The Flip Side of Exoplanet Orbits

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It was once thought that our planet was part of a “typical” solar system. Inner rocky worlds, outlying gas giants, some asteroids and comets sprinkled in for good measure. All rotating around a central star in more or less the same direction. Typical.

But after seeing what’s actually out there, it turns out ours may not be so typical after all…

Astronomers researching exoplanetary systems – many discovered with NASA’s Kepler Observatory – have found quite a few containing “hot Jupiters” that orbit their parent star very closely. (A hot Jupiter is the term used for a gas giant – like Jupiter – that resides in an orbit very close to its star, is usually tidally locked, and thus gets very, very hot.) These worlds are like nothing seen in our own solar system…and it’s now known that some actually have retrograde orbits – that is, orbiting their star in the opposite direction.

“That’s really weird, and it’s even weirder because the planet is so close to the star. How can one be spinning one way and the other orbiting exactly the other way? It’s crazy. It so obviously violates our most basic picture of planet and star formation.”

– Frederic A. Rasio, theoretical astrophysicist, Northwestern University

Now retrograde movement does exist in our solar system. Venus rotates in a retrograde direction, so the Sun rises in the west and sets in the east, and a few moons of the outer planets orbit “backwards” relative to the other moons. But none of the planets in our system have retrograde orbits; they all move around the Sun in the same direction that the Sun rotates. This is due to the principle of conservation of angular momentum, whereby the initial motion of the disk of gas that condensed to form our Sun and afterwards the planets is reflected in the current direction of orbital motions. Bottom line: the direction they moved when they were formed is (generally) the direction they move today, 4.6 billion years later. Newtonian physics is okay with this, and so are we. So why are we now finding planets that blatantly flaunt these rules?

The answer may be: peer pressure.

Or, more accurately, powerful tidal forces created by neighboring massive planets and the star itself.

By fine-tuning existing orbital mechanics calculations and creating computer simulations out of them, researchers have been able to show that large gas planets can be affected by a neighboring massive planet in such a way as to have their orbits drastically elongated, sending them spiraling closer in toward their star, making them very hot and, eventually, even flip them around. It’s just basic physics where energy is transferred between objects over time.

It just so happens that the objects in question are huge planets and the time scale is billions of years. Eventually something has to give. In this case it’s orbital direction.

“We had thought our solar system was typical in the universe, but from day one everything has looked weird in the extrasolar planetary systems. That makes us the oddball really. Learning about these other systems provides a context for how special our system is. We certainly seem to live in a special place.”

– Frederic A. Rasio

Yes, it certainly does seem that way.

The research was funded by the National Science Foundation. Details of the discovery are published in the May 12th issue of the journal Nature.

Read the press release here.

Main image credit: Jason Major. Created from SDO (AIA 304) image of the Sun from October 17, 2010 (NASA/SDO and the AIA science team) and an image of Jupiter taken by the Cassini-Huygens spacecraft on October 23, 2000 (NASA/JPL/SSI).

38 Replies to “The Flip Side of Exoplanet Orbits”

  1. is it possible that these planets have entered into the orbit from outside that particular system and thus have a different momentum and orbital direction?

    1. The odds of that happening are astronomical (pun intended!) because space is a big place; furthermore, even if it were to occur, the captured rogue planet would probably end up in a highly inclined and/or eccentric orbit.

      1. That will be true initially, but if the rogue planet happens to be captured into an orbit that brings it close to the star then tidal interactions with the star will reduce the eccentricity and inclination over time. Look at Triton’s orbit around Neptune: zero inclination and zero eccentricity.

        PS. I don’t like DISQUS; my old user name for this site no longer works.

      2. Hmm… just when I got to like DISQUS, I now find that my last reply comment to you has mysteriously disappeared!

  2. According to this paper, “Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems“, it states that “the interaction between a [star’s magnetic field] and its circumstellar disc can (but not always) have the effect of pushing the stellar spin axis away from the disc angular momentum axis toward the perpendicular state and even the retrograde state. Planets formed in the disc may therefore have their orbital axes misaligned with the stellar spin axis, even before any additional planet-planet scatterings or Kozai interactions take place. In general, magnetosphere–disc interactions lead to a broad distribution of the spin–orbit angles, with some systems aligned and other systems misaligned.”

  3. According to this paper, “Evolution of Spin Direction of Accreting Magnetic Protostars and Spin-Orbit Misalignment in Exoplanetary Systems“, it states that “the interaction between a [star’s magnetic field] and its circumstellar disc can (but not always) have the effect of pushing the stellar spin axis away from the disc angular momentum axis toward the perpendicular state and even the retrograde state. Planets formed in the disc may therefore have their orbital axes misaligned with the stellar spin axis, even before any additional planet-planet scatterings or Kozai interactions take place. In general, magnetosphere–disc interactions lead to a broad distribution of the spin–orbit angles, with some systems aligned and other systems misaligned.”

  4. The suggestion that retrograde rotation of a star started out as a perturbed highly eccentric orbit sounds plausible. Suppose we have a model stellar system with two planets. One planet is in a highly eccentric or elliptical orbit and the other in a near circular orbit. The circular orbit crosses near the elliptical orbit. There might then be a sort of gravitational billiards that causes the perturbed elliptical orbit to cross the semi-major axis and to begin to orbit in a retrograde direction. The angular momentum change or reversal of this planetary orbit is compensated for a similar change in the angular momentum of the circularly orbiting planet. The highly elliptical orbit has far less angular momentum than the circular orbit, and so a comparatively small change in its angular momentum by such an interaction could change the orientation of the orbit.

    It might mean that some early stellar systems of planets of this nature could adjust the angular momentum of such a planet so that it falls into the star.

    What is odd are the planets which orbit on planes that are completely skewed from the plane perpendicular to the angular momentum vector of the stellar rotation.

    LC

    1. New Scientist had an article — Planet found orbiting its star backwards for first time — on that possibility:

      [The planet WASP-17b] may have been thrown onto the strange path after a near-collision with another, as-yet-undetected, planet in the same system. “A near-collision with the right trajectories can make a gravitational slingshot that flings one of the planets into a retrograde orbit,” says team member Coel Hellier of Keele University.

    2. New Scientist had an article — Planet found orbiting its star backwards for first time — on that possibility:

      [The planet WASP-17b] may have been thrown onto the strange path after a near-collision with another, as-yet-undetected, planet in the same system. “A near-collision with the right trajectories can make a gravitational slingshot that flings one of the planets into a retrograde orbit,” says team member Coel Hellier of Keele University.

  5. The suggestion that retrograde rotation of a star started out as a perturbed highly eccentric orbit sounds plausible. Suppose we have a model stellar system with two planets. One planet is in a highly eccentric or elliptical orbit and the other in a near circular orbit. The circular orbit crosses near the elliptical orbit. There might then be a sort of gravitational billiards that causes the perturbed elliptical orbit to cross the semi-major axis and to begin to orbit in a retrograde direction. The angular momentum change or reversal of this planetary orbit is compensated for a similar change in the angular momentum of the circularly orbiting planet. The highly elliptical orbit has far less angular momentum than the circular orbit, and so a comparatively small change in its angular momentum by such an interaction could change the orientation of the orbit.

    It might mean that some early stellar systems of planets of this nature could adjust the angular momentum of such a planet so that it falls into the star.

    What is odd are the planets which orbit on planes that are completely skewed from the plane perpendicular to the angular momentum vector of the stellar rotation.

    LC

  6. Isn’t our view skewed it that we see only weird systems , because it is what we _can_ detect at the moment. Perhaps the majority of “normal” systems lurk there waiting for us to catch up with technical possibilites.

      1. The relatively rare likelihood of retrogrades among the rarest size of exoplanets doesn’t argue for our weirdness. I took it as a “weird” form of the lately expressed sentiment that planetary formation will have to be worked over to explain the newfound diversity.

        If anything so far it is striking how many or most parameters of our own system falls smack dab in the middle of the distributions, such as our pattern of planetary size following Kepler’s r-2 predictions, low eccentricities common et cetera.

        It does look rare as the number of planets go, but larger systems that for example Kepler hasn’t gotten to yet should naively have more planets I think (more protoplanetary disk mass). As for now, the ~ 7 (less Mercury) planets that would be observable by a lucky nearby ‘exo-Kepler’ touches the 5-6 seen in the most numerous planet systems.

        And of course you would expect many-planet systems be likelier to have inhabited planets, so there is some combined selection bias & effect to take care of.

      2. It’s too soon to say that we are in a rare system.

        1. Our few methods are not completely effective yet. 500 is not much. We don’t know if there are more planets in the systems with a few planets.
        2. My belief is that there are more systems like our own.

    1. The relatively rare likelihood of retrogrades among the rarest size of exoplanets doesn’t argue for our weirdness. I took it as a “weird” form of the lately expressed sentiment that planetary formation will have to be worked over to explain the newfound diversity.*

      If anything so far it is striking how many or most parameters of our own system falls smack dab in the middle of the distributions, such as our pattern of planetary size following Kepler’s r-2 predictions, low eccentricities common et cetera.

      It does look rare as the number of planets go, but larger systems that for example Kepler hasn’t gotten to yet should naively have more planets I think (more protoplanetary disk mass). As for now, the ~ 7 (less Mercury) planets that would be observable by a lucky nearby ‘exo-Kepler’ touches the 5-6 seen by us in the most numerous planet systems.

      And of course you would expect many-planet systems be likelier to have planets in the habitable zone, so there is some combined selection bias & effect to take care of.

      * Maybe one less problem now.

      1. And, IIRC, Venus’s orbit around the Sun(its year) takes less time than a full rotation of Venus(its day).

  7. Now retrograde movement does exist in our solar system. Venus rotates in a retrograde direction, so the Sun rises in the west and sets in the east, and a few moons of the outer planets orbit “backwards” relative to the other moons. But none of the planets in our system have retrograde orbits; they all move around the Sun in the same direction that the Sun rotates.

      1. Yes, Venus rotates the opposite direction as the other planets but still moves around the Sun in the same direction. So, were you to stand on the surface of Venus (an ill-advised move) you would notice that the Sun rises in the west. Very slowly though, since a day on Venus is 243 Earth-days long!

        I mentioned “other moons”…Phoebe, an irregularly-shaped moon of Saturn, orbits in a retrograde motion. Just one example of where this can found in our solar system: http://lightsinthedark.wordpress.com/2009/03/20/a-primordial-moon/

      2. So, were you to stand on the surface of Venus (an ill-advised move) you would notice that the Sun rises in the west. Very slowly though, since a day on Venus is 243 Earth-days long!

        Actually, although the sidereal rotation of Venus is -243.019 (retrograde rotation; therefore, negative number) Earth-days, an observer, standing on the surface of Venus, would observe a day/night cycle of 116.75 Earth-days, according to the formula:

        D = 1/(1/P − 1/R);

        where…
        P is the orbital period (224.7 days);
        R is the sidereal rotation (-243.019);
        D is the resulting day/night cycle.

        ∴ 1 ÷ [(1 ÷ 224.7) − (1÷ -243.019)] = 116.75 Earth-days

        … with the Sun rising in the West (as you stated) and setting in the East.

      3. But that’s observed subjective day/night time, as opposed to actual day/night time, correct? Otherwise where did the 243.019 come from? And so what’s the Earth’s observed/actual day/night period then?

      4. Now, I know where is the confusion. The article is mostly about retrograde orbits and then you jump in with retrograde rotation and east and west comparison. The word rotation is too general, at least for me. Spinning is better. Then, I kinda didn’t bother with east and west. 😀 It’s important to stress that it’s the rotation around its axis. Also, it’s not a good idea to mix it in the same paragraph.

        When reading quickly you get easily confused.

        2 things:
        retrograde orbit
        retrograde spin

    1. Let me see if I can clear some confusion while we wait for the experts (I’m a layman here):

      Retrograde means angular motion in the opposite direction of another (here primary positioned) body. Say, as our sun.

      It is widely expected from planetary system formation that the protoplanetary disc rotation will impart its angular momentum to the star and the planets. They will mostly orbit around the star in the same direction that the star rotates, and they will rotate in the same direction as it does.

      This is also widely observed. And it also applies to some degree to moons of planets. But here is a lot more chances for exceptions such as captured moons, and that is seen too. Thus we have moons suspected of capture that happens* to be orbiting in the “backward”, retrograde, direction from the direction its planet rotates. (Which in turn is the same direct, prograde, rotation direction as our sun have.)

      Venus is our own system’s exception. It is orbiting in the prograde direction or our suns rotation all right. But it is slowly rotating in the opposite, retrograde, direction from what our sun or we do.**
      —————————————–
      * That would depend on its trajectory at the time of capture. Some will end up in a prograde orbit, some a retrograde.

      ** Slowness and direction suggests that Venus too, like us, was hit with a massive last impactor that could have reversed its rotation depending on angle of impact.

      Or it could have tipped on its axis and leaked away parts of its angular momentum similar to what is suggested here perhaps.

  8. At the same time our system’s star mass,* star metallicity, orbit sizes, orbit eccentricities, planetary masses is well within the likeliest range of found or predicted distributions.

    It is perhaps better to say that diversity is so large that among the many planet systems ‘virtually’ (more like precisely AFAIK) every one is unique.

    And then here is yet another characteristic in which our system is exactly what you would expect. The mediocrity principle seems alive and well to me.

    * Well, M stars are more numerous of course, but our G2V [Wp] is within the observed distribution of stars having exoplanets.

  9. This is quite interesting. It explains retrograde orbits without invoking the Kozai mechanism, or at least greatly reducing the necessary size of the perturbing outer body to cause the effect. In addition, it seems to provide an alternative migratory approach for planets rather than the Type I & Type II migrations that are currently theorized.

  10. Our planet is really a special one. It is the only planet in the solar system that is populated by selfish billionaires, millionaires, by rich countries scrambling to the resources of poor nations. Triton in itself is an oddity in the solar system. Its orbit is opposite to its planet, Neptune, meaning it probably came from another system, but you do not buy that kind of an idea. Now, when talking about planets in other systems, about their weird orbits etc.., you seem to believe that massive planets occasionally tug them up. I therefore conclude that the 10th planet being proposed by Sitchin based on ancient evidence is true.true.true.

    1. There is no evidence to suppose Triton is from another system. It is much more likely the moon’s unusual orbit is likely a result of early solar system gravitational interactions. The inner edge of the Keplar belt terminates at Neptune’s orbit. This indicates the planet played a key role in clearing celestial objects in this area. The working assumption (until further explorations merit) is that Triton is one of these objects.

      There are eight known planets. There’s no evidence of a ninth or tenth. Ancient writings can interpreted any way you like – they are no substitute for modern research and observations.

      Personal theories are grounds for comment removal on UT. Just a heads up.

      1. It’s almost a certainty that Triton is a former member of the kuiper belt due to it’s size and general composition matching up well with other known members of this population. It’s composition is almost identical to Pluto’s for example, although it is slightly larger than the dwarf planet.

        As far as the 9th or 10th planet stuff goes…that all remains to be seen. We don’t have conclusive proof of either being highly likely, but there are things such as the orbit of Sedna that do suggest the possibility.

  11. Retrograde moon: A forgotten challenge
    Why some hot Jupiters orbit in the reverse? I think this question requires us to look at the retrograde moons, also, of Jupiter, Saturn. Actually, such moons are known for more than 100 years but we have never considered the point – if they can pose a problem in learning basic, college level mechanics. For more information, see the second point in my Letter to the Editor of CHANGE, May-June 2008, p. 5, entitled: Trouble with the Solar System, http://www.changemag.org…tters-to-editor.html

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