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Comment by rishav_sharan | Hacker News Reader

I always had a layman pet theory that Planet 9 is a BH, and not just any BH one but the one who's supernova remains seeded the solar system with heavier elements. Sort of like old grandpa blackhole looking after his/her small kid.

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Sorry, but no.

A supernova-born black hole ( https://en.wikipedia.org/wiki/Stellar_mass_black_hole ) would be far more massive than the sun - so the entire solar system would obviously revolve around it, not the sun.

Also, the birth of such a black hole is a supernova explosion - which would not leave any solar-system-forming remains kicking around nearby.

Question from ignorance: if the planets are "moons" around the sun and the sun rotates around P9, how would we know?

relative speed against other stars is very much detectable by red and blue shift of light. We would definitely know if sun revolves around something, meanining changes its direction.

I think we'd see the paralax being all weird as well.

Yes, and I suspect the parallax anomalies would have been noticed a few decades before the redshift / blueshift anomalies:

https://en.wikipedia.org/wiki/Stellar_parallax_method#Histor...

https://en.wikipedia.org/wiki/Redshift#History

At these distances, the sun's direction if it were orbiting a black hole would have only changed by a fraction of a degree in the hundred or so years we've been able to measure its velocity relative to nearby stars - is that data precise enough that that is not within the margin of error?

But the normal black holes have a mass greater than the sun

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