I hear that an unknown planet caused perturbations in Neptune's orbit. Is this true?

Hello,

Firstly, it was perturbations of both Uranus and Neptune that led to the
search for Pluto, but, as you state, Pluto is much too small to account
for the perturbations seen. The answer to your question is, in short,
experimental error. Modern observations of Uranus and Neptune show no
evidence for orbital perturbations that cannot be explained by the other
planets. Also, the Pioneer and Voyager spacecraft have shown no
inexplicable deviations that a tenth planet could explain.

Incidently, were you aware that Galileo actually observed Neptune long
before it was discovered. (Neptune actually was discovered based on
observed perturbations in Uranus' orbit). Jupiter happened to drift past
Neptune, and Galileo actually noted that one of the background "stars"
appeared to move, but he was too transfixed on Jupiter to realize what he
had actually seen.

Thanks for writing!

Sincerely,

Kurtis Williams

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