Ok, I promised a thread! I don't want to rehash too much of the article below, because we worked hard on it, but I do want to hit the highlights. https://twitter.com/J_Noons/status/1252272445677088772

Setting the stage: Less than two weeks after 2I/Borisov was discovered and found to be hyperbolic, we decided that if there was ever a time to go big for observing time with HST, this was it. We did the math, and decided it could work.

Specifically, the HST Cosmic Origins Spectrograph. Why? We leveraged our experience with the @ESA_Rosetta mission's instrument suite to argue that the UV wavelengths could offer us critical information about the comet's atomic and molecular abundances.

Between December and January we were able to get 4 epochs of observations on 2I/Borisov, and what we saw shocked us. There was carbon monoxide, and relative to water, there was a lot of it.

Even stranger, this ratio went up after the comet's close approach to the Sun.

The ratio wasn't just high, it was unheard of for comets observed in the inner solar system! How?

We came to the conclusion that we were seeing a truly primordial composition of a comet that formed in a system very different from our own. CO wasn't just a clue, it was the key.

The thing with CO is that it goes from ice to a gas at an incredibly low temperature. In order to keep CO ice around, you have to keep a comet at just 25 K (-248 C). That's real cold.

2I/Borisov had managed to retain large amounts of CO and water ice. That means three things:

1. 2I/Borisov had a home system that must have been chemically unlike our own. No comet observed in the inner solar system, where all volatiles (water, CO2, and CO) can sublimate, has ever shown a CO/H2O ratio like this.

2. If there is more CO than water, and both can be sublimating, than CO and water ice must have formed at the same time. Whatever system 2I/Borisov came from, it must have formed beyond the CO snowline, where carbon monoxide freezes.

3. However 2I/Borisov got ejected from this system, that ejection did not heat the interstellar comet substantially. That would have liberated the carbon monoxide from the comet, leaving much less for us to see during its passage through our system.

So, together what do these three things mean? What do they tell us about the source of 2I/Borisov?

We think that these clues point towards 2I/Borisov forming around the most common type of star in our galaxy, the humble M-type.

They:
a) form late in their respective stellar clusters, and have access to more C and O in their disks.
b) are the faintest stars, with very little heat to say, sublimate CO.
c) can easily lose weakly bound comets via planetary encounters or stellar encounters.

That's a very neat conclusion, because it would make statistical sense that the first visibly sublimating interstellar comet is coming from the most common stellar type, giving us a first-hand view at the chemical composition of an M-type protoplanetary disk!

So, in summary, the longest refrigerated delivery in the history of astronomy gave us some pretty good clues about the formation location of our first active interstellar comet.