# Where did the planet's moons come from? **URL:** \ **Category:** Factual Questions\ **Created:** [December 21, 2007, 1:56am UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779 "2007-12-21T01:56:58Z")\ **Posts on this page:** 10\ **Page:** 1
**Author:** ![solkoe](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@solkoe](https://boards.straightdope.com/u/solkoe)\ **Post date:** [December 21, 2007, 1:56am UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/1 "2007-12-21T01:56:58Z")
This is inspired from a related thread on the tilt of the planets and a question that was asked by a student of mine today. I told my class that the origin of our moon was a collision of another object with the ancient earth. One student asked how big an object that would be. Another asked how other planets got their moons. I was intrigued by the second question. If they were captured, why are they all rotating in the same direction? Any help? ---
**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, 3:59am UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/2 "2007-12-21T03:59:16Z")
Going on a single semester of astronomy, Mars’ moons are assumed to be captured objects. In the case of some of the moons around the gas giants, captured debris which eventually formed into moons as bits of debris would collide and bond. Jupiter, for instance, has a lot of minor moons which are not spherical because they have not accumulated enough mass to have the gravitational pull to form them into spheres. Not all moons rotate in the same direction: Jupiter’s “Galilean” moons and some closer moons rotate prograde (with Jupiter) and the outer moons largely rotate retrograde (opposite direction). The retrograde satellites of Jupiter and Saturn are figured to be captured items which had enough inertia of their own to keep their retrograde rotation but remain trapped in orbit. Neptune’s moon Trition is another retrograde moon while the rest of its 13 are prograde. I _believe_ that Charon is thought to be a captured object because it and Pluto have fairly different makeups but, needless to say, there’s a lot we don’t know about Pluto & Charon. ---
**Author:** ![Anne\_Neville](https://avatars.discourse-cdn.com/v4/letter/a/b9e5f3/32.png) [@Anne\_Neville](https://boards.straightdope.com/u/Anne_Neville)\ **Post date:** [December 21, 2007, 2:30pm UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/3 "2007-12-21T14:30:51Z")
> [@solkoe](#): > > One student asked how big an object that would be. About the size of Mars. > [@](#): > > Another asked how other planets got their moons. There are two different ways. A lot of the moons of the giant planets formed from a disk around the planet, like the planets formed from a disk around the Sun. The large moons of Jupiter and Saturn formed this way. Other moons are captured asteroids or Kuiper belt objects. Characteristics of captured moons are that they are in retrograde (opposite the planet’s direction of rotation) orbits, they are in orbits that are highly inclined with respect to the planet’s equator, or they are very far from their parent planet. Mars’ moons and Neptune’s moon Triton are examples of captured moons. ---
**Author:** ![IAmAMeatPopsicle](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@IAmAMeatPopsicle](https://boards.straightdope.com/u/IAmAMeatPopsicle)\ **Post date:** [December 21, 2007, 5:06pm UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/4 "2007-12-21T17:06:25Z")
They all came from Uranus ---
**Author:** ![Flander](https://avatars.discourse-cdn.com/v4/letter/f/8c91f0/32.png) [@Flander](https://boards.straightdope.com/u/Flander)\ **Post date:** [December 21, 2007, 5:51pm UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/5 "2007-12-21T17:51:52Z")
Scientists stopped calling it “Uranus” a long time ago. It’s a terribly silly name. Now it’s referred to as “Urectum”. ---
**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, 4:49am UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/6 "2007-12-22T04:49:00Z")
My understanding of current thinking among the experts is that collisions were also the cause of moons of the various smaller bodies, i.e. asteroids and Kiuper Belt Objects. That includes Pluto’s three moons. Moons are not an uncommon phenomenon in the asteroid and Kiuper Belts, but capture by such small bodies is thought to be very rare. Collisions in those belts are more common and were even more so in the early days of the Solar System when those belts were denser. ---
**Author:** ![squeegee](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/squeegee/32/14537_2.png) [@squeegee](https://boards.straightdope.com/u/squeegee)\ **Post date:** [December 22, 2007, 5:35am UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/7 "2007-12-22T05:35:56Z")
Applicable Astronomy Cast episodes (an excellent podcast, IMHO): [Ep. 17: Where does the Moon Come From?](http://www.astronomycast.com/astronomy/episode-17-where-does-the-moon-come-from/) [Ep. 61: Saturn’s Moons](http://www.astronomycast.com/astronomy/episode-61-saturns-moons/) [Ep. 57: Jupiter’s Moons](http://www.astronomycast.com/astronomy/episode-57-jupiters-moons/) There’s transcripts of the show on each page if you don’t feel like listening to the podcast. I find the astronomer, Pamela Gay, does a really good job explaining some of this stuff. ---
**Author:** ![kniz](https://avatars.discourse-cdn.com/v4/letter/k/c37758/32.png) [@kniz](https://boards.straightdope.com/u/kniz)\ **Post date:** [December 22, 2007, 6:43pm UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/8 "2007-12-22T18:43:26Z")
[QUOTE=Anne Neville] There are two different ways. A lot of the moons of the giant planets formed from a disk around the planet, like the planets formed from a disk around the Sun. The large moons of Jupiter and Saturn formed this way. Other moons are captured asteroids or Kuiper belt objects. Characteristics of captured moons are that they are in retrograde (opposite the planet’s direction of rotation) orbits, they are in orbits that are highly inclined with respect to the planet’s equator, or they are very far from their parent planet. Mars’ moons and Neptune’s moon Triton are examples of captured moons. [/QUOTE] Then was our moon (I’m on earth) formed in a third way? 😕 ---
**Author:** ![Bosda\_Di\_Chi\_of\_Tricor](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bosda_di_chi_of_tricor/32/16845_2.png) [@Bosda\_Di\_Chi\_of\_Tricor](https://boards.straightdope.com/u/Bosda_Di_Chi_of_Tricor)\ **Post date:** [December 22, 2007, 10:17pm UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/9 "2007-12-22T22:17:29Z")
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**Author:** ![exastris](https://avatars.discourse-cdn.com/v4/letter/e/b38774/32.png) [@exastris](https://boards.straightdope.com/u/exastris)\ **Post date:** [December 23, 2007, 5:08am UTC](https://boards.straightdope.com/t/where-did-the-planets-moons-come-from/430779/10 "2007-12-23T05:08:01Z")
[QUOTE=kniz] Then was our moon (I’m on earth) formed in a third way? 😕 [/QUOTE] Yes- **Anne Neville** was describing two _other_ ways planets can get moons. It’s widely accepted that Earth’s moon formed from an impact of a Mars-sized body into early Earth, as said early in this thread. As long as you’re talking to them about moons, Mars’s moon Phobos is pretty interesting\*. It’s believed to be a captured asteroid, and shares similar spectral properties with T-class asteroids (though that’s not conclusive evidence it is in fact a T-class.) Depending on your estimates of the strength of Phobos’s material, it resides within, or very close to, the Roche limit- that is, the closest a body can orbit another without being ripped apart. Its orbit is very slowly taking in closer in towards Mars, and will eventually pass this point, and be ripped to shreds by Mars’s gravity. \*didn’t mean to leave out other interesting moons- let’s face it, the Jupiter and Saturn pretty much kick ass in the cool moon department- but Phobos addresses the whole capture thing. . .