129 existed in the early solar system but is not found on present-day Earth. I popped the halite in the machine and got this amazing peak showing an abundance of xenon-129. I hadn't really expected that, so it was quite stunning. Xenon is a noble gas, which was key to dating the rock, Gilmour said. Noble gases do not readily combine with other chemicals.
Noble gases are very volatile.
The xenon-129 was a dead giveaway. After 100 million years, essentially none is left. It was present when the solar system began to form 4.5 billion years ago. It was gone 100 million years after. That means the gas was present within the first few million years of the solar system's formation.