There Are No Corkscrew Orbits

by Greg Egan


In May 2015, The Astrophysical Journal published a paper[1] by E. Oks which stated:

... it is possible for OBSS [one-planet binary star systems] to have stable planetary orbits in the shape of a helix on a conical surface, whose axis of symmetry coincides with the interstellar axis ...

Take a moment to think about this extraordinary claim! A binary star system contains two stars orbiting their common centre of mass. The interstellar axis is a line joining those two stars. So, this axis will rotate. The paper is saying that a planet can move in a conical helix (a helix that wraps around a cone, rather than a cylinder), such that the axis of the cone remains aligned with the rotating interstellar axis!

The magazine New Scientist[2] covered this with a breathless account that made it sound so exciting that I desperately wanted it to be true:

Even the craziest planets concocted by theorists still tend to trace conventionally near-circular orbits in a flat plane. Not so the corkscrew planet. Mind-bendingly, these worlds could exist in a sort of orbital limbo, spiralling about an axis between two stars in a binary system, pulled hither and thither by their competing gravities.

But wanting something to be true is not enough to make it so. If the two stars were motionless, it’s not hard to show that such “conic-helical” orbits really do exist. There is a certain amount of poetic licence in the language here: the orbits do not confine themselves precisely to a cone as a geometer would define it, but I have no objection to that. In fact, I have no problem with the entire early part of the paper which discusses the kind of stable planetary orbits that would exist, if the stars were somehow kept fixed.

However, when the author proceeds to examine how things change when the stars move in their orbit, he does not offer a convincing argument as to why the cone on which the planet’s orbit lies should rotate along with the stars. Other authors[3] who have studied binary star systems with planetary orbits at large inclinations to the plane of the stars’ orbit report nothing of the kind. And both the numerical simulations I’ve performed, and some simple analytical calculations, strongly suggest that this is not the case. (A slightly more elaborate version of these calculations is given in a paper I’ve placed on the arXiv[4].) I’ve also identified some specific problems with the paper.

[Update, 30 May 2016: On 20 May 2016, The Astrophysical Journal published an erratum[5], correcting some mistakes in the original paper: the missing factors of 2 and the radius of the planetary orbit that I noted below have now been inserted in some of the equations. Unfortunately, the corrections also introduce a brand new error, with the new version of equation (51) stating that the oscillations of the planetary motion occur entirely along the interstellar axis, with the radial oscillations being zero. This would turn the “conic-helical” orbits into “cylindrical-helical” orbits, if it were true, but it’s a mistake: a sign error in the change of coordinates from equation (48) to equation (50) has been fixed, while a sign error in the second line of equation (48), which was previously cancelling out that other error, now leads to an incorrect result in equations (50) and (51).

And none of these corrections address the most serious problem with the paper: the fact that the component of the planet’s angular momentum along the interstellar axis is not a conserved quantity as claimed.]

[Update, 25 Sep 2016: I just stumbled on another paper by Oks[6] on this subject, published in the International Review of Atomic and Molecular Physics.

In a Note Added in Proof to this paper, Oks attempts to refute the criticisms of his earlier paper given on this web page. First, he claims that, when deriving the angular momentum of the planet in the analysis below, I ought to be using a linear momentum with an additional term that corrects for the rotation of the frame. But that would only be true if I omitted the centrifugal and Coriolis forces that appear when working in the rotating frame; since I explicitly include those frame-dependent forces and the torques they generate, the correct approach is to write the linear and angular momenta of the planet in the same form as they take in an inertial frame.

Next, he states that his earlier paper never claimed that m(t), the projection of the angular momentum onto the interstellar axis, was constant. The trouble with that is that his whole analysis of the orbits relies on it, even if he never states it explicitly, since the effective potential changes if m(t) changes. He then goes on to “derive” a formula for m(t) that shows it oscillating around a non-zero average, but his derivation is circular, since it relies on a constant effective potential, which in turn relies on m(t) being at least approximately constant.

Finally, he argues that numerical simulations are unreliable, and mine in particular all the more so, because he emailed me a request to simulate a problem with a known analytical solution, and I failed his test. In fact, he didn’t explain clearly exactly what it was he wanted simulated, and rather than trying to resolve the issue he declined to discuss the matter any further. All the code used in my simulations of the “conic-helical” orbits is available online, below, and experts are welcome to read it and assess its validity for themselves.]

Contents


Numerical Simulations

Motionless stars, conic-helical orbit

The image on the left illustrates an example discussed in the paper: the binary star Kepler-16, in which one star is 3.4 times as massive as the other. The planet shown here is fictitious, and I’ve chosen the specific orbital parameters myself. It starts out located at a point 0.998 of the way along the axis from the lighter star to the heavier, and 0.192685 from the axis. Its initial velocity has a component of 1.4015 along the interstellar axis, and 4.20451 perpendicular to it. These numbers are given in scaled units, in which the distance between the stars, the gravitational constant G, and the mass of the lighter planet are all equal to 1.

The blue line in this image is the trajectory the planet would follow if the stars were motionless. This orbit is stable, and it does lie roughly on a segment of a cone centred on the interstellar axis.

You might notice that the planet is sticking pretty close to one star. The reason I haven’t put it more centrally between the two — to be, as New Scientist would have it, “pulled hither and thither by their competing gravities” — is that the paper only mounts an argument that these kinds of orbits will continue to be stable once the stars are rotating if the planet completes its orbit much faster than the stars complete theirs, and that can only happen when the planet is close to one star or the other. Figures 6 and 7 in the paper plot the necessary distances of the average plane of the planet’s orbit from the lighter or heavier star for various mass ratios, in order that the planet orbits at least ten times faster than the stars. That distance is never more than about 1% of the distance between the stars.


Rotating frame, numerical simulation

This next image, on the right, shows the result of a numerical simulation I performed, giving the trajectory of the planet if we allow the stars to rotate. For simplicity, rather than reproducing the elliptical orbits of Kepler-16, I’ve chosen to have them move in perfect circles.

The trajectory is shown in a frame that tracks the orbit of the stars, so the stars still appear motionless, but since they really aren’t, it’s necessary to include the effects of the centrifugal force and Coriolis force that arise when we compute things in a rotating coordinate system, along with the gravitational force due to the two stars.

The angular velocity with which the stars orbit each other is given, in our scaled units, by the simple formula ω=√(1+b), where b is the ratio of the masses of the stars. So for b=3.4, we have ω=2.09762.

The trajectory shown here follows the planet for one complete orbit of the stars. Clearly, it is not remaining on a cone centred on the interstellar axis. It might look a bit chaotic, but in fact it’s quite close to being an ellipse precessing around the star, where the precession arises mostly because we’ve chosen to view the orbit in a rotating frame.


Inertial frame, numerical simulation

The image on the left shows what happens if we view the same trajectory in an inertial frame, fixed to the centre of mass of the two stars. The lighter star has been omitted here, in order to show more detail. In this view, the heavier star moves around the centre of mass in a circle, while the planet winds around the star, more or less maintaining the alignment of its orbital plane.


Inertial frame, conic-helical orbit

In contrast to this, the image on the right shows what a conic-helical orbit would look like in an inertial frame. Since the orbital plane would maintain a more or less fixed alignment in the rotating frame, here it is seen swinging around along with the turning of the stars.

These are two very different kinds of motion! So why do I trust the result of this numerical simulation, rather than the claims in a paper published in a distinguished, peer-reviewed journal? One reason is that I computed the trajectory by two different methods: once by performing all the calculations in a rotating frame, with the stars fixed and centrifugal and Coriolis forces present in addition to gravity, and then the second time in an inertial frame, with the stars moving and gravity as the only force. When the results of either calculation were translated into the coordinates used for the other, the trajectories agreed.

Why should you trust this simulation, and not the peer-reviewed paper? If you have access to suitable software, you can simulate the motion yourself.

You can download the Mathematica notebook for my own calculations here, or if you don’t have Mathematica but want to scrutinise my calculations, you can read a PDF of the notebook here.


Analytical Calculations

It’s not hard to give an account of why the orbital plane of the planet will not remain orthogonal to the interstellar axis. A slightly more elaborate version of these calculations is given in a paper I’ve placed on the arXiv, “Polar Orbits Around Binary Stars”.

We will start by defining m(t) to be the projection of the planet’s angular momentum on the interstellar axis, measured in a rotating frame. Any torque that changes m(t) must be due to the centrifugal and/or Coriolis forces, because the gravitational forces from the stars will always apply torques perpendicular to the axis.

We will work in Cartesian coordinates fixed to the stars, with the x-axis as the interstellar axis, and the z-axis as the axis of rotation for the stars’ orbit.

The centrifugal force is F1 = ω2 (x(t), y(t), 0). The component of the torque along the x-axis due to the centrifugal force is:

T1 = y(t) F1,zz(t) F1,y = –ω2 y(t) z(t)

The Coriolis force is F2 = 2 ω (y'(t), –x'(t), 0). The component of the torque along the x-axis due to the Coriolis force is:

T2 = y(t) F2,zz(t) F2,y = 2 ω z(t) x'(t)

Now, suppose we approximate the motion of the planet with a circular orbit of angular velocity f and radius ρ, centred on the point (x1, 0, 0), with the plane tilted away from orthogonality with the interstellar axis by an angle α(t). We then have:

x(t) = x1 + ρ sin(α(t)) cos(f t)
y(t) = ρ cos(α(t)) cos(f t)
z(t) = ρ sin(f t)
x'(t) = –f ρ sin(α(t)) sin(f t)
T1 = –ω2 y(t) z(t) = –ρ2 ω2 cos(α(t)) sin(f t) cos(f t)
T2 = 2 ω z(t) x'(t) = –2 f ρ2 ω sin(α(t)) sin(f t)2

The component of the torque due to the centrifugal force, T1, will average to zero over one planetary orbit, because:

<sin(f t) cos(f t)> = <(1/2) sin(2 f t)> = 0

But the component of the torque due to the Coriolis force, T2, will not average to zero, because:

<sin(f t)2> = <(1/2)(1 – cos(2 f t))> = 1/2

So <T1> = 0, but <T2> = –f ρ2 ω sin(α(t)). So if the orbit is tilted even slightly, T2 will result in a persistent torque that will act to reduce the projection of the angular momentum onto the interstellar axis:

<m'(t)> = <T2> = –f ρ2 ω sin(α(t))

Note that m(t) itself is given by:

m(t) = y(t) z'(t) – z(t) y'(t) = f ρ2 cos(α(t))

The solution for α(t) that is compatible with the torque we have computed is then:

α(t) = ω t
m(t) = f ρ2 cos(ω t)
m'(t) = –f ρ2 ω sin(ω t) = <T2>

So to a first approximation, m(t) will be a cosine with angular frequency ω.

What if the orbit is initially not tilted at all? Then although the average torque over a full planetary orbit will be zero, if we look at the average torque over one quarter-cycle, there will be a small, but non-zero torque, with the same sign as that derived above. So the initial tilt of zero will not persist.

Problems with the paper

Effective potential

The paper starts out by describing an effective potential for a planet that is orbiting the interstellar axis between two motionless stars, including both the gravitational potential energy and the part of the kinetic energy associated with a fixed value for m, the projection of the planet’s angular momentum on the interstellar axis. This is a standard method for separating out the orbital motion, leaving only the motion in the other two directions to be influenced by the effective potential. The image on the right shows the contours for this potential in a plane through the axis, for two stars with a mass ratio of b = 3.4 and a planet with an angular momentum of m = 0.810145; there is an energy valley in which the planet can orbit, at a distance of 0.998 along the axis (where 1 is the full length of the axis).

This section of the paper contains no errors that I could find. The problems start when the author tries to use exactly the same effective potential to analyse the situation when the stars are allowed to move around in their orbit. But if m changes, the effective potential changes, and the original version can no longer be used to describe the planet’s motion.

Why would m not change? (I’ve already explained in the previous section why I believe that it would.) Perhaps there is an intuitive appeal to the idea that the planet is “trapped” in a valley of the effective potential, which is fixed relative to the stars. So, as the stars move, the valley moves with them, and the planet has to come along for the ride, with the extra forces it feels in the stars’ rotating frame merely perturbing it slightly, so that it moves back and forth within the valley. The trouble with that analysis is that the extra forces don’t just nudge the planet itself. Because the effective potential depends on m, when a force acts on the planet in a way that changes m, the whole energy landscape shifts.

In any case, the author never really makes an argument for the constancy of m. He requires the planet to complete its own orbit much more rapidly than the stars complete theirs, so that the change in the orientation of the interstellar axis is a relatively slow process compared to the planet’s motion, but that in itself guarantees nothing: a slow process can still have a large cumulative effect. He offers no calculations in relation to this, and no reference to any established theorem; he simply refers to the stars’ rotation as being adiabatic (that is, gradual) as if that settles the matter, and then proceeds to assume that m will remain at least approximately constant.

Having made this assumption, the author then models the effect of the Coriolis force on the planet as a driven two-dimensional harmonic oscillator about the equilibrium point (the valley in the effective potential for the motionless stars), and derives amplitudes for the size of the oscillations in each direction. Because m isn’t actually constant as the stars orbit, this whole analysis is pretty much irrelevant to the real physics of the planet’s trajectory, but we will follow through its implications and see where they lead.

For a start, the author makes two errors in this calculation, first omitting a factor of 2 in the Coriolis force in equation (44), and then omitting a factor of the planet’s orbital radius in equation (46). So his final formulas for the forced oscillations in equation (51) are missing both those factors.

[Update, 30 May 2016: On 20 May 2016, The Astrophysical Journal published an erratum[5] which inserts the missing factors ... but then introduces a new error into the results by claiming that the radial oscillations of the planet have an amplitude of 0. Checking the algebra shows that equation (48) in the original paper has a sign error, which remains uncorrected in the erratum, but now leads to errors in equations (50) and (51). But it’s also obvious on purely physical grounds that the new equation (51) can’t be correct, because it amounts to saying that the response of a two-dimensional driven harmonic oscillator is colinear with the driving force — the Coriolis force, which is directed along the interstellar axis — even though the interstellar axis is not a principal axis of the oscillator.]


Parameter regions

The paper never offers a concrete example of an orbit for which the oscillations described by equation (51) are relatively small (a prerequisite for the whole analysis), but it does deal with some other considerations.

The plot on the left is for w, the distance of the orbital plane along the axis from the lighter star to the heavier, and b, the mass ratio of the stars. The blue, red and green regions show how the valid parameter space shrinks as three necessary restrictions are imposed in turn:

So, these two green slivers in the parameter space show the possibilities so far. But what happens if we go on to consider, also, the need for the forced oscillations about the equilibrium to be small?


Ratio of radial oscillations to orbit radius Ratio of radial oscillations to orbit radius

The plots on the right show the ratio of the radial oscillations to the complete radius of the orbit, if we take the erroneous formula for them in equation (51) of the paper seriously. The two plots correspond to the two cases for the green sliver: either for very small values of w (orbits close to the lighter star), or values near 1 (orbits close to the heavier star).

This ratio never drops below about 3. In other words, equation (51) of the paper implies that, for the regions of the parameter space allowed on other grounds, the oscillations in the planet’s orbital radius are always at least 3 times larger than the equilibrium orbital radius.

To put it another way, if we require the radial oscillations to be smaller than the orbital radius, the region of the parameter space to which the paper’s analysis is claimed to be applicable, according to its own criteria, is the empty set.


Ratio of radial oscillations to orbit radius Ratio of radial oscillations to orbit radius

If we correct for the missing factors in the paper’s version of the oscillation amplitudes, the situation becomes farcical in a different way.

The plots on the left show that the correct value for the radial oscillations becomes reasonably small when the orbit is very close to the lighter star (w is small), and the heavier star is much heavier (b is large).

But in that region of parameter space, we might as well be talking about the Earth as the lighter object and the sun as the heavier one. What the analysis in the paper would then amount to is the claim that if we put a satellite into orbit from pole-to-pole around the Earth, the plane of the orbit would be locked into facing towards the sun throughout the year. We have more than enough experience of the actual orbits of such satellites to know that this is not the case.


References

[1] “Stable Conic-Helical Orbits of Planets Around Binary Stars: Analytical Results” by E. Oks, The Astrophysical Journal, 804:106 (11pp), 2015 May 10. Online at IOP Web Site.

[2] “Worlds of Pure Imagination” by Adam Hadhazy, New Scientist No. 3039, 19 September 2015, pp 34–38.

[3] “The Kozai Mechanism and the Stability of Planetary Orbits in Binary Star Systems” by K.A. Innanen, J.Q. Zheng, S. Mikkola & M.J. Valtonen, The Astronomical Journal, Volume 113, No. 5, pp 1915–1919, May 1997. Full article available at Smithsonian Astrophysical Observatory/NASA Astrophysics Data System.

[4] “Polar Orbits Around Binary Stars” by Greg Egan, on the arXiv preprint server.

[5] “Erratum: Stable Conic-Helical Orbits of Planets Around Binary Stars: Analytical Results” by E. Oks, The Astrophysical Journal, 823:69 (1pp), 2016 May 20. Online at IOP Web Site.

[6] “Astrophysical Analogue of One-Electron Rydberg Quasimolecules: One-Planet Binary Star Systems” by N. Kryukov and E. Oks, International Review of Atomic and Molecular Physics, 6 (1), January-June 2015. Online at Serials Publications Pvt. Ltd. web site.



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