In 'The Ultimate Computer' the Enterprise destroyed another Constitution class ship, the Excalibur, with just a couple of phaser shots at distance.
While it may not shatter a planet to rubble like the Death Star, I do think the TOS Enterprise was fully capable of devastating a planet, turning a thriving ecosystem into an airless wasteland easily.
While it may not shatter a planet to rubble like the Death Star, I do think the TOS Enterprise was fully capable of devastating a planet, turning a thriving ecosystem into an airless wasteland easily.