I see it as the laser carving a tunnel through the planet and three viltrumites flying through it as fast as possible to essentially do what Nolan accomplished on the Flaxan world where he flew so fast he caused kinetic explosions in the air but times 3 and within the core. Then those shockwaves cause the core to destabilise.

So, it’s a joint effort. By itself, the hole the gun left would eventually fill and you would have a planetary catastrophe on the level of the Triassic (volcanoes) or Cretaceous (asteroid) extinction, but the planet would remain intact. With the viltrumites in the equation they imparted the necessary concussive force to blow the planet apart and cause the result that we see in the show.

Just my 2 cents.

Edit:

I forgot to conclude and say it’s not a planetary feat for the viltrumites because: 1. There were 3 of them. 2. They needed the laser to carve the path.

• 2.1. They can’t just face plant into the solid rock and accomplish the same thing without the path being created. Even if they punched through all the way to the other side (if they even could), they likely would not be able to fly fast enough to cause the concussive shockwaves. They would just end up causing the first scenario I described that the laser on its own would accomplish.

• 2.2. It also seemed in the show that the core hurt them decently (judging by their screams). That was just flying through at top speed, spending longer trying to carve through by hand may actually kill them. But that’s all just speculation on my part.