I have been getting some interesting mail and comments about the Alien Sun issue. Several people are pointing out that I have been saying the Sun is native to the Milky Way, but I have no proof of that.
They're right. I don't. I've been a little sloppy in my terminology. The way I have been phrasing things makes it sound like the Sun was definitely formed in the Milky Way Galaxy, and we don't know that for sure. What I should be saying is that we know it didn't form in the Sagittarius Dwarf galaxy. That much is positive.
But this isn't the first galaxy the Milky Way has cannibalized. It's not even the only one we're eating right now! And over time, no doubt the Milky Way has eaten several other galaxies. The Milky Way has been around for at least ten billion years, long enough to ingest other galaxies and discard the evidence. If a dwarf galaxy happened to impact our own in the plane of the disk, it might be very hard to tell what stars were originally "ours" and which came from the interloper. And given that the Sun is only 4.6 billion years old, it's possible that some of the gas from which it formed came from Out There.
So I cannot say for sure that the Sun was born in the Milky Way, or is made from 100% Milky Way parts. I don't know if we could ever prove it... or not. Given our location, our orbit, and other factors, it seems likely the Sun has been here the whole time. But there is always room for doubt.
So let me be clear: the Sun is not from the Sagittarius dwarf galaxy the Milky Way is currently eating. We know that. And if I had to bet, I would put money on the Sun having lived its whole life here in the Milky Way. But I'll try to be more judicious in my phrasing in the future.