Proper noun
Planet 9
(astronomy) A theoretical ninth planet in the Solar system, the gravitational effects of which would explain the improbable orbital configuration of a group of trans-Neptunian objects that orbit mostly beyond the Kuiper belt.
“All this is optimistically hoping that Planet 9 does exist and turns out to be a black hole,” Dr. Witten said in an email, “and that technology develops enough that a suitable scaled version of Breakthrough Starshot is possible.” Source: Internet