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 and it’s pretty exciting to be able to get a close up look at something from outside the Solar System. You might be asking yourself just how can astronomers tell if something is truly an object from outside the Solar System?

In principle it’s actually pretty easy for astronomers to figure out where an object came from. (I’m sure the astronomers working on this would disagree about how easy it is, and there have actually been several cases in the past where we’ve been less certain about whether objects were from outside the Solar System or not.) Its orbit tells us a lot and with enough photos of an object over time you can work out its orbit pretty accurately. In this case over a hundred observations were made over the course of a few weeks and put together to work out the orbit, and by September 24 we knew the orbit accurately enough that astronomers could say for sure that this is an interstellar object.

Being able to work out the orbit depends on our understanding of gravity. How objects move in the Solar System only really depends on two things: gravity (that’s actually a little simplistic, because gravity depends on the masses of all the other objects nearby and how far away they are) and how fast they’re moving. Lucky for us we understand gravity really well (and we can just see how fast its moving). We’ve actually known almost everything we need to know to work out objects’ orbits since Isaac Newton published his theory of gravity in 1687. You might know that Newton’s theory of gravity doesn’t tell the whole story and we need Einstein’s General Relativity to fill in the gaps, but actually even Newton’s theory of gravity is good enough for most purposes. NASA has sent probes to every planet in the Solar System – and even to outside the Solar System – using Newton’s theory of gravity.

After a few measurements of 2I/Borisov came in, it was pretty obvious that it was moving so quickly on such an extremely elongated orbit that it couldn’t possibly have originated from inside the Solar System, for the simple reason that its on a path that means it will escape from the Solar System. It’s not in interstellar space at the moment, but it will eventually end up in interstellar space and that means it must also have started there, because it couldn’t have ended up on its current trajectory if it had started in the Solar System.

Here’s a great animation from NASA showing 2I/Borisov’s orbit from different angles:

Featured image credit: ESO/M. Kornmesser

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