Bombs are special purpose weapons for destroying missile bases. (And bombing colonies into extinction, but that is generally a seceondary purpose.) You can attack planets with beams, but you need a significant tech lead to overcome the halving effect if the defender has decent shield tech. Ideally one has both beams and bombs, but if the tech tree does not cooperate you use what you have.
If I do not have a good bomb in my tree, then teching for the next higher rung in Weapons and hoping for a better bomb tech is usually my first response. But if I go another rung and still do not have a bomb, things get tough. Missiles can work, if you have good ones and your target's shields are not
too strong. Similarly, torpedoes can sometimes work. But if the enemy has good shield tech.... Well, spying is a possibility. Trade with another AI can sometimes get you a bomb tech, although the price is likely to be painfully expensive.
Worst case, you can do as Sullla said and go for swarm tactics with your transports. You can lose a lot of pop that way, though, especially if the enemy has good missile tech.
![frown frown](https://www.realmsbeyond.net/forums/images/smilies/frown.gif)
Sometimes you just will not be able to crack their planetary defenses. Maybe you can still win by council, if you have enough planets and planetology tech to grow your pop big enough.