# Planet X - where would it have formed? **URL:** \ **Category:** Factual Questions\ **Created:** [January 23, 2016, 11:44am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965 "2016-01-23T11:44:47Z")\ **Posts on this page:** 12\ **Page:** 1
**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:** [January 23, 2016, 11:44am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/1 "2016-01-23T11:44:47Z")
So, if the speculation is correct that there is a large Neptune sized planet way out there, how would it have formed? Could it be a captured isolated planet? A planet that formed in the Solar System but somehow got ejected? ---
**Author:** ![NeonMadman](https://avatars.discourse-cdn.com/v4/letter/n/58f4c7/32.png) [@NeonMadman](https://boards.straightdope.com/u/NeonMadman)\ **Post date:** [January 23, 2016, 7:13pm UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/2 "2016-01-23T19:13:12Z")
It’s the Death Star. 😛 ---
**Author:** ![excavating\_for\_a\_mind](https://avatars.discourse-cdn.com/v4/letter/e/e8c25b/32.png) [@excavating\_for\_a\_mind](https://boards.straightdope.com/u/excavating_for_a_mind)\ **Post date:** [January 24, 2016, 1:04am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/3 "2016-01-24T01:04:12Z")
With the demotion of Pluto, wouldn’t that be Planet IX? ---
**Author:** ![Rigamarole](https://avatars.discourse-cdn.com/v4/letter/r/77aa72/32.png) [@Rigamarole](https://boards.straightdope.com/u/Rigamarole)\ **Post date:** [January 24, 2016, 1:14am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/4 "2016-01-24T01:14:13Z")
> [@excavating\_for\_a\_mind](#): > > With the demotion of Pluto, wouldn’t that be Planet IX? I don’t know if you’re making a joke, but of course the term is in reference to the letter X and not the Roman numeral. Anyway, as to the OP’s question, I think the null hypothesis would be that it formed at the same time and in the same general way as the rest of the solar system, unless there was evidence for thinking otherwise. I also don’t know what you mean by it “somehow got ejected” from the solar system. It’s still well within the solar system (if it exists). Our solar system is much bigger than most people realize - the distance from the sun to Pluto is only about 1/50,000th of it. There’s a ton of room out there for other stuff we haven’t found yet. ---
**Author:** ![Enola\_Straight](https://avatars.discourse-cdn.com/v4/letter/e/dec6dc/32.png) [@Enola\_Straight](https://boards.straightdope.com/u/Enola_Straight)\ **Post date:** [January 24, 2016, 1:23am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/5 "2016-01-24T01:23:27Z")
Suppose we had a neptune/uranus type planet ejected from it’s orbit…what size would it be when all the various gasses freeze to ice? ---
**Author:** ![Dajn](https://avatars.discourse-cdn.com/v4/letter/d/c2a13f/32.png) [@Dajn](https://boards.straightdope.com/u/Dajn)\ **Post date:** [January 24, 2016, 5:03am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/6 "2016-01-24T05:03:25Z")
> [@NeonMadman](#): > > It’s the Death Star. 😛 That’s almost surely what it is. Whe should be able to send up small group of f 15’s to take it down. ---
**Author:** ![Isilder](https://avatars.discourse-cdn.com/v4/letter/i/cc9497/32.png) [@Isilder](https://boards.straightdope.com/u/Isilder)\ **Post date:** [January 24, 2016, 6:22am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/7 "2016-01-24T06:22:34Z")
> [@Northern\_Piper](#): > > So, if the speculation is correct that there is a large Neptune sized planet way out there, how would it have formed? > > Could it be a captured isolated planet? A planet that formed in the Solar System but somehow got ejected? If it was “ejected” it would surely have a highly elliptical orbit. There’s nothing out there to suggest it could form by accretion in situ, so it would have to be captured. This would only occur if it the speed, relative to the sun, was just right, there being nothing out there to alter the speed… See, too slow, and it falls in to the sun, too fast and its not in orbit, it just keeps on going… ---
**Author:** ![AK84](https://avatars.discourse-cdn.com/v4/letter/a/85e7bf/32.png) [@AK84](https://boards.straightdope.com/u/AK84)\ **Post date:** [January 24, 2016, 6:48am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/8 "2016-01-24T06:48:12Z")
What chance that is is the remains of a companion star? ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [January 24, 2016, 9:55am UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/9 "2016-01-24T09:55:27Z")
> [@Isilder](#): > > If it was “ejected” it would surely have a highly elliptical orbit. At first, yes, but such an orbit could be modified subsequent to “ejection”. Ejection from the closer-in parts of the Solar System is, by far, the most likely way it could get there. > [@](#): > > There’s nothing out there to suggest it could form by accretion in situ, This definitely didn’t happen. There wasn’t enough gas and dust that far out during the formation of the Solar System. > [@](#): > > so it would have to be captured. This would only occur if it the speed, relative to the sun, was just right, there being nothing out there to alter the speed… See, too slow, and it falls in to the sun, too fast and its not in orbit, it just keeps on going… Nope, couldn’t happen, at least not that way. You need some third object to carry away excess energy. It is possible there was a third object and it was captured, but very very unlikely > [@AK84](#): > > What chance that is is the remains of a companion star? Zero. Its projected mass is way too low for that. Also, stellar remains (white dwarfs and neutron stars) are warm enough that any that close would have been discovered years ago. ---
**Author:** ![The\_Hamster\_King](https://avatars.discourse-cdn.com/v4/letter/t/8edcca/32.png) [@The\_Hamster\_King](https://boards.straightdope.com/u/The_Hamster_King)\ **Post date:** [January 24, 2016, 3:56pm UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/10 "2016-01-24T15:56:43Z")
> [@Isilder](#): > > If it was “ejected” it would surely have a highly elliptical orbit. This hypothetical is addressed in the article on Planet X in this month’s Scientific American. Apparently if it was ejected early enough in the formation of the Solar System (in the first 10 million years) then the remaining dust in the primordial disk could have circularized it’s orbit. ---
**Author:** ![FluffyBob](https://avatars.discourse-cdn.com/v4/letter/f/ecb155/32.png) [@FluffyBob](https://boards.straightdope.com/u/FluffyBob)\ **Post date:** [January 24, 2016, 5:51pm UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/11 "2016-01-24T17:51:18Z")
If it is in the plane of the ecliptic, doesn’t that pretty much necessitate its formation from the primordial disk? ---
**Author:** ![Quartz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/quartz/32/267_2.png) [@Quartz](https://boards.straightdope.com/u/Quartz)\ **Post date:** [January 24, 2016, 6:46pm UTC](https://boards.straightdope.com/t/planet-x-where-would-it-have-formed/743965/12 "2016-01-24T18:46:12Z")
A nitpick, but the proposed planet is not necessarily _large_ but is _massive_. It could be a rocky super-Earth.