EDITOR'S PICK
This is Part 3 of a series on interstellar comets.
Read Parts1and2. So why should we expect interstellar comets like 3I/ATLAS and 'Oumuamua and even to some extent Borisov to be different-different? Well, for one, they came from somewhere else.
They were literally born in a foreign solar system.
And not all solar systems are created equal.
The star they formed around might have been hotter or cooler than the Sun.
The protoplanetary disk — the spinning cloud of gas and dust that eventually clumps into planets and comets — would have had its own unique recipe.
Different ratios of hydrogen to helium to metals.
Different temperatures at different distances from the star.
Different amounts and kinds of dust grains acting as seeds for everything that came after.
A comet born in a system with a higher metal content is going to carry that signature with it forever, like an accent you never quite lose.
So when ATLAS shows up loaded with nickel, that might just be where it's from. And for another, they got kicked out. Comets don't just decide to go interstellar.
Something has to boot them.
Usually that something is a giant planet — a Jupiter-sized bully whose gravity can fling a small icy body clean out of its home system and into the void between stars.
This happens all the time; our own Jupiter has been evicting comets for billions of years.
But here's the thing: the comets that get kicked out might not be a random sample of all the comets in a given system.
They might come from specific regions — the ones closest to the giant planet's gravitational reach.
They might skew smaller, because smaller objects are easier to fling.
They might skew toward different compositions depending on where in the disk they formed before getting the boot.