Does defense actually help a planet before the invasion?
I have attacked zero defense and attack planets with my ship having over 1000 attack points. The attack does 20 hits to the defender's population.

Against a planet with 60 defense, I hit for 20 points again.

Is the 20 pretty much standard?
does planetary defense actually help?

20 is the max damage you can do on a planet. If the planet has a lot more defense then you will do less damage.

From what I could tell planetary defense is better then a ship's. I could have 2000 attack and the planet has 1200 defense and I will only deal 10 damage. multiply that by a few hypersphere's worth of population and you have a planet that will take an inordinate amount of energy to invade.

yes planetary defense does help against the damage, but with damage being so high on ships along with it being the predominant tree next to energy for people researching, odds are they're going to get the 20 damage in unless you have a legion guarding it. Where the defense comes into play the most is in their invade roll. The higher your defense is on the planet, the lower the chance they have of passing the invade roll. Structures like the soldier outpost wich give defense strictly against the invade roll for little space can be vary important. I just spend 3 days last week invading some guys planet and kept failing the invade roll because he had a decent amount of defense on it. Sometimes that failed invade will be the only warning you have to prevent a planet from being taken and having time to respond.