1930: Skylark Three, a novel by Edward E. Smith, originally published as a serial in Amazing Stories, text available at Project Gutenberg. The Fenachrone planet is destroyed in Part III of the serial, which appeared in Amazing Stories, October 1930, available at the Internet Archive.
"I agree with you that the time has come to destroy the planet of Fenachrone. As for pursuing that vessel through intergalactic space, that is your problem. You must figure out some method of increasing our acceleration. Highly efficient as is this system of propulsion, it seems to me that the knowledge of the Norlaminians should be able to improve it in some detail. Even a slight increase in acceleration would enable us to overtake them
[. . . .]
"Well, then, let's mop up on that planet. Then we'll go places and do things."
Seaton had already located the magazines in which the power bars of the Fenachrone war-vessels were stored, and it was a short task to erect a secondary projector of force in the Fenachrone atmosphere. Working out of that projector, beams of force seized one of the immense cylinders of plated copper and at Seaton's direction transported it rapidly to one of the poles of the planet, where electrodes of force were clamped upon it. In a similar fashion seventeen more of the frightful bombs were placed, equidistant over the surface of the world of the Fenachrone, so that when they were simultaneously exploded, the downward forces would be certain to meet sufficient resistance to assure complete demolition of the entire globe. Everything in readiness, Seaton's hand went to the plunger switch and closed upon it. Then, his face white and wet, he dropped his hand.
"No use, Mart — I can't do it. It pulls my cork. I know darn well you can't either — I'll yell for help."
"Have you got it on the infra-red?" asked Dunark calmly, as he shot up into the projector in reply to Seaton's call. "I want to see this, all of it."
"It's on — you’re welcome to it," and, as the Terrestrials turned away, the whole projector base was illuminated by a flare of intense, though subdued light. For several minutes Dunark stared into the visiplate, savage satisfaction in every line of his fierce green face as he surveyed the havoc wrought by those eighteen enormous charges of incredible explosive.
"A nice job of clean-up, Dick," the Osnomian prince reported, turning away from the visiplate. "It made a sun of it — the original sun is now quite a splendid double star. Everything was volatilized, clear out, far beyond
their outermost screen."
"It had to be done, of course — it was either them or else all the rest of the Universe," Seaton said, jerkily. "However, even that fact doesn't make it go down easy. Well, we're done with this projector. From now on it's
strictly up to us and Skylark Three. Let's beat it over there and see if they've got her done yet — they were due to finish up today, you know."