You say "how could some later arrival from distant stars know details of the planet's implosion?" Sad story, that. You see, the last Earthlings were not those secreted in some underground hideaway, consumed finally by the crumbling crust and spastic magma. No, the last Earthlings were the six who were on something called the International Space Station. See, a typical boson-started planetary implosion doesn't attain sufficient gravitational pull to grow much beyond its original planetary mass. Rather, the original planet just shrinks down to an infinitesimal point, still bound by the laws of celestial mechanics. Terra's singularity continued to orbit its star; indeed it still does. This primitive orbital facility the Earthlings had put up continued to circle the tiny hole in space long after its few denizens had expired. The recordings they had made showed us the progress of the implosion - a seldom-recorded event, since its cause usually wipes out the people who cause it. They likewise recorded the extinction of the human species some eleven months later. Four of the six chose their own death, stepping out the station's airlock. The final two, as their last writings testify, succumbed to a failing air factory - fixable, but to what end? At most they could have recycled air and water until their limited food ran out. For this was not a self-sufficient orbital colony, but a mere foothold. A shame really - according to material translated from the long-cold station, this human people were on the verge of stepping out to colonize a neighboring world. That further-out orb would have been a harsh home, but you can bet it would NOT have harbored high-mass physics experimentation!