system
Given the following post, respond as if you're commenting on Reddit:
human
Why haven't we found any meteorites older than 4.5b years old?

Isn't it possible for meteorites to come from interstellar space? Or even intergalactic space? Why aren't there any meteorites older than our solar system?
gpt
It's possible, but the odds quite low.  The solar system is generally believed to have ejected between 10 and 100 Earth masses in rocks during it's formation (there's a lot of uncertainty, but it's pretty reliable that something like this happened, since the same process forms the Oort Cloud.)

It was mostly in kilometer sized rocks, so there would've been ~ 10^14 of them.  With 10^11 stars, figure 10^25 such rock/ice balls floating through the galaxy.  The galaxy is something like 3x10^17 kilometers across, and maybe 10^16 kilometers high, call it 10^51 cubic kilometers.  About one interstellar comet per 10^26 cubic kilometers, meaning each is surrounded by an area 10^9 kilometers across (about the distance from the Sun to Saturn).  Finding the one interstellar comet in that volume (among the perhaps million such stellar comet/asteroids of that size in the same area) is very hard.  

Put another way, the Earth traces out tube about 7000 km in radius, and 60 kilometers long, every second.  That's 10^9 cubic kilometers per second, or one impact from a kilometer sized interstellar comet every 10^17 seconds (which is about the age of the Earth).  

So probably once in the Earth's history, we've had a kilometer sized interstellar comet has hit us.  This should scale like the size to the 2.5 power, so 300 100 m objects (every 10 million years).  But a similar sized object from within the solar system hits us every 10 or 100 years.

The same ratio comes up - between a hundred thousand and a million solar meteorites for every interstellar meteorite.  So if we took an age measurement of enough meteorites, we should find one that's interstellar.  But that's a lot, a lot of tests, which aren't easy or automated.  There just isn't enough labs, people, funding, or recovered meteorites.  The chance is just too low.