Planet Nine

proper noun

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.

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Meanings

proper noun

alternative Planet 9 · alternative Planet IX

  1. astronomyA 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.
Cite this entry
APA

Planet Nine. (n.d.). In WatkinsX Dictionary. Retrieved November 3, 2026, from https://dictionary.watkinsx.com/word/Planet%20Nine

MLA

“Planet Nine.” WatkinsX Dictionary, https://dictionary.watkinsx.com/word/Planet%20Nine. Accessed November 3, 2026.

Chicago

WatkinsX Dictionary, s.v. “Planet Nine,” accessed November 3, 2026, https://dictionary.watkinsx.com/word/Planet%20Nine.

Source: Planet Nine on Wiktionary, CC BY-SA 4.0.