Prograde Orbit of Exosolar Planets

[dougettinger]

Do all the Exosolar Planets observed to date (assuming the necessary information is observed) have prograde orbits with respect to their binary star orbits ?
I would think that for binaries and their planets to be created angular momentum would have to be conserved making prograde orbits a necessary condition.

[bystander]

No, see:

RAS/ESO: Turning Planetary Theory Upside Down
http://asterisk.apod.com/vie ... 31&t=19042

[Chris Peterson]

Do all the Exosolar Planets observed to date (assuming the necessary information is observed) have prograde orbits with respect to their binary star orbits ?
I would think that for binaries and their planets to be created angular momentum would have to be conserved making prograde orbits a necessary condition.

Yeah, that's what you'd think. But it isn't the case- there are a number of examples of planetary systems that appear to have retrograde orbits with respect to the direction of rotation of their parent star (or stars). You can be sure this is causing lots of head scratching and hypothesizing. It certainly argues that the standard models of stellar system formation need some work.

[hstarbuck]

How about a stable figure eight orbit in some kind of resonance (possibly with some negative feedback to keep it stable??)? I'm pretty sure that there are no observations of this, but are there simulations that work? Then, part of the year you would be in prograde and the other part, around the other star, you would be in retrograde.
This isn't quite what I was thinking--all the masses seem to be the same. From http://www.scholarpedia.org/article/Three_body_problem

[Chris Peterson]

How about a stable figure eight orbit in some kind of resonance (possibly with some negative feedback to keep it stable??)? I'm pretty sure that there are no observations of this, but are there simulations that work? Then, part of the year you would be in prograde and the other part, around the other star, you would be in retrograde.

I think a configuration like that is too unlikely to explain the number of retrograde orbits that have been seen.

[neufer]

1) The main angular momentum constituent of the solar system:
Jupiter has an inclination of over 6° vis-a-vis the solar equator.

1) Earth, itself, has an inclination of over 7° vis-a-vis the solar equator.

2) Our solar system used to have a planet with an inclination of almost 35° to the invariant plane.

<<Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered, by astronomer Heinrich Olbers on March 28, 1802. Pallas was at first considered a planet, as were the other early asteroids 1 Ceres, 3 Juno, and 4 Vesta, until the discovery of many additional asteroids led to their re-classification. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.

With a mass estimated to be 7% of the total mass of the asteroid belt, Pallas is one of the largest asteroids. Its diameter is some 530–565 km, comparable to or slightly larger than that of 4 Vesta, but it is 20% less massive, placing it third among the asteroids. It appears to be the largest irregularly shaped body in the Solar System, not completely rounded under its own gravity. The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite meteorites. >>

[Chris Peterson]

Our solar system used to have a planet with an inclination of almost 35° to the invariant plane.

It is not difficult to explain orbital inclinations of a few degrees for massive planets, or even a few tens of degrees for minor planets. It is very difficult, however, to understand how planets can end up with completely retrograde orbits.

[BMAONE23]

Would Solar rotational dynamics allow for a stable rotation and still allow the star to slowly tumble (rotate along 2 axis')

[Chris Peterson]

Would Solar rotational dynamics allow for a stable rotation and still allow the star to slowly tumble (rotate along 2 axis')

Maybe. More likely than tumbling, however, would be some kind of interaction that just tipped the star's axis. It happens to planets (like Venus). It seems to me a simpler explanation for retrograde planetary orbits that the star got flipped, than that planets somehow ended up orbiting against the orbital direction of the presolar nebula.

[wonderboy]

Are retrograde planetary ortbits slowing down? I'm just trying to understand.

If it is -going against the grain- of the main stars rotation, would this not cause the orbit of the planet to be slowed?

Paul.

[dougettinger]

Would Solar rotational dynamics allow for a stable rotation and still allow the star to slowly tumble (rotate along 2 axis')

Maybe. More likely than tumbling, however, would be some kind of interaction that just tipped the star's axis. It happens to planets (like Venus). It seems to me a simpler explanation for retrograde planetary orbits that the star got flipped, than that planets somehow ended up orbiting against the orbital direction of the presolar nebula.

I am not sure how a flipped star in a binary system causes retrograde orbits of neighboring planets. Flipping would cause retrograde spin of the star, but how are the orbits affected by this process?

Doug Ettinger
Pittsburgh, PA

[dougettinger]

1) The main angular momentum constituent of the solar system:
Jupiter has an inclination of over 6° vis-a-vis the solar equator.

1) Earth, itself, has an inclination of over 7° vis-a-vis the solar equator.

2) Our solar system used to have a planet with an inclination of almost 35° to the invariant plane.

<<Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered, by astronomer Heinrich Olbers on March 28, 1802. Pallas was at first considered a planet, as were the other early asteroids 1 Ceres, 3 Juno, and 4 Vesta, until the discovery of many additional asteroids led to their re-classification. The Palladian orbit, at 34.8°, is unusually highly inclined to the plane of the main asteroid belt, and the orbital eccentricity is nearly as large as that of Pluto, making Pallas relatively inaccessible to spacecraft.

With a mass estimated to be 7% of the total mass of the asteroid belt, Pallas is one of the largest asteroids. Its diameter is some 530–565 km, comparable to or slightly larger than that of 4 Vesta, but it is 20% less massive, placing it third among the asteroids. It appears to be the largest irregularly shaped body in the Solar System, not completely rounded under its own gravity. The Palladian surface appears to be a silicate material; the surface spectrum and estimated density resemble carbonaceous chondrite meteorites. >>

I am missing the point. Inclined orbits have some affect on angular momemtum of the system. But what is the connection between inclined orbits and angular momentum being conserved in the formation of a binary system with retrograde planetary orbits?

Doug Ettinger
Pittsburgh, PA

[neufer]

Our solar system used to have a planet with an inclination of almost 35° to the invariant plane.

It is not difficult to explain orbital inclinations of a few degrees for massive planets, or even a few tens of degrees for minor planets. It is very difficult, however, to understand how planets can end up with completely retrograde orbits.

The earth endured a collision with a Mars sized object to create the moon.

If these two young planets had had a near miss instead then the smaller one of them could easily have gone retrograde.

In such a scenario, the retrograde planet most likely would have ended up fairly close to the sun thereby making it a more easily observable exoplanet for someone else.

[Chris Peterson]

I am not sure how a flipped star in a binary system causes retrograde orbits of neighboring planets. Flipping would cause retrograde spin of the star, but how are the orbits affected by this process?

The rotational direction of the star is what defines "prograde" and "retrograde" for that star system. Retrograde planetary orbits are those that are in an opposite direction to the rotation of their star. So all you need to do is flip the rotational axis of the star 180° and you have created a system where the planets are in retrograde orbits.

I'm not actually talking about binary systems here, just single stars. It's more complex in a binary system. I could imagine that such a system started with three stars, and they interacted in a way that resulted in one being ejected, with a transfer of angular momentum such that the remaining pair end up with retrograde orbits around each other. But while that sounds perfectly feasible, it doesn't seem like planets would survive such an event.

[Chris Peterson]

If these two young planets had had a near miss instead then the smaller one of them could easily have gone retrograde.

I don't see that as likely. I think the energy required to reverse the orbital direction of a planet (as opposed to its rotational direction) would probably destroy the planet. I guess you could construct some sort of slingshot orbit, where the planet got close to a second star; that could certainly change the direction of an orbit. But it would be tricky without ejecting the planet from the system completely. Given that a number of these retrograde systems have been seen, I think the cause should be something simpler.

[wonderboy]

It probably would destroy it, and end up shattering into pieces. What if this planet in question was a moon of a larger planet which the sun swallowed. Apparently models have shown that gas giants in our solar system swallowed other planets etc etc. Say that the moon in question was highly elipitical or had quite a distant orbit, when the sun swallowed the planet, the moon was at a distance which rendered it safe, it was orbiting in the opposite direction to the sun and entered the area of the sun at such an angle that it ended up captured by the sun.

what if some (not all) retrogade planets are moons of planets swallowed by the main star in such a fashion?

Paul


This post has been edited to sort out my dodgy explanation.

[dougettinger]

If these two young planets had had a near miss instead then the smaller one of them could easily have gone retrograde.

I don't see that as likely. I think the energy required to reverse the orbital direction of a planet (as opposed to its rotational direction) would probably destroy the planet. I guess you could construct some sort of slingshot orbit, where the planet got close to a second star; that could certainly change the direction of an orbit. But it would be tricky without ejecting the planet from the system completely. Given that a number of these retrograde systems have been seen, I think the cause should be something simpler.

I support Chris on this matter of collisions not causing retrograde orbits. It is rather apparent that major collisions of some sort have occurred in our solar system and no retrograde orbits of planets and major satellites exist.

Doug Ettinger
Pittsburgh, PA

[dougettinger]

I am not sure how a flipped star in a binary system causes retrograde orbits of neighboring planets. Flipping would cause retrograde spin of the star, but how are the orbits affected by this process?

The rotational direction of the star is what defines "prograde" and "retrograde" for that star system. Retrograde planetary orbits are those that are in an opposite direction to the rotation of their star. So all you need to do is flip the rotational axis of the star 180° and you have created a system where the planets are in retrograde orbits.

For binary systems I thought the orbital direction of the minor star around the major star defined what was prograde.

I'm not actually talking about binary systems here, just single stars. It's more complex in a binary system. I could imagine that such a system started with three stars, and they interacted in a way that resulted in one being ejected, with a transfer of angular momentum such that the remaining pair end up with retrograde orbits around each other. But while that sounds perfectly feasible, it doesn't seem like planets would survive such an event.

This is a stupid question because I should have known this already. Is there adequate observed data to support known rotation or spin of stars in binary systems as well as systems where exosolar planets have been discovered ?

Doug Ettinger
Pittsburgh, PA

Doug Ettinger
Pittsburgh, PA

[neufer]

If these two young planets had had a near miss instead then the smaller one of them could easily have gone retrograde.

I don't see that as likely. I think the energy required to reverse the orbital direction of a planet (as opposed to its rotational direction) would probably destroy the planet. I guess you could construct some sort of slingshot orbit, where the planet got close to a second star; that could certainly change the direction of an orbit. But it would be tricky without ejecting the planet from the system completely. Given that a number of these retrograde systems have been seen, I think the cause should be something simpler.

Well, you are probably right as regards the earth whose orbital velocity (30 km/s) exceeds its escape velocity (11 km/s);
but what about a planet like Jupiter whose escape velocity (60 km/s) well exceeds its orbital velocity (13 km/s)?

The back of Jupiter presents a large cross section to objects coming up behind it with orbital velocities ~ 16 km/s and which then get left with retrograde orbital velocities ~ 4 km/s. Such repositioned retrograde gas giants with small perihelia orbits will be the ones most easily visible to distant exoplanet astronomers even though they are unrepresentative of the solar system as a whole.

[bystander]

Podcast: Rotation
Universe Today - 23 April 2010

Everything in the Universe is spinning. In fact, without this rotation, life on Earth wouldn't exist. We need the conservation of angular momentum to flatten out galaxies and solar systems, to make planets possible. Let's find out about the physics involved with everything that spins, and finally figure out the difference between centripetal and centrifugal force.

Astronomy Cast: Ep. 181: Rotation.


xkcd: Angular Momentum

[dougettinger]

Podcast: Rotation
Universe Today - 23 April 2010

Everything in the Universe is spinning. In fact, without this rotation, life on Earth wouldn't exist. We need the conservation of angular momentum to flatten out galaxies and solar systems, to make planets possible. Let's find out about the physics involved with everything that spins, and finally figure out the difference between centripetal and centrifugal force.

Astronomy Cast: Ep. 181: Rotation.


xkcd: Angular Momentum

I enjoyed your cartoon and the podcast. I am an amatuer astronomer and scientist like yourself. In Oklahoma what makes clouds begin to spin into tornadoes? I know of three things and perhaps you can add others. They are the gravity field of the Earth, the surface of the Earth, and the Corolis affect. So what things inside a giant molecular cloud (GMC) cause it to begin to spin coherently and then fragment into other coherently rotating cells like tornadoes in Oklahoma? A shock wave from a supernova would make things even more random and incoherent. The surface of the landscape in Pittsburgh is less coherent and, hence, we have less tornadoes. Can you prop up this nebular hypothesis for me perhaps with a good analogy? What causes the coherent rotation to center around a point and reach out several hundred AU's to cause a protostar disk ?

Doug Ettinger
Pittsburgh, PA

[Chris Peterson]

So what things inside a giant molecular cloud (GMC) cause it to begin to spin coherently and then fragment into other coherently rotating cells like tornadoes in Oklahoma? A shock wave from a supernova would make things even more random and incoherent.

It takes nothing more than gravity and the net angular momentum of the cloud. Self gravity causes condensation, and the conservation of angular momentum requires the condensed structure to increase its rotation rate (just like an ice skater pulling in his arms to increase his spin rate). A shock wave doesn't make things more random; it increases the density in some areas which increases the effect of that region's self gravity. Diffusion increases randomness; shock waves reduce it.

[bystander]

In Oklahoma what makes clouds begin to spin into tornadoes? I know of three things and perhaps you can add others. They are the gravity field of the Earth, the surface of the Earth, and the Corolis affect.

http://en.wikipedia.org/wiki/Tornado#Life_cycle
http://en.wikipedia.org/wiki/Tornadogenesis

[dougettinger]

You did not appreciate my analogy of why horizontal disks form in the Earth's atmosphere and for the same reasons cannot form in a GMC. Other factors are needed.

Doug Ettinger

[Chris Peterson]

You did not appreciate my analogy of why horizontal disks form in the Earth's atmosphere and for the same reasons cannot form in a GMC. Other factors are needed.

Analogies are dangerous things, that need to be used with care. The mechanisms involved in creating tornadoes and stellar accretion discs are very different. I don't think you can reasonably compare the two.