In this case, “En dehors de […] ” means “Besides […]”
[…] will be a list or a restriction : the biggest planets, the players of a team, the cakes on this tables…
In your case you can say something like :

“I knew that, besides the biggest planets, which were named, there are hundreds others. Some are so small that you’ll have a hard time seeing them with a telescope.”

In your case, it means that those other planets and bodies are just not part of this select list. It’s merely an historic point : the biggest planets have been named for hundreds of years, because they were big enough or close enough to be seen, then rules appeared to decide what qualified as a planet and how to name those space faring bodies.

The reasons could be different :

“En dehors des gateaux sur cette table, ils sont tous très bon”

“Apart from the cakes on this table, they are all really good.”

“En dehors” is understood with respect to the whole preposition: “big planets with given names” (“grosses planètes comme la Terre, Jupiter, Mars, Vénus, auxquelles on a donné des noms“).

They are neither outside in terms of distance, nor are they different, “planetologically” speaking. I understand “en dehors” as a reference to the common knowledge. Most (well, maybe not…) believe than some planets orbit around the Sun. By a recent IAU definition (2006, still controversial), there are eight solar planets (Pluto has been ruled out, as Ceres, etc.).

Outside (the solar system), there are (common knowledge) stars. Well, not only, and subtle physical effects let us think that other (big and small) planets orbit around other suns. Size (only) does not matter. Besides is a nice translation.

Caveat (for fun): commercial link on Oh man, we are not Alone (Wassup Aliens)