How do we know when something is from outside the Solar System?

You probably saw the recent news that astronomers have spotted an object from interstellar space making its way through the Solar System. It’s called 2I/Borisov and it’s the second object like this that we’ve seen. The first was called 1I/’Oumuamua and was only spotted a couple of years ago, so this is all very new… Continue reading How do we know when something is from outside the Solar System?