Can Nearby Binary Star Systems Mimic Planets?

The vast majority of the known exoplanets have been discovered by the radial velocity method. This method employs the effects of a planet’s gentle tug on its parent star which is perceived as a “wobble” in the star’s motion. A new study, conducted by Morais and Correia, looks at whether this effect can be mimicked by another, distinctly non-planetary, source: Binary stars.

Conceptually, the idea is rather straightforward. A star of interest lies in a triple star system. It is the third member and in a larger orbit around a tight binary system. As the tight binary system orbits, there will be periods in which they line up with the star of interest giving a minutely greater pull before relaxing the pull later in their orbit. This remote tug would show a distinctly periodic effect very similar to the effects expected from an inferred planet.

The obvious question was how astronomers could miss the presence of binary stars, close enough to have a notable effect. The authors of the paper suggest that if the binary pair orbited sufficiently close, it would be unlikely that they could be resolved as a binary. Additionally, if one member were sufficiently faint (an M dwarf), it may not appear readily either. Both of these instances are plausible given that some three fourths of nearby main sequence stars are M class, and about half of all stars are in binary system.

Next, the team asked how important these effects may be. They considered the case of HD 18875, a binary system in which a distant star (A) has a 25.7 year period around a tight binary (Ba + Bb) that orbit each other with a period of 155 days. This system was noteworthy because a hot Jupiter planet was announced around the A star in 2005, but challenged in 2007 when another team could not repeat the observations.

The new study attempted to use their understanding and modeling of three body systems to see if the binary interaction could have produced the spurious signal. Using their model, they determined that the effects of the system itself would have produced effects similar to those of a planet of 4 Earth masses located at 0.38 AU. A planet of such mass is well below the limit of a hot Jupiter and the distance is somewhat larger than usual as well. Thus, the nearby B-binary could not have been responsible. Furthermore, such minute effects of this type are generally interpreted as “super-Earths” and have only become prevalent in observations in the past few years.

Thus, while the unconfirmed planet around HD 18875 A might not have been caused by the nearby binary, the work in this new paper has shown that effects of nearby binaries will become increasingly important as we start detecting radial velocities indicative of less and less massive planets.

8 Replies to “Can Nearby Binary Star Systems Mimic Planets?”

  1. Yes, this is correct, same effect has been known since the early 1960s. The issue is often labelled under apsidal motion, which I assume is what this “…remote tug would show a distinctly periodic effect very similar to the effects expected from an inferred planet.” would be. In orbital terms this means that the nodes of the orbit slowly increases or decrease overtime. The orbital elements of i (inclination) and ω (angle of the apsides, ‘lowercase omega’), strongly influence the observed degree of the effects on apsidal motion (dω/dt). Such effects have a consequence on the radial velocities over time ’ this mimicking change possible mistaken for planets..
    For multiples, it has been known for a while that there exists a linear relationship between both the period and the components radial velocities (identifying exoplanets), unless of course there is this significant apsidal motion.
    Observations of this effect are often greatly magnified in contact or eclipsing binaries, where the distorted components I.e. like teardrop-shape or ellipsoidal stars, distorted by their close gravity fields. An example is CO Lac, whose apsidal rotation period is about 42 years, with the binary orbiting each other once every 1.542 days.

    As for the paper, the triple they study also have fairly eccentric orbits and display non-coplanar motion (meaning where the forces exerted on the stars in not regular, but significantly change the velocities (motion) of the components in their orbits.) of the third component orbiting the inner binary.

    Note: An interesting thing about orbital element in visual binaries (like alpha Centauri, for example), is that the orbit is derived by many measures over decades or centuries of the separation and the angle from north. This is a slow and laborious task, whose obtained data is often grossly (and notoriously) imprecise. It could be vastly improved if we could know the radial velocity of both stars at certain points during the visual orbits as well. (In the past has been near impossible to do.)

    This is why they say in the conclusion;

    “Although the radial velocity variations due to a binary are distinct from those due to planet(s), in practice, the measured effect depends on the instrument’s precision and the observation time span.”

    It is also interesting this effect works only in certain scenarios and conditions; and observationally, this is made worse by the fact that the close binary might not be resolved by us on the ground, but might cause this orbital effect.

    All interesting stuff!

  2. Maybe it is weekend blues, but color me unimpressed. The number of triples wouldn’t be reflected by binaries, which to add harm to hurt AFAIU are inordinately common by way of planetary formation.

    So yes, some exoplanet candidates won’t be exoplanets, and worse they will prove to be mimics. But systematic effects are mundane, bad to have, good to know, but seldom killers of subjects.

    1. I wasn’t trying to imply that it would kill any subject. Just that it was an interesting effect of which most people weren’t aware that is something to watch out for.

    2. About 35% of all stars have three or more components. Triples are fairly common place, most being a close pair to other more distant components. The ratio of the semi-major axis (a) — the mean distance of the stars which orbit in an eccentric orbit (expressed in arcsec, or if the distance is known, in AU) — of the triple and the closer binary are anywhere between 8:1 and 500:1. [From observation NOT from theory!] This ratio value is often called q, and when examined by computer simulations, there is a place where the triple become unstable in its orbit. ([See definition below!)]

      I.e. The stability criterion Hence, there is a minimum critical value of the coefficient of stability as δq equalling to about 0.01 (1%).

      Another common stability value is calculated as “q(2) / a”, sometimes referred as ξ1 or Xo), being the parameter used in classifying many observed visual systems. This latter is a convenient method to describe stability in multiple systems. (Whilst not discussed exactly this way in this Morais and Correia paper, some portions of the existence of exoplanets in binaries might depend on understanding what kind of conditions are required so they can survive over long periods of time. [If you want to read more about stability, then search in the ADS for the definitive paper; Anosova, J.P. “Dynamical Evolution of Triple Systems With Big Differences in Massive Bodies. A Criterion for Stability.”; A&A.Sup.Ser., 238, 223-238. (1996) Anosova did 1.5 million planar hierarchical triple computer simulations examining all the varied cases!]
      The main issue IMO with exoplanets in binary systems; Is how did the planets form in such a very gravitational active environment. (It seems unlikely to me, but hell, others better than me seems to think it indeed possible. I’ve yet to see the definitive proof though!)
      The second question if they did survive the stellar formation process, then how do they survive the perturbation effects by the binary, and as in this story, triple stars?
      Another important ‘big-picture’ issue is that binary and triple stars have non-circular orbits (unlike the solar system); whose high eccentricity means significant perturbations on smaller objects. I.e. Binaries with gross eccentricities (e=0.7) will destroy most bodies, as the gravitation effects quickly throw out the gas and dust (and planets), or place objects into collision courses.
      Much work needs to be done on solar system stabilities, because it is required firstly to know what scenarios are stable and those which are not.
      The second question is the number of rouge planets wandering between stars. If they are turfed out in large numbers by binary and multiple star systems, their numbers may be even larger than attached planets orbiting stars.

      Note: The definition of stable and unstable triples are;
      For stable triples, the third body is always isolated from each of the other two stars (inner binary).
      For unstable triples, components are not isolated from each other during sometime in their dynamical evolution.

  3. Spectroscopy can sometimes tell you if there’s a close binary companion, even if it is not resolvable. See whether the the spectrum of the star has a fainter spectrum of another star superimposed on it. Of course, it is very difficult sometimes to make sense of the jumble of spectral lines.

    1. Actually, the reverse is true. Most of the close binaries are of similar mass and size. The close proximity often means the main spectral lines (sometimes as emission lines) are visible, but the gas surrounding both stars haze-out the fainter ones. We have detected about 2000 spectroscopic binaries, and a fair portion of those are visual or speckle systems. A few are eclipsing binaries.
      Really most of the close binaries we observe are found as eclipsing binaries. Many do have more distant companion(s), making them triples or multiples.

      In this story, why we are interested in spectroscopy is to measure the radial velocities of the two in the orbits. From the patterns in these radial velocities over time suggests another body is causing unexplained or unusual deviations from the orbital behaviour of the two stars. This is not exactly an easy task!

  4. And then there was no longer such a thing as a ‘hot Jupiter’.

    Joking of course — would be fun though. ;D

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