Universe Today
This is Part 3 of a series on interstellar comets. Read Parts 1 and 2.
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. The population of comets that gets ejected is not necessarily the same population that stays behind. It's a biased sample, and we're only seeing the exiles.
And lastly, interstellar comets are different because they've had different lives.
Think about a comet in our own solar system. It forms in the early chaos, gets nudged out to the Oort Cloud — that vast, cold shell of icy bodies way out past the planets — and then it just sits there. For billions of years. It's cold, it's dark, but it's not totally alone. It still receives a steady trickle of sunlight. Incredibly feeble, barely a whisper, but not zero. It's still gravitationally tethered to the Sun. It's still part of the family, just the distant branch.
An interstellar comet? It got flung out of its home system entirely. It's been drifting through the space between stars for — how long, exactly? The same billions of years? More? Less? We don't know. And during all that time, it received no starlight at all. It was bathed in nothing but the faint glow of the cosmic microwave background and the occasional cosmic ray. Its surface evolved differently: different temperature histories, different radiation exposure, different accumulation of interstellar dust grains settling onto it like cosmic snowfall. By the time it arrives in our solar system, it has been shaped by an entirely different set of experiences than anything that grew up here.
And if you think I'm just getting into the nerdy weeds here, well, that's where all the magic of science happens. It's the kind of stuff that makes the universe so rich and varied and interesting. Details matter, folks.
Now, I know I said that last point was the last one. I lied.
The real last point is that we have to remember we don't get to observe interstellar comets when they're, you know, being interstellar. We only see them when they get close to us, which means when they get close to the Sun. (The same is true for all comets, by the way — we have no direct observations of comets in the Oort Cloud. We only suspect it's out there because the comets have to come from somewhere.)
So you pile on the fact that interstellar comets were born in different homes. Plus the fact that comets that get kicked out might be different from comets that stay. Plus the billion-plus years of different histories. And then you add on the fact that interstellar comets are really, really fast — which changes how they interact with the Sun entirely.
3I/ATLAS was traveling at about 33 kilometers per second. That's roughly 74,000 miles per hour — twice as fast as a typical comet. It came screaming toward the Sun much faster, spent less time looping around it, and left much faster.
We know comets by what they do when they get close to the Sun: they warm up, their surfaces change, they grow a coma and a tail, and they start evaporating. That's the comet experience. But 3I/ATLAS has a different relationship with the Sun. It spent less time doing all the things we normally associate with being a comet. Less leisure time.
And that can explain a lot of its weirdness. Why it took so long to develop a coma and tail? Less time in the warm zone. And that mysterious acceleration on the way out? By the time ATLAS finally did warm up enough, it was already headed for the exit. It started outgassing — the polite term for comet farts — but it was already so far from us that we couldn't see the gas itself. All we could detect was the effect of the gas pushing it, like a sort of comet rocket. The gas was the engine; we just couldn't see the exhaust.
To be continued...
