Never let fear get in the way of greed. The Earth government leaders didn't get to where they are by showing any caution or common sense. Even as the intelligent and scientifically savvy portion of humanity cowers in terror and wonderment at the scale of power these aliens display, our leaders would be boastfully filling the airwaves with invitations to parlay and offers of favorable trading nation status, as if such things might matter to the gods who just moved in next door. Among all the pomp and flamboyant posturing, the American President, being more business minded than most, would include an invoice for the Earth genetic resources which these new illegal aliens had purloined during their property grab.
When their invitation, offers and overdue notices are inevitably ignored, our wonderful leaders would take offense, demanding of their scientific advisers create the means for swift military retaliation. Declarations of War would be signed on live television. We must teach the newcomers whose boss! Old-time battle hymns would begin to play.
But those scientists, who came to their offices through much more learned paths than their bosses, are not so quick to take offense, especially when global annihilation is on the line. They would energetically try to convince their masters...
Please don't invoke Armageddon!
They just created a planet!
They are way out of our league!
Then, once their urgent pleas failed, they would quietly poison their bosses and start the first sensible government that our world has ever known.
Turning their eyes away from the strange and new arrangement of stars, out of respect for the beings who rearranged them, the Scientist-Leaders would start trying to figure out what can be learned from what they watched the aliens do.
"Obviously, some of our previous theories are wrong. Conservation of energy and the insulating effects of vacuum have just been disproven. But conservation of mass holds up because the combination of those three planets is just about perfectly Earth sized."
"How did it cools so quickly?"
"Where did all the energy to move the planets come from? Where did all the energy from the melting and merging go as the planet cooled off?"
With these issues and questions as a starting point, our new leadership would apply the entire planet's bounty to continuing our exploration of the universe and its laws. All this, in the hopes that someday we might become advanced enough to be of interest to the beings who now live next door.