He was wandering around the solar system only for a couple hours until he came across a (dwarf planet, a planet, a moon, one of your OC's, etc...) hanging out off at the edge Kuiper belt (or anywhere else), he's never seen you around here before, then Earth had the thought if he should come up to you or not in this exact moment of time, but he couldn't stay here for more than 2-3 hours since his life won't survive much longer if he stays out here longer (or anywhere else again-) than that...