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Source/Further reading:

BBC, why has noone seen Planet Nine? https://www.bbc.com/future/article/20...

NASA, Planet 9 indepth:https://solarsystem.nasa.gov/planets/...

New Scientist, primordial black hole? https://www.newscientist.com/article/...

Smithsonian, Hunt for Planet Nine takes a hit: https://www.smithsonianmag.com/smart...

Scientific American, Planet Nine could be a mirage: https://www.scientificamerican.com/ar...

Search for Planet Nine blog: https://findplanetnine.blogspot.com/2...

Smithsonian, history of the hunt for Planet X: https://www.smithsonianmag.com/scienc...

The search for TransNeptunian Worlds: https://solarviews.com/eng/hypothet.h...

Nat Geo, the hunt for Planet Vulcan: https://www.nationalgeographic.com/sc...

1870s NYT article on Vulcan’s existence: https://timesmachine.nytimes.com/time... Nibiru myth: https://www.space.com/15551nibiru.html 

Britannica, discovery of Neptune: https://www.britannica.com/place/Nept...

Geographics on Pluto:    • Pluto: The Frozen World  

Nemesis Star: https://www.space.com/22538nemesiss...

Sedna orbit and Planet Nine: https://www.astrobio.net/newsexclusi...

Mike Brown, Alan Stern rivalry: https://www.theguardian.com/science/2...

Finding mostdistant ever objects by looking for Planet 9: https://www.sciencealert.com/farfarou...

Durer’s Rhino: https://www.historytoday.com/archive/...