It would most likely require an expedition to Earth, and some reason to go near that area. That being said, I tend to explain my players the Pandora Gates as Stargates, so having one on Earth is quite the homage to the series XD. My wild theories on the subject: The ETI, in the analogous way of Stargate's Ancients, sent a "seeder" ship here and there, placing Pandora Gates where the stars where right... I mean, when a predefined set of conditions was met XD. This original gate in the Solar System crashed and was recovered, then put into the Area 51 some millions of years later, and the Bracewell probe could have reached the Solar system using that gate (in fact, it would be a two-parts probe: one part taht would be left behind, the Bracewell probe in Venus, and another that would take readings of the system and go back through the gate to the point of origin. Now how would that original gate crash? The question can be answered by a few options. Option one: the asteroid belt between Mars and Jupiter comes from a planet that failed to achieve a stable situation desintegrating (or broke after suffering an asteroid impact, or whatever) and the gravitational destabilization plunged the gate into Earth. Or the comet/asteroid that took down the dinosaurs also took down the gate. Wanna more history to it? Add WW2 and the nazi's program of supernatural/paranormal gathering and finding, say they found the gate in Antartica... and ask yourself if they managed to use it to go to an exoplanet close enough to Earth's biosphere to survive. And of course, the gate would be in the Area 51 because the americans found it after the war inside all the documents they got. Or it was in South America, and they found it while searrching for running nazis. Of course, it can also have no relation whatsoever to that over-exploited part of human history ^^