# High-speed planets: Why don't they fly apart? **URL:** \ **Category:** Factual Questions\ **Created:** [September 9, 2009, 3:43am UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387 "2009-09-09T03:43:40Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 9, 2009, 3:43am UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/1 "2009-09-09T03:43:40Z")
I keep hearing about Jupiter-sized planets orbiting stars at incredible speeds; like hours or days for each orbit. Why don’t they fly apart? ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [September 9, 2009, 3:47am UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/2 "2009-09-09T03:47:40Z")
Many probably do. But the only ones we know about are the ones that don’t. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [September 9, 2009, 4:01am UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/3 "2009-09-09T04:01:47Z")
> [@panache45](#): > > Many probably do. But the only ones we know about are the ones that don’t. But why not? A body the mass of Jupiter would have enormous forces on it at those speeds. What holds them together? ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 9, 2009, 4:02am UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/4 "2009-09-09T04:02:23Z")
> [@Johnny\_L.A](#): > > Why don’t they fly apart? A planet can fly apart from rotating too fast on its own axis, but not from orbiting another body at high speed. In orbit, you are in freefall - forces are in balance. (Okay, right, I’m ignoring tidal forces.) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 9, 2009, 5:13pm UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/5 "2009-09-09T17:13:13Z")
> [@](#): > > A planet can fly apart from rotating too fast on its own axis, but not from orbiting another body at high speed. In orbit, you are in freefall - forces are in balance. Not exactly: If you’re extremely close to your parent object, then the difference in force between the part of the planet near the star and the part away from the star can be enough to tear the planet apart. If the planet and the star are the same density, the threshold at which this happens (called the Roche limit) is about 1.5 times the radius of the star. The exosolar planets we’ve observed, while often pretty close to their star, have none of them been _that_ close to their star. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [September 9, 2009, 5:47pm UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/6 "2009-09-09T17:47:35Z")
> [@Chronos](#): > > The exosolar planets we’ve observed, while often pretty close to their star, have none of them been _that_ close to their star. According to this [g force calculator](https://www.msu.edu/~venkata1/gforce.htm), a planet orbiting a star at 1 million km would have to complete an orbit every 100 minutes to generate a centrifugal force of 0.11 X g. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 9, 2009, 6:25pm UTC](https://boards.straightdope.com/t/high-speed-planets-why-dont-they-fly-apart/509387/7 "2009-09-09T18:25:39Z")
> [@Chronos](#): > > If you’re extremely close to your parent object, then the difference in force between the part of the planet near the star and the part away from the star can be enough to tear the planet apart. Whence the disclaimer regarding tidal forces.