# How did those planets get that way? **URL:** \ **Category:** Factual Questions\ **Created:** [December 21, 2007, 1:06am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774 "2007-12-21T01:06:30Z")\ **Posts on this page:** 18\ **Page:** 1
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [December 21, 2007, 1:06am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/1 "2007-12-21T01:06:30Z")
[This thread](http://boards.straightdope.com/sdmb/showthread.php?t=448340) got me thinking about the axis tilt of the planets. Venus is about 180 degrees over, Uranus 90. [Here’s a table with all the actual figures.](http://www.astronomynotes.com/solarsys/plantbla.htm) Now, clearly there’s a bunch of variation, and our sample size is small, but 6 of the 8 major planets have tilts less than 30 degrees, suggesting (perhaps) that the variation among those 6 is just that, variation, but the two outliers are rather extreme cases for which there might be some other explanation. How did axis tilt occur on any of the planets occur? Why aren’t we all straight and tall like good old Mercury? And then why are Venus and Uranus so far out of the grouping? What happened to them? ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [December 21, 2007, 1:41pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/2 "2007-12-21T13:41:25Z")
There was another SD column about “what if there were no moon” which insinuated that moons help keep planets at a stable tilt. Venus has none, Uranus’s is/are vanishingly small IIRC ---
**Author:** ![MrDibble](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mrdibble/32/114_2.png) [@MrDibble](https://boards.straightdope.com/u/MrDibble)\ **Post date:** [December 21, 2007, 3:22pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/3 "2007-12-21T15:22:04Z")
As to the first question - from the moment the planets were formed as rotating bodies, they were subject to impacts from other bodies, which would lead all of them to be off “true” ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [December 21, 2007, 3:32pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/4 "2007-12-21T15:32:26Z")
That answers the second question as well. It’s theorised that Uranus collided with another planet-sized body early in its development which “knocked it over.” Uranus’s magnetic field is also wonky; the magnetic poles are about 60 degrees off from its axis. Likewise for Venus, there’s a theory that Venus collided with a major object and, rather than break apart like the Earth & moon, it absorbed the impact. But the force of the hit not only put the planet off-kilter but also forced it into a unique retrograde orbit around the sun. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [December 21, 2007, 3:45pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/5 "2007-12-21T15:45:02Z")
Another thing about Venus is the cratering seems to be no more than 300-500 million years old, indicating that some catastophic resurfacing probably occurred at some point. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [December 21, 2007, 4:21pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/6 "2007-12-21T16:21:23Z")
One theory about Venus (which is not universally accepted, btw) is that it’s crust is too well insulated. Venus doesn’t have plates like the Earth does. The crust of Venus is thick, solid and unmoving. Instead of constantly shedding heat like the Earth does, heat just builds up inside of Venus. When enough heat builds up, the entire crust melts, and once melted, then the heat can finally escape. When enough heat escapes, the crust then cools, and the whole cycle starts all over again. ---
**Author:** ![msmith537](https://avatars.discourse-cdn.com/v4/letter/m/d9b06d/32.png) [@msmith537](https://boards.straightdope.com/u/msmith537)\ **Post date:** [December 21, 2007, 4:28pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/7 "2007-12-21T16:28:52Z")
One theory is that our own Moon was formed when another body colided with the Earth during it’s formative years. ---
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [December 21, 2007, 6:10pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/8 "2007-12-21T18:10:26Z")
[QUOTE=Jophiel] Likewise for Venus, there’s a theory that Venus collided with a major object and, rather than break apart like the Earth & moon, it absorbed the impact. But the force of the hit not only put the planet off-kilter but also forced it into a unique retrograde orbit around the sun. [/QUOTE] I don’t think this is right–people say that Venus is in retrograde ROTATION, meaning that it’s flipped over 180 degrees on its axis (what we’re talking about in this thread). I have never seen anything to indicate Venus is in retrograde REVOLUTION (orbit) around the Sun. ---
**Author:** ![Jophiel](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jophiel/32/66_2.png) [@Jophiel](https://boards.straightdope.com/u/Jophiel)\ **Post date:** [December 21, 2007, 6:20pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/9 "2007-12-21T18:20:13Z")
[QUOTE=toadspittle] I have never seen anything to indicate Venus is in retrograde REVOLUTION (orbit) around the Sun. [/QUOTE] My mistake then. I’m going off of a semester of astronomy from a year ago. I probably screwed up. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 22, 2007, 3:38am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/10 "2007-12-22T03:38:38Z")
No collision is needed for Venus to rotate backwards: [link](http://physicsworld.com/cws/article/news/2661) ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 22, 2007, 3:51am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/11 "2007-12-22T03:51:12Z")
[QUOTE=toadspittle] [This thread](http://boards.straightdope.com/sdmb/showthread.php?t=448340) got me thinking about the axis tilt of the planets. Venus is about 180 degrees over, Uranus 90. [/quote] If Venus’ axis is 180 degrees from vertical with respect to the ecliptic, how would you know it unless you saw it before it got tilted? Like a Jackson Pollock painting, which way is “up”? ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [December 22, 2007, 4:13am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/12 "2007-12-22T04:13:12Z")
[QUOTE=Musicat] If Venus’ axis is 180 degrees from vertical with respect to the ecliptic, how would you know it unless you saw it before it got tilted? [/QUOTE] It’s a matter of definition. As **Freddy the Pig** mentioned in the other thread, a planet’s “north” pole is the one about which the planet rotates counter-clockwise. As defined by rotation, Venus is upside down compared to the rest of the planets. I don’t know enough about rotational physics to say if it were more likely that a collision flipped Venus over (that is, its original north pole is still its north pole, but the planet is upside down); or if it somehow caused the planet to rotate “backwards” (that is, the planet maintained its original orientation but is rotating in the opposite direction). ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [December 22, 2007, 4:19am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/13 "2007-12-22T04:19:40Z")
[QUOTE=Colibri] It’s a matter of definition. As **Freddy the Pig** mentioned in the other thread, a planet’s “north” pole is the one about which the planet rotates counter-clockwise. As defined by rotation, Venus is upside down compared to the rest of the planets. [/quote] I understand what you’re saying, but it seems a bit presumptiously arrogant to say the “right” way to rotate is with the North Pole pointing “up”. Kind of like saying the Earth is the center of the solar system. I guess I never thought of Venus as upside down, but with a retrograde rotation. ---
**Author:** ![Colibri](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/colibri/32/1841_2.png) [@Colibri](https://boards.straightdope.com/u/Colibri)\ **Post date:** [December 22, 2007, 4:50am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/14 "2007-12-22T04:50:56Z")
[QUOTE=Musicat] I understand what you’re saying, but it seems a bit presumptiously arrogant to say the “right” way to rotate is with the North Pole pointing “up”. Kind of like saying the Earth is the center of the solar system. [/QUOTE] Who said anything about “right?” Which way is “north” is completely arbitrary, unlike whether the Earth is the center of the solar system. There’s nothing arrogant about defining it in terms of how our own planet rotates. It just so happens that that’s the way that most of the other planets in our solar system rotate. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [December 22, 2007, 2:27pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/15 "2007-12-22T14:27:53Z")
[QUOTE=Musicat] I understand what you’re saying, but it seems a bit presumptiously arrogant to say the “right” way to rotate is with the North Pole pointing “up”. Kind of like saying the Earth is the center of the solar system. [/QUOTE] Well, when the vote is 6 to 1¹ with one abstention², I think we can say north is counter-clockwise rotation when viewed from above. _Most_ of the planets have their rotations the same way, more or less. ¹ Poor little Pluto got kicked out of the club. ² I’m counting Uranus with its sideways pole as abstaining ETA: fixing the footnotes. ---
**Author:** ![Northern\_Piper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/northern_piper/32/5304_2.png) [@Northern\_Piper](https://boards.straightdope.com/u/Northern_Piper)\ **Post date:** [December 22, 2007, 10:01pm UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/16 "2007-12-22T22:01:37Z")
In defining which way is up, it might help to consider the rotation of the Sun, which is the single biggest object in the Solar System. Which way does the Sun rotate? ---
**Author:** ![Gorsnak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/gorsnak/32/7587_2.png) [@Gorsnak](https://boards.straightdope.com/u/Gorsnak)\ **Post date:** [December 23, 2007, 12:09am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/17 "2007-12-23T00:09:01Z")
[QUOTE=Musicat] I understand what you’re saying, but it seems a bit presumptiously arrogant to say the “right” way to rotate is with the North Pole pointing “up”. Kind of like saying the Earth is the center of the solar system. I guess I never thought of Venus as upside down, but with a retrograde rotation. [/QUOTE] It’s not presumptuous in the least. The vast, vast majority of mass in the solar system is orbiting and revolving all in the same direction - due to the way the dust cloud that it formed out of was swirling. You see there’s this whole conservation of angular momentum thing going on. Venus is clearly spinning the “wrong” way, and so something out of the ordinary must have happened to cause this. I’d think that knocking the planet upside down would take a lot less energy than stopping it’s original spin and then starting it in the other direction, but I’m no astronomer. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [December 23, 2007, 8:28am UTC](https://boards.straightdope.com/t/how-did-those-planets-get-that-way/430774/18 "2007-12-23T08:28:11Z")
[QUOTE=Colibri] Who said anything about “right?” Which way is “north” is completely arbitrary, unlike whether the Earth is the center of the solar system. [/quote] Actually, there’s two ways of defining the north pole of a solar system body: the natural way (counter-clockwise rotation about the pole) and the [IAU way](http://adsabs.harvard.edu/full/1993CeMDA..57..473L). For both Venus and Uranus, the IAU north poles are the south poles of the natural north poles.