https://en.wikipedia.org/wiki/Planet_Nine
Resolves once it's confirmed beyond a doubt that it does, or we've explored the solar system to an extensive enough degree that we couldn't possibly have missed it.
Pretty sure that still has the same resolution problem...
Perhaps Planet Nine existed in the solar system last year but was flung out without anyone noticing. No amount of solar system exploration can prove whether or not it was there last year.
"Resolves once it's confirmed beyond a doubt that it exists in the solar system, or..." seems like a simpler resolution criteria. I highly doubt the odds of Planet Nine both existing and being flung out of the solar system after market creation yet before being discovered are high enough to affect the practical odds of the market at all. While if Planet Nine is never discovered then someone could always argue that it might have existed and we might have missed it while this market was open, and you could never prove them wrong.
https://pubs.aip.org/physicstoday/article/72/3/70/916004/The-Planet-Nine-hypothesisThe-putative-planet There are too many minor planets orbiting in the same direction to be a fluke, and I think the Planet Nine hypothesis makes the most sense
@JonLamb No, it just needs to be any planet that could reasonable be considered Planet Nine. i.e. if scientists announce "we found Planet Nine!", that counts.