[Space] Making a solar system with 3 (semi-) habitable planets?

[kdtipa]

Originally Posted by johndallman

It isn't. Even without considering the perturbations from other planets, two planets of similar mass in the same orbit 180 degrees apart are unstable. The slightest disturbance from perfectly opposite - such as Earth's orbit not being perfectly circular - will produce an unbalanced attraction, and they'll drift towards each other.

The best bet for solving Vicky's problem is a double planet. The larger one will have more than 1G of surface gravity, and the smaller less, but you should be able to get them both credibly habitable if your players don't look too hard.

Check out the scientific analysis section: "Counter-Earth"

Nothing in there saying it isn't possible... just that there isn't one there. I was wrong though... that it would "always" be on the opposite side of the sun though. Because of the elliptical orbit, there would be points where we could see it.