The Chironian Spheres in The Shattered Sphere destroy planets by using them as near light speed projectiles against attacking neutronium-based life forms. And even that only sometimes kills the things. The gravity control technology of the setting can rip a planet apart for raw materials, or squeeze one down into a black hole.
In the Belisarius series, a Great One is described as being able to destroy a planet by ramming into it fast enough (97% or so of light speed). And at the end one does so to a large moon, leaving an expanding cloud of vapor and fragments.
Anyone remember if there are any planet-killers in Olaf Stapledon’s Star Maker? I remember that at some point it turns out that some or all stars are actually sentient and capable of burning the hell out of any planet orbiting them by shooting enormous frigging jets of plasma at them.
In one of Jack Williamson’s Legion of Space novels he reveals “AKKA” which simply makes things “go away”. They use it against an enemy fleet and take out the Moon as collateral damage.
Yeah, and that particular reveal should go on the list of “things that were better left as mysteries”. Didn’t like the way he ended that series.
In his tenure as the writer of Astonishing X-Men, Joss Whedon gave The BreakWorld a planet-killer in the form of a giant bullet, shielded from magical counter-measures and capable of plowing through a Sentinal like it was tissue paper. It was “stopped” by Shadowcat “phasing” it as it passed through the planet.
[QUOTE=Star Maker]
A sphere of incandescent material, expanding from the sun with incredible speed, swallowed up and destroyed every girdle of planets. The star then subsided.
[/QUOTE]
From Chapter 11: Stars and Vermin, Disaster In Our Galaxy.