# Space News: Uranus rocked, planets smashed



## Brian G Turner (Jul 3, 2018)

I know it's only been a few days since the last roundup, but there have been too many exciting news stories to ignore:


*1. Uranus was struck by planet*

Uranus spins at an odd angle compared to the rest of the planets in the solar system, suggesting a major collision in the past. Now a study suggests that Uranus was struck a glancing blow by a planet twice the mass of Earth: 'Cataclysmic' collision shaped Uranus' evolution



> The research could also help explain the formation of Uranus' rings and moons, with the simulations suggesting the impact could jettison rock and ice into orbit around the planet. This rock and ice could have then clumped together to form the planet's inner satellites and perhaps altered the rotation of any pre-existing moons already orbiting Uranus.
> 
> The simulations show that the impact could have created molten ice and lopsided lumps of rock inside the planet. This could help explain Uranus' tilted and off-centre magnetic field.



The big question is: if another planet hit Uranus, where did that go?

*2. Asteroid belt had few parents*

Studies of the asteroid belt between Jupiter and Mars suggest most of the rocks there may have originated from the destruction of 5 minor planets: Study reveals secret origins of asteroids and meteorites



> at least 85 percent of 200,000 asteroids in the inner asteroid belt—the main source of Earth's meteorites—originate from five or six ancient minor planets. The other 15 percent may also trace their origins to the same group of primordial bodies, said Stanley Dermott, lead author and a theoretical astronomer at the University of Florida.



This follows on from recent studies suggesting that many of the asteroids shared familiar chemical compositions, suggesting shared sources. Perhaps one of them really was Phaeton?


*3. Japanese reach asteroid - samples soon*

Meanwhile, the Japanese probe Hayabusa 2 has arrived in orbit around the asteroid Ryugu - with the intention of taking a sample to return with to Earth. Phil Plait provides a further explanation of the strange dynamics of the diamond-shaped asteroid: What would it be like to stand on the surface of Ryugu?

Also worth mentioning is that the BBC reports that Ryugu has a retrograde orbit, which may suggest an extrasolar origin.


*4. Ceres' gleaming crater up close*

NASA's Dawn spacecraft, now in orbit around the dwarf planet Ceres, has made its closest approaches yet - allowing it to take clearer picture of the mysterious Occator Crater: Dawn's latest orbit reveals dramatic new views of Occator crater






*5. Our galaxy just got a lot bigger*

A study of metallic elements in stars suggests the Milky Way might be bigger than originally thought: How Long Would It Take to Cross the Milky Way at Light Speed?



> When looking beyond the previously assumed boundary of the Milky Way's disk, scientists were surprised to see stars with compositions resembling those of disk stars.
> 
> The new study estimates the size of the Milky Way's disk at 200,000 light-years across. Past studies have suggested the Milky Way is between 100,000 light-years and 160,000 light-years across.



That's a heck of a change, and likely to have all kinds of ramifications, not least when accounting for the number of stars and mass within the Milky Way. Perhaps our future merger with Andromeda will be more one-sided than we expected?


*6. BONUS! Maths story: Drop Pi for Tau*

The number pi is famous, but there's a growing movement to demote it for tau, aka 2pi - because 2pi appears in far more formulas than just pi: Why Tau Trumps Pi



> At its heart, pi refers to a semicircle, whereas tau refers to the circle in its entirety. Mathematician and poet Mike Keith ... said that thinking in terms of pi is like reaching your destination and saying you're twice halfway there.




Also, don't forget other stories posted to our Science & Nature section, not least: First confirmed image of a newborn planet.


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## night_wrtr (Jul 3, 2018)

Brian G Turner said:


> The big question is: if another planet hit Uranus, where did that go?



The simulation in that article seems to show that it absorbed the other planet completely after a second strike. Very cool. It gets hit and the pieces are dragged back in with Uranus' gravity for the second one. The impact and absorption shot a lot of debris out. I wonder if that explains why it has 27 moons!


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## Venusian Broon (Jul 3, 2018)

A bit of fun about the last story, from about 6 years ago:






I agree wholeheartedly with the final score, being a fully-blooded Pi man 

None of this Tau nonsense.


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## Vertigo (Jul 4, 2018)

On the pi front isn't Tau already in use in relativity and nothing to do with pi? Isn't that just going to cause confusion?


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## Venusian Broon (Jul 4, 2018)

Vertigo said:


> On the pi front isn't Tau already in use in relativity and nothing to do with pi? Isn't that just going to cause confusion?



Exactly, another great argument


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## Brian G Turner (Jul 5, 2018)

night_wrtr said:


> The simulation in that article seems to show that it absorbed the other planet completely after a second strike.



Sorry, yes, I hurried that line in as an afterthought - I meant to ask where it came from, rather than where it went. 

Existing models on our solar system's formation struggle enough to account for the planets we have, let alone an additional one twice the mass of Earth.

IIRC the most accurate model to date predicts an additional planet forming, before gravity slingshots it out of the solar system - but that is described as "Neptune-like", which would put it at around 17 Earth masses, not 2: Five-planet Nice model - Wikipedia


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## Venusian Broon (Jul 5, 2018)

Brian G Turner said:


> Sorry, yes, I hurried that line in as an afterthought - I meant to ask where it came from, rather than where it went.
> 
> Existing models on our solar system's formation struggle enough to account for the planets we have, let alone an additional one twice the mass of Earth.
> 
> IIRC the most accurate model to date predicts an additional planet forming, before gravity slingshots it out of the solar system - but that is described as "Neptune-like", which would put it at around 17 Earth masses, not 2: Five-planet Nice model - Wikipedia



I know you want to have some sort of body come from outside the initial solar system ....

But we are talking about practically an infinite number of configurations to try and explain what we see now. I doubt they have even scratched the surface on what may have first occurred in terms of different initial conditions, both in terms of positions and actual number of proto-planets and mass distribution. _I would not take the results of the NICE model to be correct or the last word on the solar system's initial set up and development._

It was developed to explain certain characteristics we see now, but only a subset of observations that they had at the time. I am sure that putting in more data, say assuming that Uranus was hit by such a planet (note twice the size of earth is really virtually nothing in terms of mass of the solar system) could still result in a set of initial conditions that would give rise to our current system. And would not require a planet to come in from outside.

It should also be noted that although they talk about the fifth giant planet being Neptune-like they also need the rest of the disk to have something like 15 Earth masses of material outside the giant planets at minimum for the model to operate (at least, I guess, to make it more _probable_). In fact reading between the lines, the more mass the better. And one of the issues is that there does not seem to be as much of this mass about today (Perhaps some of it collided with Uranus )

Remember we are talking about a chaotic many-body system. You _cannot_ solve any equations of motion for more than three relatively massive objects. Therefore one cannot just rewind time on a system as complex as the solar system as if it is clockwork and make 100% definitive statements about initial conditions. Note that the discussion of the NICE model talks exclusively about probabilities as to what may have happened - as it should.


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## Brian G Turner (Jul 5, 2018)

Indeed, the composition of the solar system is a problem, but I take the Nice-5 model as illustrative of this rather than a definitive solution. The collision with Uranus makes for an interesting piece of the puzzle.


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## thaddeus6th (Jul 5, 2018)

Thanks for keeping us up to date with news of Uranus.

On a more serious note, that's really rather interesting. Given that, and the potential Earth-Moon collision in the past, it seems a bit peculiar. Given all the space within the solar system, collisions between large celestial bodies should surely be rarer?

I have vague memories of one planet (Neptune, I think) having an off-kilter orbital path, whereas all the others are on the same plane. It could be Uranus, I suppose, but if it's Neptune I wonder if that got whacked as well.


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## Joshua Jones (Jul 5, 2018)

I will set aside all the potential Uranus jokes for a moment...

If they are talking about a glancing blow, I could see where some of the bits would wind up inside the planet and others be ejected and captured as moons and ring structures. I would think that the remainder would either wind up as comets, Kupiter objects, or ejected into interstellar space, maybe becoming Alpha Centauri's Oumuamua...


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## Old_Man_Steve2016 (Jul 11, 2018)

Saturn: Hey Uranus! You're on your side! 
Uranus: Anybody get the number of that truck that just hit me?


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