We would expect that planets that form close together would be similar to one another. We would also expect that planets that form close together would stay close together and planets that form far apart would stay far apart. Therefore, we would not expect planets that orbit close together to be very different from each other.
Well, we expected a lot of things about planets.
The Kepler-36 solar system is arguably the most recent in a long line of exoplanet oddities. It has two planets that orbit very close together: the closest together of any known planets. The orbit of Kepler-36c is only 11% larger that that of Kepler-36b, a difference of only a few times the distance to the Moon. The larger planet would look 5 times the size of the full Moon in the sky at that distance.
The two planets’ gravity strongly affects each other’s orbits: the orbits are probably only stable because they are in an orbital resonance. Every time Kepler-36b orbits 7 times, its neighbor Kepler-36c orbits 6 times. That’s weird enough by itself, but the really strange thing is that the two planets look nothing alike. Both planets are super-Earths, a few times as massive as Earth. But Kepler-36b is almost as dense as iron, meaning it has to be pretty much all rock and metal. Kepler-36c, on the other hand, is less dense than water! The only way that could happen is if it’s a very small gas giant–a mini-Neptune.
How did planets that are so different wind up right next door to each other? The only guess is that they formed in different places and migrated to their current locations. We wouldn’t have expected them to be able to get so close, but there they are. Who knows what other strange new planetary discoveries await us in the next few years?