Feasibility of a retrograde planet

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Feasibility of a retrograde planet

Post by ciceron on 11th August 2009, 10:39 am

I was wondering if a planet could have a retrograde orbit in any fashion stable in a crowded neiboughood like Sun's system.

Also , the ways a retrograde orbit could be achieved are of some interest to me. Free-floater captured , slingshot'd by a close star encounter...

Is there any way this kind of orbit would be stable at the Gy mark ?

And , are there known planets to present this behavior ?

Thank you

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Re: Feasibility of a retrograde planet

Post by Lazarus on 11th August 2009, 12:54 pm

There have been a fair few papers considering retrograde resonances in extrasolar planetary systems.

In fact this appeared on the arXiv today...
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Re: Feasibility of a retrograde planet

Post by ciceron on 11th August 2009, 1:48 pm

the math is a little over my head , but it seems indeed intriguing the coincidence Smile

I'll have to ask for earth's twin detection next


The dynamical fits presented in the present paper tend to
show that, over the eight studied multi-planetary systems,
six of them are liable to be regulated by a mechanism involving
a counter-revolving configuration with a retrograde
MMR. Except for the HD37214 and HD160691 systems for
which the retrograde fits are indeniably bad, the whole of
other fits are slightly better than fits in prograde configurations.
Nevertheless, it remains necessary to perform new
series of observations in order to enlarge the observational
data samples and, as a consequence, to obtain more precise
results.

That was my gut feeling , but i couldn't even begin to formulate the math for it. Nice to know there is work in progress on that.

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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 11th August 2009, 7:13 pm

There's at least one protoplanetary disk (can't think of the system's name) that has one part of it rotating prograde, and another part retrograde.

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Re: Feasibility of a retrograde planet

Post by Lazarus on 12th August 2009, 2:32 pm

...and now WASP-17b... WHAT HAVE YOU DONE, CICERON???
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Re: Feasibility of a retrograde planet

Post by ciceron on 12th August 2009, 2:48 pm

Oops...sorry..i'll write a hundred lines "I will not mess with the celestial mechanics" on the blackboard Wink

Now , back on track , what are the chances of a catastrophic event serius enougth to turn a planet orbit all the way backwars , or could cause a debris ring to do a retrograde on its star?

On the other hand , i was under the impression that angular momentum has to be conserved , so , althougth the orbits of other planets can be stabilized , the whole system should act funny , there should be something readly detectable on this kind of systems , an anomaly on the angular momentun versus spectral class. Any studies or thougths on this matter ?

Be rigth back , have to walk the wormhole Wink

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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 13th August 2009, 8:56 am

Nice work Ciceron =). Now go ask about the feasibility of an Earth-like planet around Alf Cen Laughing Laughing.

IIRC, if you take any of the planets in the solar system, and make them go retrograde all of the sudden, it actually adds to the stability of the system because the planets spend less time in closer proximity during their close approaches (and thus less gravitational influence over time).

I can imagine planet-planet scattering could induce a retrograde planet, but I can't begin to imagine the odds. I VERY seldom see this when toying around in Gravity Simulator.

So perhaps WASP-17 (and other such... oddballs) had a counter-rotating disk. Is it possible for a stationary, non-rotating cloud to collapse in radial sections? The inner cloud collapses inward into a disk followed by the outer cloud collapsing shortly afterward into another, outer disk? Could this allow for their angular momenta to be different? I can't see how this would be stable in the long run though.

Maybe WASP-17 b got hit hard by something.The planet is very anomalously large. I would think that the impactor would have to be retrograde to produce enough force to do that.

But then how did the impactor get into a retrograde orbit?

I'm stumped. Crying or Very sad

(Post Scriptum)
Lazarus wrote:...and now WASP-17b... WHAT HAVE YOU DONE, CICERON???
That is the most hilarious thing I have read in the past month, thanks Smile

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Re: Feasibility of a retrograde planet

Post by ciceron on 13th August 2009, 1:26 pm

In fact , i was wondering EXACTLY if a double star system wouldn't be the perfect place for a retrograde planet , more to the point , would it make it easier of more difficult to be detected , say , around Alfa Centaur B ?

I suppose it would make the RV detection more difficult , but if the planet is not in a transing eclising orbit , and the RV variation is masked by other bodies in a regular orbit , we woldn't be capable of detection till the DARWIN mission

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Re: Feasibility of a retrograde planet

Post by Lazarus on 13th August 2009, 6:44 pm

And now HAT-P-7b... the plot thickens...
Sirius_Alpha wrote:I can imagine planet-planet scattering could induce a retrograde planet, but I can't begin to imagine the odds. I VERY seldom see this when toying around in Gravity Simulator.
Would be interesting to know what kind of setups you have tried... Coplanar or mutually-inclined? How many planets? 2? 3? more?
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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 13th August 2009, 6:49 pm

Lazarus wrote:Would be interesting to know what kind of setups you have tried... Coplanar or mutually-inclined? How many planets? 2? 3? more?

All coplanar. Usually I spawn a few hundred particles between orbits of various planets in multi-planet system to see how long they would last, sort of like an asteroid belt.

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Re: Feasibility of a retrograde planet

Post by Lazarus on 23rd August 2009, 4:37 pm

I suspect that the Kozai mechanism may help with getting planets into retrograde orbits - this effect would not occur in coplanar simulations. Might be worth starting the planets off with small mutual inclinations?
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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 25th August 2009, 3:59 am

IRAS 16293-2422 is the protostar with the counter-rotating disks, by the way (Remijan & Hollis 2006).

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Re: Feasibility of a retrograde planet

Post by Lazarus on 26th August 2009, 1:09 pm

Going back to the experiments with Gravity Simulator, going for a combination of planet scattering (by putting the planetary orbits too close together) and Kozai oscillations induced by a binary star companion seems to work quite well at producing retrograde orbits.
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Re: Feasibility of a retrograde planet

Post by ciceron on 26th August 2009, 5:45 pm

Sounds like Alfa Centauri A-B to me Wink
Anyone willing to make a model including retrograde planets around B in a crowded-like system and predict what the ligth curve migth be when a kepler-like resolution telescope take a peek there?

I demand the planet to be named "Rogue" if there is indeed a planet like that in this system

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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 26th August 2009, 6:26 pm

All else held constant, the light curve of a retrograde transiting planet is identical to the light curve of a prograde transiting planet.

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Re: Feasibility of a retrograde planet

Post by ciceron on 27th August 2009, 9:02 am

well , of course , the individual retrograde planet would have the secondary and primary dips reversed , but it wouldn't be discernable from the others. silly me.
Appart from doppler effects that i don't think would be easy to perform for a terrestrial planet arround Alfa Cent , in the near future , how could you tell if it is indeed retrograde?

And still , i wonder , shouldn't there be some funny thing about the stellar rotation in a system that gets one massive enougth object going backwards? I mean , conservation of angular momentum should do very bad things to that kind of systems. Or , if not bad things , things at least interesting to note at the stelar level, if we are talking about jovian-mass bodies in near orbit around red dwarfs, in the mid M range like Proxima

well , thank you for reading my rants. -my math is not good enougth to figure the answers by myself , so i will continue to imagine systems weird enougth to resemble what is awaiting out there for us to peek at them.

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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 27th August 2009, 9:26 am

ciceron wrote:Appart from doppler effects that i don't think would be easy to perform for a terrestrial planet around Alfa Cent , in the near future , how could you tell if it is indeed retrograde?
As far as I know, the only way to do this is to measure the Rossiter-McLaughlin effect. If (as will likely be the case), the hypothetical Alf Cen Ab or Bb is in a non-transiting orbit, this is of course impossible. With only knowing the radial velocity, you don't have a clue what direction the planet is actually moving (other than toward or away). Remember that as long as HAT-P-7 b has been known from radial velocity measurements, no one suggested it was in a retrograde orbit until it's Rossiter-McLaughlin effect was measured. However, even though we know WASP-17 b and HAT-P-7 b are in retrograde orbits, we don't know just how the orbit looks in the plane of the sky (whether the planet(s) transit from north-to-south in the sky, or from east-to-west in the sky). Essentially, we don't know the full orbit in 3D.

Furthermore, as far as I know there's no way to determine exactly how the stellar rotation axis is inclined relative to Earth. We can in some cases measure the rotation axis's inclination, but we don't know if we're seeing the north stellar pole, south stellar pole, or which direction the star is rotating. Remember that all we get to see is a dot of light that might as well be infinitesimally small (except for nearby stars (Altair) or epic huge stars (Alpha Ori) that have been spatially resolved).

So in summary, we don't even know which way the orbit of the planet is orientated, or which way the stellar rotation axis is orientated.

I guess given sufficient technological development, after the orbit of a planet is astrometrically fully known in 3D, you could use an amazing telescope and watch star spots move across the surface to determine for sure the stellar rotation axis.

Ciceron wrote:And still , i wonder , shouldn't there be some funny thing about the stellar rotation in a system that gets one massive enougth object going backwards? I mean , conservation of angular momentum should do very bad things to that kind of systems. Or , if not bad things , things at least interesting to note at the stelar level, if we are talking about jovian-mass bodies in near orbit around red dwarfs, in the mid M range like Proxima
Anything that orbits a body faster than the body rotates will have its orbit raised. An example is the moon/Earth system. Lunar gravity pulls up the oceans, Earth's rotation puts the oceans slightly ahead of the sub-Lunar point on Earth, and the raised mass of the oceans has a gravitational exertion on the moon. Since the mass is ahead of the moon (albeit slightly, but it is ahead of the sub-lunar point on Earth's surface), the moon gets a little gravitational "push" in the direction it's already orbiting. Increasing orbital velocity increases semi-major axis... thus the moon drifts away.

Anything that orbits a body slower than the body rotates will have its orbit lowered. An example is Phobos. It raises a little bit of tides on Mars, but since the moon is below synchronous orbit of Mars (and thus orbits Mars faster than Mars rotates), the tidally raised area "drags" behind Phobos, exerting a gravitational pull against the moon's direction of travel. This lowers it's orbital velocity. Slower orbital velocity decreases semi-major axis... Phobos falls into Mars.

(and for completion, any bodies that are tidally locked will not experience a chance in semi-major axis)

The Neptune/Triton system could be thought of as being similar to WASP-17 b (or any retrograde orbit.... ). Triton raises a tide on Neptune, but since Neptune is rotating in the opposite direction as the moon is orbiting, the tide gets pushed behind the sub-Triton point on Neptune. This has the same effect as Phobos does on Mars, in that Triton is slowed by the gravitational pull of the tidally raised part of Neptune. This is why Triton's orbit is decaying.

Stellar rotation rates are usually on the order of weeks. 35 or so days for our sun. Hot Jupiters of course orbit their stars much faster. So I am sure that most hot Jupiters are destined for tidal obliteration, with the retrograde ones to die faster.

Expect a retrograde planet to have a very low eccentricity. Triton's is (as far as I know) indistinguishable from zero. I think Wikipedia says it's measured out to 16 decimal places or something (not that Wikipedia or my memory are 100% reliable...). Tidal circularisaton seems to be quite efficient for retrograde orbits.

Edit:
I would assume a more massive planet would exert a more massive tide, which would exert a more massive gravitational force on the planet, causing its orbit to drop faster than a planet with lower mass. Maybe that's why they're talking about WASP-18 b's orbital period changes being detectable in only a decade, being 10 Jupiter-masses and all.


Last edited by Sirius_Alpha on 27th August 2009, 9:45 am; edited 4 times in total (Reason for editing : Spelling, added bit about stellar rotation rates. Just can't quit adding stuff.)

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Re: Feasibility of a retrograde planet

Post by ciceron on 29th August 2009, 5:39 am

thank you for your extensive explanation. I'm thinking of some of the implications of your explanation about what happens with the tides and semi-major axis variation.

I was refreshing my math from a pair of decades past , about conservation of angular momentum with the help of wikipedia
http://en.wikipedia.org/wiki/Angular_momentum
And that pointed me to the question of what the effects would be on a complex system like Alfa Cent.

My next world question will be more exotic, i hope Smile

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Re: Feasibility of a retrograde planet

Post by Lazarus on 29th August 2009, 7:43 am

Sirius_Alpha wrote:Expect a retrograde planet to have a very low eccentricity. Triton's is (as far as I know) indistinguishable from zero. I think Wikipedia says it's measured out to 16 decimal places or something (not that Wikipedia or my memory are 100% reliable...). Tidal circularisaton seems to be quite efficient for retrograde orbits.
WASP-17b, retrograde planet, orbital eccentricity = 0.129
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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 29th August 2009, 9:00 am

Lazarus wrote:WASP-17b, retrograde planet, orbital eccentricity = 0.129
Hmm. Do you think this may just be an exception? Or was I wrong about retrograde orbits being more efficient at tidal circularisation?

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Re: Feasibility of a retrograde planet

Post by ciceron on 29th August 2009, 9:02 am

maybe the event that made it retrograde is recent and the orbit hasn't had time to shed the eccentrity.

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Re: Feasibility of a retrograde planet

Post by Lazarus on 29th August 2009, 5:19 pm

Triton's low orbital eccentricity is a bit of a problem, as apparently tides would take longer than the age of the solar system for reasonable post-capture orbits to get to such a low value. Probably something in the environment around Neptune helped the process, perhaps a circumplanetary disc helped things along...
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Re: Feasibility of a retrograde planet

Post by Sirius_Alpha on 29th August 2009, 7:38 pm

Triton's interaction with a circumplanetary disk at Neptune would be interesting, since Triton is not co-planar with the rest of Neptune's moon system.

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