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Comment by akolbe | Hacker News Reader

Basically, the average distance between one planet and another will always be greater than the distance between the respective planet and the sun. (Note that in the table on the last page, all average planet distances for earth are > 1 AU.) Thus whichever planet is closest to the sun will always be the closest on average to any other planet.

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This is true for the Solar System because all our planets have near-circular orbits. Suppose though if you had two planets with highly elliptical orbits and similar arguments of perigee. They will spend most of their time far away from their star but relatively close to one another.

I'm not sure if such an elliptical orbit would be possible while still classifying them as planets and not dwarf planets though.

IIRC we've found some big exoplanets in wild orbits. They tend to form in a circular-ish orbit because that's how you get a big enough chunk of the dense bits of an accretion disk, but interactions with other planets/stars can put eg gas giants into wild orbits after formation.

EG https://astronomynow.com/2019/08/28/exoplanet-found-in-unusu...

Outside of binary planets orbiting each other, is this achievable as a (mostly) stable configuration?

Intuitively, if the two planets have orbiting periods that are not basically identical, then after long enough they will also have long stretches of time where they are on opposite sides of the star (with a slight caveat if the periods are rational multiples of each other, but in either case their positions will be asymptotically uncorrelated). On the other hand, if they are close and have the same period, I'd expect their gravitational pulls would eventually merge them together unless they become a binary planet system.

But I have no physics/astrophysics background so this could easily all be stupid.

Almost true. Good thinking though. There are exceptions to this rule, so it's more of a guidance than the actual rule. For example you could have two planets orbiting the Sun at the relatively similar distance from the Sun and a small distance from each other.

You couldn't actually have two separate planets with similar orbital distances like that though.

Right. They would necessarily have different orbital periods because the size of an orbit is directly proportional to the time it takes to complete one orbit: https://en.wikipedia.org/wiki/Orbital_period#/media/File:Sol...

This is why objects in geostationary orbit can only exist at a particular distance from the earth: https://en.wikipedia.org/wiki/Geostationary_orbit

Well, that's not the issue here. The issue is two planets can't exist near each other because they can't possibly have formed that way 4.5B years ago and kept existing at that point since then. Their gravitational influences would have long since caused them to collide into each other and form a single planet. There's a reason there's big gaps in the orbital distances of every planet; it's simply not possible to pack them in too tightly.

That's still the same scenario, just slower to become apparent. You need to change other things, like they orbit each other as they go around the Sun, to change this.

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