It was anomalies in the orbit of Uranus that led to the disconvery of Neptune
what is the subject of "led" ?
"anomalies" ??? nope...
let me try to write a correct version:
It was an anomaly in the orbit of uranus that led NASA to the discovery of
NASA found anomalies in the orbit of uranus and this finding led
to the discovery of neptune.
They were anomalies in the orbit of Uranus that led to the disconvery of Neptune
what is pronoun "they" referring to? thats right..they has no referent..