# Influence of an inner gas giant over a solar system

I'm designing a solar system with three planets in its hot zone, two in its cold zone and three in its habitable zone. All of the planets are rocky worlds of various sizes, except for the middle world of the habitable zone which I'd like to be a gas giant.

Our system giant's gravitational forces have a lot of influence over the behaviour of everything orbiting the Sun, keeping everything more or less balanced and reshaping things every once in a while.

Would having a single gas giant in such place as the middle of the habitable zone cause too much of an orbital havoc for there to be stability on the system?

## It would be fine if you arrange the orbits properly.

This system wouldn't necessarily be problematic if you can maintain stability via particular orbital resonances. We've discovered numerous compact systems whose stability is believed to be maintained on long timescales in this manner. Kepler-90 is a great example of a compact system kept in place by resonances; it's a roughly 2 billion-year-old Sun-like star that harbors eight planets within $$\lesssim1$$ AU. The outermost (and most massive) is roughly 2/3 the mass of Jupiter, orbiting right around 1 AU.

The system you're proposing is quite similar - presumably just with a gas giant in the middle, not in the edge, and with an increased spacing of planets. You might be concerned by this rearrangement, but note that the gas giant is only $$\sim0.3$$ AU away from the next planet over. Some models predict that the Sun's habitable zone is $$\sim0.5$$ AU across, making it quite feasible to have a similar spacing here (though of course spacings much less than $$0.3$$ AU are still rather stable).

L.Dutch cites the asteroid belt as a reason for worrying about stability; I'm not as concerned. The asteroid belt was formed in part due to unstable resonances relative to Jupiter's orbit, rather than stable resonances. The Kirkwood gaps are one set of notable results. But bear in mind that the presence of an additional gas giant can also lead to stable resonances, as I noted above, and may actually make it easier for compact, stable systems of terrestrial planets to form (see Hands & Alexander 2016).

Would having a single gas giant in such place as the middle of the habitable zone cause too much of an orbital havoc for there to be stability on the system?

Pretty much yes, unless the habitable zone is really extended to that the perturbations induced by the gas giant can be negligible. This is what happened in our solar system in the asteroid belt

The asteroid belt formed from the primordial solar nebula as a group of planetesimals. Planetesimals are the smaller precursors of the protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of fusing together, the planetesimals and most of the protoplanets shattered. As a result, 99.9% of the asteroid belt's original mass was lost in the first 100 million years of the Solar System's history. Some fragments eventually found their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits.

Just look at how big is the gap between the orbits of Mars and Jupiter when compared to the gap between the orbits of the inner planets, and compare it with the width of the habitable zone, roughly comprised between Venus and Mars orbits.

• Well that may be a problem, yes. What if the gas giant were as small as a gas planet gets? Mar 23, 2021 at 9:06