Theories Proven Wrong

Basically, before the discovery of extrasolar planets, theories on a solar system (using our Sol System as a model) dictated the following:

1) Gas Giant cannot exist before the Snow Line, or at least close in. (Proven: FALSE)
- Hot Jupiters.

2) Neptune size planets are rare. (Proven: FALSE)
- Kepler-11 system & HD 6930 system.

3) Planet radius will not exceed Jupiter size radius. (Proven: FALSE)
- As for Bloated Gas Giants.

4) Planets form nearly circular orbits. (Proven: FALSE)
- HD 96167 b

5) Planets cannot exist around Neutron stars and White Dwarf stars (Post-stars). (Proven: FALSE)
- PSR B1257+12 system.

6) With minor exceptions, everything in a star system should orbit in the same plane and in the same direction. (Proven: FALSE)

Any other theories you know, I will add.
Not sure of these:
- Planets that orbit a binary star or cirumbinary orbits can't exist.
- Planets don't orbit ahigh inclined orbit.
- Planets don't form like stars, via gas collapse.

Reference:
Three Theories of Planet Formation Busted, Expert Says, National Geographic.