If planet busting power levels where as low as some calculations put them, then the planet would have been destroyed in goten and trunks's recent sparring match (It wasn't). Clearly, there is more to it than "Hit it hard and the planet blows up"

The three ways to blow up a planet are as follows:

1: High power attack to the core. It makes planets blow up much more effectively than the size of any given laser would imply. This is how Vegeta was going to blow up earth and how Frieza and Buu destroyed Vegeta/Namek/Earth. The only difference was the amount of effort they put into the attacks. Any arguments against this must also contain explanations for why sometimes mountains explode and other times the planet goes boom.

2: Massive suicide explosion. Similar to what Cell tried, a suicide explosion can get much more power out than any other move that can be done.

3: Hit it extremely hard and the planet blows up. Like Beerus and champa's flashback fight over who ate more of the cake. You just need to be really, really strong.