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The planet killer from Star Trek episode:
The Doomsday Machine.
They managed to make it look scary. In those times, it is scary.
Another idea I have to destroy planets or even stars - if it is possible to completely block gravity. Scientists will tell you this is outright impossible because gravity is a property of space-time and inextricably linked to it. However, I have doubts on the sentiment because gravity waves don't travel at infinite speed but still limited by the speed of light.
But if it is possible, you're not just blocking the Earth's gravity but the gravity of the entire universe or approximately "half" of it. but even just a half a universe, if you managed to get yourself to the very edge of the Universe, the gravity you'll be subjected to would be equivalent to the Event Horizon of a black hole.
But by completely blocking gravity, you have replicated the conditions at the very edge of the universe. It may be the key to "folding" space-time. Traversing vast distances in an instant.
Ironically, the technology may also trigger the collapse of a planet or even a star into a black hole or cause something with a smaller mass like a moon or a portion of the planet to explode with the yield of an anti-matter bomb.
I believe the tech exists in Star Trek with the shield tech. They do have a much of tech in there the writers have "failed" to exploit but not entirely. At least the "Q" entities know how to use the deflector dish to fold space-time. But never demonstrated its utility as a planet or even star-killing weapon.
Stuff in Star Trek have tendency to become reality. It will become a real problem if some nation invents the warp drive first and realizes its planet-killing potential - or tactically, wiping out whole fleets of ships or armies with a simple beam without radioactivity nor toxic after-effects. The perfect weapon of mass destruction.