planet x
/ˈplænɪt ɛks/
Word Rarity
Uncommon
Frequency: Zipf frequency 4.57 in modern English
16
Scrabble Score
7
Letters
2
Syllables
7
Unique Chars
Meaning
name
A hypothetical planet in the Solar system, beyond the currently known outermost planet. (The term was applied to Eris for a short while, when Pluto was still considered the ninth planet.)
name
A theoretical planet responsible for reported gravitational perturbations of Neptune's orbit. (This was initially believed to be Pluto. The reported perturbations were an error.)
Obsolete
name
A theoretical planet responsible for observed gravitational perturbations of Uranus' orbit. (This planet was discovered and is now known as Neptune.)
Obsolete
Etymology
X represents an unknown as opposed to the Roman numeral for 10 (initially, Planet X would have been the ninth planet).