It's the most extraordinary cosmic present any astronomer could wish for. Just six days before Christmas, an ancient traveller that has been careening through the interstellar medium of the Milky Way galaxy for billions of years is set to make a once-in-a-universe approach to our planet. This is not a returning comet we have tracked for decades; this is 3I/ATLAS, and its fleeting visit is giving scientists an unprecedented peek into the birth of an alien world.
This interstellar vagabond is only the third large interstellar visitor that we have ever discovered — an asteroid or comet that originated outside of our solar system and was discovered passing through. For planet hunters, this is the gold standard. Since it presumably formed in a protoplanetary disk of gas and dust swirling around another star, it offers pure, untainted data on how planetary systems beyond our own take shape.
