This planet I tried to invade had zero defense and zero attack. How did I fail? I buffed up my ship and weakened the planet with an arty. please tell me how I fail when it has no defense or attack at all.

Invasion chance is capped at 90%

Basically you sent in the red shirts, and the were beaten back by the native rabid squirrels.

really.. your not a new player, you should know this by now.

I'm more concerned about:

You can still check the wiki or the forums

same here:
Planetary defense: 0
Planetary attack: 0
No planet defense arti/trap nor effect nor guarded..

Yet:
Invasion was a failure. Planetary defenses were strong enough to ward off your advance You had a 90% chance of success.

Now I do understand that 90% is not 100%, but when the planet has no defense... well, that is no defense.

that other 10 % if for "unforseen circumstances" .. like there may not be defense, but your ship landed on a crater and the volcano exploded..
It's to make it a bit more fair.

Plus there may not be any defense, but clearly there are always a few people left, perhaps in your state of glee at demolishing that planets pop, you felt safe in sending Red-Shirt to the planet.

Problem is, that when he got there, he saw what he thought was an enemy combatant, fired his phazer and killed himself(he actually saw his reflection in a mirror, phazer fire bounced back on him)... invasion critically failed, you lost a dumbass.

in every thing you do there Room for Error Never a thing called perfect . Maybe a wrap storm or a drunk pilot messed it up.

Chuck Norris was on the planet when you attempted invasion. Sorry bud.

+1
yet today, with triggered defense trap my crew made it!