Flipping hot Jupiters: Why some planets orbit the wrong way

May 11, 2011
The transiting giant planet orbits very close to the star and in a direction opposite to the stellar rotation. This peculiar configuration results from gravitational perturbations by another much more distant planet (upper left). Credit: Credit: Lynette Cook

(PhysOrg.com) -- In the last few years astronomers have observed that in some extrasolar systems the star is spinning one way and the planet, a "hot Jupiter," is orbiting the star in the opposite direction. A Northwestern University research team is the first to model how these huge planets got so close to their stars -- thanks to gravitational perturbations by a much more distant planet -- and how the planets' orbits can flip in the process.

More than 500 extrasolar planets -- planets that orbit stars other than the sun -- have been discovered since 1995. But only in the last few years have astronomers observed that in some of these systems the star is spinning one way and the planet, a "hot Jupiter," is orbiting the star in the opposite direction.

"That's really weird, and it's even weirder because the planet is so close to the star," said Frederic A. Rasio, a theoretical astrophysicist at Northwestern University. "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 ."

Figuring out how these huge got so close to their led Rasio and his research team to also explain their flipped orbits. Using large-scale , they are the first to model how a hot Jupiter's orbit can flip and go in the direction opposite to the star's spin. Gravitational perturbations by a much more result in the hot Jupiter having both a "wrong way" and a very close orbit. (A hot Jupiter is a huge Jupiter-like planet in very close proximity to the central star.)

"Once you get more than one planet, the planets perturb each other gravitationally," Rasio said. "This becomes interesting because that means whatever orbit they were formed on isn't necessarily the orbit they will stay on forever. These mutual perturbations can change the orbits, as we see in these extrasolar systems."

Details of the study will be published May 12 by the journal Nature.

In explaining the peculiar configuration of an extrasolar system, the researchers also have added to our general understanding of planetary system formation and evolution and reflected on what their findings mean for the solar system.

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

Rasio, a professor of physics and astronomy in Northwestern's Weinberg College of Arts and Sciences is the senior author of the paper. The first author is Smadar Naoz, a postdoctoral fellow at Northwestern and a Gruber Fellow.

The physics the research team used to solve the problem is basically orbital mechanics, Rasio said, the same kind of physics NASA uses to send satellites around the .

"It was a beautiful problem," said Naoz, "because the answer was there for us for so long. It's the same physics, but no one noticed it could explain hot Jupiters and flipped orbits."

"Doing the calculations was not obvious or easy," Rasio said, "Some of the approximations used by others in the past were really not quite right. We were doing it right for the first time in 50 years, thanks in large part to the persistence of Smadar."

"It takes a smart, young person who first can do the calculations on paper and develop a full mathematical model and then turn it into a computer program that solves the equations," Rasio added. "This is the only way we can produce real numbers to compare to the actual measurements taken by astronomers."

In their model, the researchers assume a star similar to the sun, and a system with two planets. The inner planet is a gas giant similar to Jupiter, and initially it is far from the star, where Jupiter-type planets are thought to form. The outer planet is also fairly large and is farther from the star than the first planet. It interacts with the inner planet, perturbing it and shaking up the system.

The effects on the inner planet are weak but build up over a very long period of time, resulting in two significant changes in the system: the inner gas giant orbits very close to the star and its orbit is in the opposite direction of the central star's spin. The changes occur, according to the model, because the two orbits are exchanging angular momentum, and the inner one loses energy via strong tides.

The gravitational coupling between the two planets causes the inner planet to go into an eccentric, needle-shaped orbit. It has to lose a lot of angular momentum, which it does by dumping it onto the outer planet. The inner planet's orbit gradually shrinks because energy is dissipated through tides, pulling in close to the star and producing a hot Jupiter. In the process, the of the planet can flip.

Only about a quarter of astronomers' observations of these hot systems show flipped orbits. The Northwestern model needs to be able to produce both flipped and non-flipped orbits, and it does, Rasio said.

Explore further: Lucky star escapes black hole with minor damage

More information: "Hot Jupiters From Secular Planet–Planet Interactions" Nature (2011)

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Greene
not rated yet May 11, 2011
This brings up an interesting question for me. Does the plane of the ecliptic for various star systems in a galaxy align with the plane of the ecliptic for the host galaxy?
Johannes414
2.1 / 5 (9) May 11, 2011
A more simple explantion would be that the current theory on star and planet formation is incorrect or incomplete. But instead astronomers, like Ptolemy, would rather develop a complex rescue device to save the theory.
Nik_2213
5 / 5 (3) May 11, 2011
"Does the plane of the ecliptic for various star systems in a galaxy align with the plane of the ecliptic for the host galaxy?"

No. IIRC, they are randomly inclined, which is why Doppler exo-planet detection techniques can only give an upper limit to their mass. eg "M sin i < 10 MJup"

You need to do a lot of careful spectroscopy to figure out the actual axis direction.

This angle issue also applies to the 'transit' method of detection, which can only pick up ~10% of 'hot jupiters' and even less if they're further out...
El_Nose
not rated yet May 11, 2011
How does frame dragging in the gravity well effect the overall speed of the planet -- is the planet's momentum a heck of a lot greater than normal because the star is actually dragging it slightly in the opposite direction and it is so close???
Skeptic_Heretic
4.5 / 5 (2) May 11, 2011
How does frame dragging in the gravity well effect the overall speed of the planet -- is the planet's momentum a heck of a lot greater than normal because the star is actually dragging it slightly in the opposite direction and it is so close???

That would explain why the hot jupiter's have migrated so close to their host stars however, I think I have a reasonable hypothesis for this behavior.

The majority of stars surveyed in star forming regions are binary or trinary. It is suspected that most stars are formed in large groups of greater than 3 and slowly shift apart as they revolve around the galaxy. It would be reasonable to me to discover that 'planet trading' may be a common aspect as one approaches more densely populated star regions.
Nik_2213
5 / 5 (3) May 11, 2011
IIRC, frame-dragging is a very small effect compared to that of solar tides...
gunslingor1
1 / 5 (1) May 11, 2011
2 questions:

1) I think this would reasonably explain the closeness of these planets to the star, but I do not at all see how this explains the flip?

2) If the effects of multiple planets in a system is really that great, why the heck is our's so darn stable with a record 8 1/2 planets?
Skeptic_Heretic
3 / 5 (2) May 11, 2011
If the effects of multiple planets in a system is really that great, why the heck is our's so darn stable with a record 8 1/2 planets?
There are a great many stable planetary systems that we've discovered outside of our own.

We also have a lot of evidence that the stability of our system is the result of a multitude of catastrophic events as evidenced by the status of Neptune, Venus, Mercury, and Uranus.
gunslingor1
3 / 5 (2) May 11, 2011
Yeah, I understand all that, but if what they are saying is accurate, shouldn't the orbit of Jupiter (real Jupiter, not extra-Jupiter), shouldn't it be decaying relatively rapidly? I say relatively meaning that before the sun dies, we would have a hot Jupiter. But eveything I've studied about our system predicts very little change in orbits until our sun starts the end of its life. I don't know, I'm just an engineer with a novice knowledge of physics, plus, this article is severely lacking.... This site should realy start including equations and more simulations.
sender
1 / 5 (1) May 11, 2011
Hope this proves precession is a force of nature to be harnessed for renewable power applications.
omatumr
3 / 5 (3) May 11, 2011
More than 500 extrasolar planets have been discovered since 1995.


What about the extrasolar planets discovered earlier?

Extrasolar planets were first discovered in 1992 [1].

They were confirmed in 1994 [2].

With kind regards,
Oliver K. Manuel

1. A. Wolszczan and D. Frail (1992). "A planetary system around the millisecond pulsar PSR1257 + 12". Nature 355 (6356): 145147.

2. A. Wolszczan (1994). "Confirmation of Earth Mass Planets Orbiting the Millisecond Pulsar PSR B1257+12". Science 264 (5158): 538542.
El_Nose
not rated yet May 11, 2011
1) our own solar system is relatively balanced -- most all of the movement has already taken place and the planets are happy where they are

2) planetary orbits are strongly modeled by the kepler equations and more strongly by adding in gravity of other large objects and A LOT of trigonometry to boot

Jupiter isin't moving closer because it has enough momentum to be perfectly happy where it is... now if it takes on more mass it will move a little closer to the sun and we all ( the other planets ) will adjust.

the biggest reason we are so stable is that our orbit around the milky way doesn't take us too close to any other stars
Silverhill
5 / 5 (2) May 11, 2011
Johannes414 said:
A more simple explantion would be that the current theory on star and planet formation is incorrect or incomplete. But instead astronomers, like Ptolemy, would rather develop a complex rescue device to save the theory.
Did you not read this part?
Some of the approximations used by others in the past were really not quite right. We were doing it right for the first time in 50 years...
Their model was not (necessarily) more *complex*, but it was more *correct*. (Or, based on your phrasing, *less incorrect* and *less incomplete*.)
pauljpease
not rated yet May 11, 2011
Maybe I'm just being dense but I'm not sure I understand what they mean by a "flipped" orbit. Are they talking about the direction of ROTATION of the planet, compared the direction of rotation of the star, or the direction of REVOLUTION of the planet relative to the direction of rotation of the star. I can believe this if its the former (a plant spinning one way, then stopping and spinning up the other way), but find it harder to believe if the latter (a planet revolving in one direction, then stopping, then revolving in the other direction).
that_guy
3.7 / 5 (3) May 11, 2011
@gunslinger - have you heard the phrase "There's more than one way to skin a cat"? There's multiple ways for sytems to enter stable or unstable configurations. And yes, our theory of planet formation is almost definitely incomplete as one of the other commentors suggested.

@paul - I suppose the true definition of backward orbit would be antegrade to the star's rotation, but I'm not sure they are capable of determining that. The article states that they are comparing two planets in a system and have determined that they are orbiting opposite each other, so one is *definitely* orbiting 'backwards'.

only about a quarter of these systems show a flipped orbit

Only?? I'm fairly sure that a quarter of the systems is a huge amount compared to expectations.
BillFox
1 / 5 (1) May 11, 2011
Wouldn't this theory assume that at some point in time, the planet is not moving around the star at all while it reverses? That angular momentum they talk about is the very force keeping all planets (and satellites for that matter) from falling into their gravitational anchors... Poor science, bad programming, whatever you wish to call this load.
RealScience
1 / 5 (1) May 12, 2011
BillFox - When the inner planet has an eccentric, needle-shaped orbit, it is moving very slowly at when it is farthest from its star. It thus take very little energy to change its motion from very slow clockwise to very slow counterclockwise.
And at that point it can pass close to the outer planet, whose gravitational pull can supply the energy needed.
In doing so the outer planet is boosted to a higher orbit by conservation of angular momentum.
You are correct that if the tranfer robs the inner planet of all of its angular momentum, it will fall into the central star (Jupiter does this to comets on a fairly frequent basis).
But if the transfer is more than the inner planet's angular momentum before the encounter, then the inner planet will end up with a retrograde orbit.

antialias
5 / 5 (1) May 12, 2011
why the heck is our's so darn stable with a record 8 1/2 planets?

Wether 8 planets is a lot or nor is something we don't know. currently we can only detect planets which fit very specific criteria (either very massive or massive and close to the parent star or the plane of rotation happens to be just so aligned so that we see it edge on)

If we were to sctutinize a planetary system like ours from afar we would probably not detect any planets with current methods.

Are they talking about the direction of ROTATION of the planet, compared the direction of rotation of the star, or the direction of REVOLUTION

Direction of revolution. An opposite direction of rotation isn't totally unheard of (e.g. Venus, Uranus (and Pluto) rotate in the opposite direction from what is considered 'usual')
kevinrtrs
1.5 / 5 (8) May 12, 2011
First of all we need to keep in mind that this "result" is what comes out of a simulation and that no matter how accurate they get to modelling the movements currently observed, it does not mean that it's actually how the planets got to where they are in reality. The model is just one possibility and it could be completely WRONG.
Like Johannes1 said - they need to come up with lots of complicated explanations because the current planetary formation model is simply totally inadequate. Questions arise quickly:
1. How did the planet start migrating inwards from where it supposedly formed?
2. What stopped it from crashing into the star?
3. Where did that initial pertubation come from in the first place and can they actually still observe it? It should still be there, given their assumption that it happened over a long period of time.

We ...have...evidence that the stability of our system is the result of a multitude of catastrophic events..

More like major miracles, you mean.
antialias
5 / 5 (3) May 12, 2011
Wouldn't this theory assume that at some point in time, the planet is not moving around the star at all while it reverses?


Not necessarily. Think about a planet rotating around a star. Now start tipping the plane of rotation - for whatever reason. If you tip it 180 degrees then you will end up with a retrograde orbit and the planet never having changed its speed. a 'swing-by' of another massive object could do the trick.

1. How did the planet start migrating inwards from where it supposedly formed?

Many reasons are possible:
- Collisons which chang the momentum
- The stellar system moved through a dust cloud which caused 'friction losses'
- Near misses or gravitational interaction with other planets, stars....

2. What stopped it from crashing into the star?

Simple celestial mechanics: If the angular momentum isn't reduced to exactly zero it will just go to a lower orbit and stay there.

3.

See the answer to 1. - most of which we can't observe directly
gunslingor1
3 / 5 (2) May 12, 2011
Interesting discussions, thanks
Skeptic_Heretic
5 / 5 (4) May 12, 2011
We have evidence that the stability of our system is the result of a multitude of catastrophic events.
More like major miracles, you mean.
I don't consider the utter destruction of the crust of Mercury, the massive impacts on Mars and Venus, nor the slingshotting of Neptune and Uranus to the outer solar system (thanks to Jupiter) to be miraculous as I understand basic physics.

Get an education so someone with knowledge like myself won't be able to scam you out of your cash and free time.
that_guy
3.5 / 5 (4) May 12, 2011
We need a lot more commenters like anti-alias.