# What determines the orbital period of planets?

**URL:** <https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710>\
**Category:** Factual Questions\
**Created:** [August 7, 2006, 5:03pm UTC](https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710 "2006-08-07T17:03:24Z")\
**Posts on this page:** 5\
**Page:** 2

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**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\
**Post date:** [August 7, 2006, 11:21pm UTC](https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710/21 "2006-08-07T23:21:27Z")

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[QUOTE=psychonaut]  
Likewise, if the Earth suddenly doubled (or halved) in mass, would its orbital path and/or the length of the sidereal year be affected at all?  
[/QUOTE]

Whatever the result, it wouldn’t bode well.

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**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\
**Post date:** [August 7, 2006, 11:55pm UTC](https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710/22 "2006-08-07T23:55:42Z")

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[QUOTE=Bryan Ekers]  
Whatever the result, it wouldn’t bode well.  
[/QUOTE]  
That’s sinking pretty low.

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**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:** [August 7, 2006, 11:55pm UTC](https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710/23 "2006-08-07T23:55:46Z")

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[QUOTE=psychonaut]  
if the Earth suddenly doubled (or halved) in mass, would its orbital path and/or the length of the sidereal year be affected at all?  
[/QUOTE]

No. The orbital period of a planet doesn’t depend on the mass of the planet (assuming the mass of the planet is much less than the mass of the star, as is the case with the Earth and Sun).

> [@Lemur66](#):
>
> Right, which means if you speed up the planet it will take longer to go around the sun. While if you slow it down it will go faster.

I thought about this when I was watching the re-issued _Star Wars_ (I was also taking a celestial mechanics class at the time).

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<div class="post-metadata">

**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\
**Post date:** [August 8, 2006, 12:04am UTC](https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710/24 "2006-08-08T00:04:56Z")

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[QUOTE=David Simmons]  
That’s sinking pretty low.  
[/QUOTE]

Oh, I can get [way](http://boards.straightdope.com/sdmb/showthread.php?p=4908583#post4908583) lower that that.

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**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\
**Post date:** [August 8, 2006, 1:01am UTC](https://boards.straightdope.com/t/what-determines-the-orbital-period-of-planets/367710/25 "2006-08-08T01:01:40Z")

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[QUOTE=Bryan Ekers]  
Oh, I can get [way](http://boards.straightdope.com/sdmb/showthread.php?p=4908583#post4908583) lower that that.  
[/QUOTE]  
Quoting Elaine when talking to Jerry, “Just when I think you’ve reached absolute bottom, you manage to drain a little more out of the tank.”

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