Series of bumps sent Uranus into its sideways spin

Oct 07, 2011
Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

(PhysOrg.com) -- Uranus’s highly tilted axis makes it something of an oddball in our Solar System. The accepted wisdom is that Uranus was knocked on its side by a single large impact, but new research to be presented on Thursday 6th October at the EPSC-DPS Joint Meeting in Nantes rewrites our theories of how Uranus became so tilted and also solves fresh mysteries about the position and orbits of its moons. By using simulations of planetary formation and collisions, it appears that early in its life Uranus experienced a succession of small punches instead of a single knock-out blow. This research has important ramifications on our theories of giant planet formation.

Uranus is unusual in that its spin axis is inclined by 98 degrees compared to its orbital plane around the Sun. This is far more pronounced than other , such as Jupiter (3 degrees), Earth (23 degrees), or Saturn and Neptune (29 degrees). is, in effect, spinning on its side.

The generally accepted theory is that in the past a body a few times more massive than the Earth collided with Uranus, knocking the planet on its side. There is, however, one significant flaw in this notion: the moons of Uranus should have been left orbiting in their original angles, but they too lie at almost exactly 98 degrees.

This long-standing mystery has been solved by an international team of scientists led by Alessandro Morbidelli (Observatoire de la Cote d’Azur in Nice, France), who will be presenting his group’s research on Thursday 6th October at the EPSC-DPS Joint Meeting in Nantes, France.

Morbidelli and his team used simulations to reproduce various impact scenarios in order to ascertain the most likely cause of Uranus’ tilt. They discovered that if Uranus had been hit when still surrounded by a protoplanetary disk – the material from which the moons would form – then the disk would have reformed into a fat doughnut shape around the new, highly-tilted equatorial plane. Collisions within the disk would have flattened the doughnut, which would then go onto form the moons in the positions we see today.

However, the simulation threw up an unexpected result: in the above scenario, the moons displayed retrograde motion – that is to say, they orbited in the opposite direction to that which we observe. Morbidelli’s group tweaked their parameters in order to explain this. The surprising discovery was that if Uranus was not tilted in one go, as is commonly thought, but rather was bumped in at least two smaller collisions, then there is a much higher probability of seeing the moons orbit in the direction we observe.

This research is at odds with current theories of how planets form, which may now need adjusting. Morbidelli elaborates: “The standard planet formation theory assumes that Uranus, Neptune and the cores of Jupiter and Saturn formed by accreting only small objects in the protoplanetary disk. They should have suffered no giant collisions. The fact that Uranus was hit at least twice suggests that significant impacts were typical in the formation of giant planets. So, the standard theory has to be revised.”

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Nanobanano
1.7 / 5 (6) Oct 07, 2011
I can offer an alternative explanation, which I've explained in the past.

If Uranus had a close encounter with one or more object(s) from a different orbital plane (i.e. above or below it's own orbit the way Pluto and Sedna are,) the gravitation would cause it's spin axis to be tilted, and moreover, it would capture objects as moons along a plane orthogonal to it's own orbital plane.

This is actually to be expected over alleged cosmic time scales when you have an asymetric group of orbiting objects on many different orbiting planes: i.e. planets, moons, asteroids, comets, etc.

The gravity and other force enteractions will tend to push all objects more and more towards the plane where the most mass was: which happens to be the same as the "eight" major planets.

Over time, orbital planes would become tilted enough that planetesmals which eventually became the enclined moons would have close encounters with Uranus.
Nanobanano
1 / 5 (4) Oct 07, 2011
Well, you might say, "Well then why aren't Saturn and Jupiter tilted, if that's the case."

Well, because they are so much more massive that it would take a much, much larger impact or close encounter to alter their spin axis. In fact, about the only way to alter Jupiter's spin axis significantly would be if something the size of Saturn had a head on collision with it at one of it's poles...So J and S could absorb a non-planar collision and show little evidence of it due to mass...

Now Uranus is far enough out there, and massive enough, to encounter many, many TNO (trans-neptunian objects) during it's life time, which are on different planes above and below itself.

Whenever it encounters these objects, some are collisions, some are captured as moons, some are ejected from the system. In all cases, some of their non-planar angular momenta is transfered to the planet, tilting it's axis.
Nanobanano
1 / 5 (4) Oct 07, 2011
Then you will say, "Well, why doesn't that happen to Neptune."

Well, Neptune's orbit is much larger, and so it takes much longer time scales for "events" to happen that far out from the Sun.

We know Neptune and Pluto orbit on different orbital planes, but that they also "cross" so that they change in relative distance to the Sun.

Over cosmic time scales, the gravitational attraction between Neptune and Pluto should cause their obits to seek the same plane. I mean several billion years, not thousands, millions or even one billion. Like if they manage to survive the Sun's red giant phase...

So eventually, if the universe the SS is around that long, there would be a close encounter between Neptune and Pluto, and they will either collide, or Pluto will be captured as a non-planar moon, or one of them will be ejected (probably Pluto.) Neptune's axis will be tilted from the normal when this happens.
bwvandorn
not rated yet Oct 07, 2011
When a "solution" to a small problem requires revision to a larger standard theory, I tend to doubt the solution. If Uranus & Neptune formed between the orbits of Jupiter & Saturn, as some current theories propose, then Uranus being perturbed by a close encounter with Jupiter or Saturn on it's outward migration requires no impacts at all. A close encounter with Saturn may also help explain Saturn's 29 degree tilt as you would expect little or no tilt (such as Jupiter's 3 degrees) for such a relatively large planet.
emsquared
1 / 5 (1) Oct 07, 2011
"Series of bumps sent Uranus into its sideways spin"

That's what she said!!

C'mon... did no one else honestly pick up on that.
mikedraghici
1 / 5 (3) Oct 07, 2011
I wrote an article on how the earth got it's tilt and fault lines, also from an impact, if you search google for this title you can read it "Cosmic Impact Site That Created Earths Axial Tilt and Fault Lines"
RealScience
4.5 / 5 (2) Oct 07, 2011
Coalescing bodies are likely to have L4 and L5 companions of considerable mass coalescing at the same time.
Lagrange point 4 and Lagrange point 5 orbits are metastable in multi-planet systems, so such companions are likely to eventually strike their larger siblings.
The speed of an L4/5 companion impacting would be just right to have formed earth's moon (this was first suggested at least a decade ago).
An L4 and an L5 collision would be good candidates to tilt Uranus.
antialias_physorg
2.8 / 5 (4) Oct 08, 2011
Another alternative could be that Uranus isn't an original member of our solar system but formed (along with its moons) on its own somewhere else.
Gas and material can coalesce gravitationally anywhere - not only in the vicinity of a sun.

In that case we could see any kind of axis inclination/moon orbit without having to resort to impact theories. (Not saying that the impact theories are wrong, just that we should keep in mind the larger picture - i.e. the rest of the galaxy)
Nanobanano
2.7 / 5 (3) Oct 08, 2011
Another alternative could be that Uranus isn't an original member of our solar system but formed (along with its moons) on its own somewhere else.
Gas and material can coalesce gravitationally anywhere - not only in the vicinity of a sun.


Well, see, that's unlikely. Not saying it's impossible, but Uranus orbits on the same plane as the other 7 major planets. It's just tilted very much on it's axis.

So for example, it's more likely that Pluto or Sedna came from some place else, rather than Uranus.

I'm not saying it's impossible, just saying that statistically it's highly unlikely for a capture event from another star system or a rogue planet to just so happen to line up perfectly with the local orbital planes.

In my theory, everything was created near the Sun on it's own orbital planes, and they migrated to the averages, this "squeezed" some of the planetesmals' orbital planes down into Uranus' gravitational environment, and it captured them as moons.
Nanobanano
3 / 5 (3) Oct 08, 2011
I admit there are problems with my theory as well, because Uranus' moons should not all orbit on or even near the "exact" same plane, but should be rather chaotic, perhaps oriented as much as plus or minus 30 degrees from one another. however, it's possible over cosmic time scales that their gravity caused them to seek an average plane as well....

Then again, the impact theories do not explain that either.
antialias_physorg
3 / 5 (3) Oct 08, 2011
Well, see, that's unlikely. Not saying it's impossible, but Uranus orbits on the same plane as the other 7 major planets. It's just tilted very much on it's axis.


Planets that would orbit on any other plane would very quickly be brought down to that plane. Any time a planet orbits above the plane there would be a net downward gravitational force and vice versa. Being close to the plane is the only stable configuration for a planet (native to our solar system or not)

Smaller panets should be more likely to have some sort of extraplanar orbit because they are much more affected by flybys of other, massive objects. Se eccentricity of smaler planes would be less surprising than those of larger planets.

Of course then there is Venus. If we say Uranus is tilted by 93 degrees then Venus is even more of a poser since it is tilted 177 degrees.
omatumr
1.7 / 5 (6) Oct 09, 2011
Given the chaos of supernova debris, it is remarkable that the spin axis of most plants is still so close to their orbital plane around the Sun.

www.omatumr.com/Origin.htm

"Neutron Repulsion", The APEIRON Journal, in press (2011)

http://arxiv.org/...2.1499v1
Shootist
2 / 5 (3) Oct 09, 2011
Of course then there is Venus. If we say Uranus is tilted by 93 degrees then Venus is even more of a poser since it is tilted 177 degrees.


Why does it require an impact or large scale interactions with other large bodies? Mars has been modeled to have a plus/minus 90 degree wobble in its axis of rotation. Why not Venus and Uranus? Perhaps we've just happened to catch them at their oddball best?
d_robison
not rated yet Oct 11, 2011
@Nanobanano

One large problem with this theory is where are these bodies that are large enough to cause Uranus to tilt as much as it has? If there were such an object would it not effect Neptune as well? The general pattern for objects in the far edge of the solar system (that aren't Uranus and Neptune) are small icy bodies; the largest of which, Eris, has a fairly inclined orbit, but it has a mass of only .23 moons (not nearly enough to tilt Uranus through gravitational forces alone).

Gravitational forces could have had some effect on Uranus' extreme axis tilt, but the contribution is likely small. Object collisions (especially in early solar system formation) are frequent and extremely powerful.
d_robison
not rated yet Oct 11, 2011
@Nanobanano

One large problem with this theory is where are these bodies that are large enough to cause Uranus to tilt as much as it has? If there were such an object would it not effect Neptune as well? The general pattern for objects in the far edge of the solar system (that aren't Uranus and Neptune) are small icy bodies; the largest of which, Eris, has a fairly inclined orbit, but it has a mass of only .23 moons (not nearly enough to tilt Uranus through gravitational forces alone).

Gravitational forces could have had some effect on Uranus' extreme axis tilt, but the contribution is likely small. Object collisions (especially in early solar system formation) are much more frequent and extremely powerful.
antialias_physorg
5 / 5 (1) Oct 11, 2011
One large problem with this theory is where are these bodies that are large enough to cause Uranus to tilt as much as it has?

They either crashed into Uranus and just merged with it or (if it was glancig blows) were ejected from the solar system. If the objects were puverized to less than a few dozen or maybe a few hundred kilometers in diameter 'particles' then they could still be around and we just haven't detected them yet. It's very hard to detect planets that far out wih such little mass.

The question is: Statistically multiple impacts should average out. The more impacts we propose the more likely the resulting change in tilt is close to zero.
d_robison
not rated yet Oct 11, 2011
One large problem with this theory is where are these bodies that are large enough to cause Uranus to tilt as much as it has?

They either crashed into Uranus and just merged with it or (if it was glancig blows) were ejected from the solar system. If the objects were puverized to less than a few dozen or maybe a few hundred kilometers in diameter 'particles' then they could still be around and we just haven't detected them yet. It's very hard to detect planets that far out wih such little mass.

The question is: Statistically multiple impacts should average out. The more impacts we propose the more likely the resulting change in tilt is close to zero.


If they merged with Uranus that would be considered a collision which is not what Nanobanano is claiming. If they did not collide with Uranus, and were massive enough to tilt it essentially 90 degrees, then one would expect a much more eccentric or inclined orbit rather than a fairly circular and non-inclined orbit
JonSilveus
not rated yet Oct 12, 2011
"This long-standing mystery has been solved" - I definitely do not see such arrogance to be qualified here. For one, they had to "tweak their data" to get it to match their desired outcome, so then it would be equally (if not more) plausible for the scenario to have been different than what they wished it. I do understand tweaking models can have positive results, but the opposite is also common (particularly in the case of paradigm paralysis).

I am surprised to not see binary companion postulations on the perturbations since a long period companion (such as Tyche) could well fit the needed modulations. Or is such still a taboo with too many?

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