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What are some possible ways this could happen?

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    $\begingroup$ One planet? Four? Five? Is Mars terrestrial? Mercury? Pluto? They don't fit the standard description of gas giants. Are those the only two options? What are you trying to do? It's likely that you could explain almost anything by having another star come by and poach some of your planets or your star poaching planets from elsewhere. $\endgroup$ – Brythan Feb 27 '16 at 2:53
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    $\begingroup$ See Kepler-444. Kepler-37 is also a possibility $\endgroup$ – HDE 226868 Feb 27 '16 at 13:44
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    $\begingroup$ Note, Kepler-444 is thought to have two dwarf companions but otherwise seems to fit the criteria if we ignore age. $\endgroup$ – Mazura Feb 27 '16 at 15:50
  • $\begingroup$ @Mazura Good point. $\endgroup$ – HDE 226868 Feb 27 '16 at 16:26
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Possible? yes, though highly unlikely. Much more plausible would be a system that only has gas giants that have ejected the other planets.

Way back when: Jupiter and Saturn had harmonically resonant orbits and this is surmised to have helped eject all the other garbage out of our system (anything without a stable orbit got pushed and pulled and tossed out). Without gas giants to protect your system, you're not going to be able to charge good rent money.

Something similar might have happened in your system, but instead the gas giants ejected themselves. Note, this still leaves yours in 'the cheap seats' but gets around the unlikely event of a system having had to form without gas giants.

I've nothing to back this up, but I'd assume that's why most stars are binary: you either don't have enough 'stuff' to start a star at all, so much that you get two stars, or like ours: gas giants as well.

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  • $\begingroup$ The Universe S06E03 How the Solar System Was Made $\endgroup$ – Mazura Feb 27 '16 at 7:37
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    $\begingroup$ I feel like you might be confusing the Jumping Jupiter scenario a bit, although I haven't yet watched the whole video. What was ejected in the Jumping Jupiter model is that a third ice giant was ejected, not a bunch of planetesimals. The Kuiper Belt might be excited, resulting in a quick increase in the semimajor axes of KBOs (see this video), but it's not like a lot of small stuff is ejected. More importantly, only one giant planet is ejected, not all five. $\endgroup$ – HDE 226868 Feb 27 '16 at 13:32
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    $\begingroup$ Regarding the final paragraph (sorry if I'm jumping in this a bit much) - the masses of stars can vary drastically. Except in the case of cloud fragment masses on the order of that of low-mass red dwarfs, you can generally form two stars from the amount of matter present. $\endgroup$ – HDE 226868 Feb 27 '16 at 13:37
  • $\begingroup$ The video I linked is only relevant. I can never seem to find the one that talks about how they 'cleared' our solar system during the LHB with an orbital resonance. Your Wiki link sure sounds like it though, only much more in-depth, and it at least helps my theory somewhat. $\endgroup$ – Mazura Feb 27 '16 at 15:38
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There are a number of presentations on planet formation in the SETI Weekly Colloquium Series covering the latest understanding and speculation on the subject.

I suggest you start with one of the formation models presented, well-peppered with real nomenclature and details, and also cite the specific mechanisms and situations for getting rid of the other planets and leaving the configuration you want. That is, mix and match among the real mechanisms to craft the scenario you want.

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  • $\begingroup$ What a wonderful bit of information! Thank you! $\endgroup$ – Jim2B Feb 27 '16 at 6:25
  • $\begingroup$ It's an amazing resource for WorldBuilding interests, I must say. $\endgroup$ – JDługosz Feb 27 '16 at 6:29
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It depends on the size and composition of the proplanetary disc . If it is fairly small , with a low amount of hydrogen and helium , then I'd say yes , it would be possible , even with a large proplanetary disk , if the percent composed of hydrogen is low enough , than yes , you could get a solar system with no gas giants , but a side affect may be a vastly larger amount of terrestrial planets and asteroids

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    $\begingroup$ Without hydrogen, how did you form a working star? $\endgroup$ – JDługosz Feb 27 '16 at 2:59
  • $\begingroup$ Slightly correction (by the way, @JDługosz is spot-on, unless you were to have all the hydrogen form the star, which seems unlikely): It might be better to refer to the disk as a protoplanetary disk. While accretion does happen close to the star (as Wikipedia notes), the vast majority will not be accreted, unlike in a traditional accretion disk. $\endgroup$ – HDE 226868 Feb 27 '16 at 13:40
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A star is made of gas. Some of it ends up in the planets, it would be awfully hard to make a solar system without gas giants. However, something could have happened to them. While I can't see a mutual ejection scenario working (where does the energy come from??) you could have an ejection/destruction scenario (one planet leaves, the other ends up too close to the central star) or ejection by an encounter with an outside body passing through.

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