observed extrasolar systems we see are the result of this kind of planet-scattering in a system that was once much like our own, with planets orbiting around a single original star.

Intriguing stuff, and it makes good sense, given that stars forming in groups are going to be in a crowded environment during the first 100 million years after their birth. That makes single stars into binaries easily enough, with obvious implications for stripping the system of one or more planets, and greatly affecting the orbits of those that remain.

The paper is Malmberg et al., “The instability of planetary systems in binaries: how the Kozai mechanism leads to strong planet-planet interactions,” submitted to Monthly Notices of the Royal Astronomical Society and available online as a preprint.

Comments on this entry are closed.

  • philw December 7, 2006, 20:58

    Great stuff. Ever since 51 Peg this has been a roller coaster ride with paradigms continually overturned. With most stars birthed in clusters, chaotic planetary systems may be the norm. Can’t wait for COROT and KEPLER data.

  • andy December 8, 2006, 14:29

    On the other hand, 55 Cancri and Upsilon Andromedae show that planetary systems can occur in at least some wide binaries.

  • Administrator December 8, 2006, 15:30

    Yes, absolutely. What the paper is arguing, though, is that wide binaries in which the planetary system and companion star are not co-planar with the primary are probably systems that have experienced serious disruptions. Some planets might well emerge unscathed from the mix but the system at large would show the effects. You can start with a system something like our own and then extrapolate the results of a binary in such an orbit to see how some extrasolar planetary systems may have formed.

  • andy December 8, 2006, 16:06

    It is interesting to note that HD80606b, the most eccentric planet currently known, is in a wide binary (secondary is HD80607).

    However, the Kozai mechanism is quite sensitive to deviations from the ideal model. For example, Uranus is able to maintain satellites in circular orbits when the Kozai mechanism predicts the eccentricity would rise to above 0.98. Stability is apparently a result of the oblateness of Uranus, tidal interactions, and moon-moon interactions. It all depends on the relative timescales of the various effects – it is unsurprising that Uranus and Neptune were disrupted in the binary star simulations, as the timescales for interactions would presumably be much longer than the Kozai timescale.

    It would be very interesting to see whether 55 Cancri and Upsilon Andromedae posess any outer planets beyond the ones currently known. Also both of these systems have eccentric outer planets. In particular, Upsilon Andromedae shows evidence of having planet-planet interactions in the past: could the Kozai mechanism have been responsible for causing the third and now-lost fourth planet of this system to have interacted in this way? The current configuration has strong interactions between planets two and three, which presumably would damp any Kozai mechanism, particularly if the relative inclination of the secondary star didn’t exceed the Kozai angle by a large amount. The hot Jupiter is subject to strong solar tides which would presumably protect it.

  • Administrator December 8, 2006, 16:43

    That’s fascinating re Neptune and Uranus, especially your thought on Uranian oblateness and tidal interactions, etc. Like you, I’d love to see how this plays out as we develop a better picture of Upsilon Andromedae in particular.

  • ljk December 11, 2006, 4:25

    Astrophysics, abstract

    From: Franco Joos [view email]

    Date: Fri, 8 Dec 2006 12:17:32 GMT (118kb)

    Limb polarization of Uranus and Neptune. II Spectropolarimetric observations

    Authors: F. Joos, H.M. Schmid

    Comments: Accepted by A&A 24.11.2006

    We have detected a strong limb polarization for Uranus and Neptune. With spectropolarimetric observations we characterize the spectral dependence of this limb polarization and explore the diagnostic potential for investigating the distribution and properties of the scattering particles. We present disk resolved spectropolarimetry of Uranus and Neptune covering the wavelength range from 530nm to 930nm and compare the spectropolarimetric signal for different limb sections and the center of the planetary disk. As an additional benefit we obtained center-to-limb disk profiles for the intensity and polarization for various wavelengths.


  • ljk January 16, 2007, 1:00

    Astrophysics, abstract

    From: Daniel Malmberg [view email]

    Date (v1): Fri, 1 Dec 2006 21:02:07 GMT (69kb)

    Date (revised v2): Mon, 15 Jan 2007 17:05:57 GMT (67kb)

    The instability of planetary systems in binaries: how the Kozai mechanism leads to strong planet-planet interactions

    Authors: Daniel Malmberg, Melvyn B. Davies, John E. Chambers

    Comments: 5 pages, 2 figures, accepted for publication in MNRAS Letters

    In this letter we consider the evolution of a planetary system around a star inside a wide binary. We simulate numerically the evolution of the planetary orbits for both co-planar and highly-inclined systems. We find that the Kozai mechanism operates in the latter case. This produces a highly eccentric outer planet whose orbit crosses those of some of the inner planets. Strong planet-planet interactions then follow resulting in the ejection of one or more planets. We note that planetary systems resembling our solar system, formed around single stars in stellar clusters may exchange into binaries and thus will be vulnerable to planet stripping. This process will reduce the number of solar-system like planetary systems, and may produce at least some of the observed extra-solar planets.


  • ljk February 14, 2007, 11:25

    Astrophysics, abstract

    From: Mauro Barbieri [view email]

    Date: Tue, 13 Feb 2007 17:08:36 GMT (89kb)

    Planets in binary systems: is the present configuration indicative of the formation process?

    Authors: F. Marzari, M. Barbieri

    Comments: 5 pages, 5 figures Accepted for publication on A&A

    The present dynamical configuration of planets in binary star systems may not reflect their formation process since the binary orbit may have changed in the past after the planet formation process was completed. An observed binary system may have been part of a former hierarchical triple that became unstable after the planets completed their growth around the primary star.

    Alternatively, in a dense stellar environment even a single stellar encounter between the star pair and a singleton may singificantly alter the binary orbit. In both cases the planets we observe at present would have formed when the dynamical environment was different from the presently observed one.
    We have numerically integrated the trajectories of the stars (binary plus singleton) and of test planets to investigate the abovementioned mechanisms.

    Our simulations show that the circumstellar environment during planetary formation around the primary was gravitationally less perturbed when the binary was part of a hierarchical triple because the binary was necessarely wider and, possibly, less eccentric.

    This circumstance has consequences for the planetary system in terms of orbital spacing, eccentricity, and mass of the individual planets. Even in the case of a single stellar encounter the present appearance of a planetary system in a binary may significantly differ from what it had while planet formation was ongoing. However, while in the case of instability of a triple the trend is always towards a tighter and more eccentric binary system, when a single stellar encounter affects the system the orbit of the binary can become wider and be circularized.