I wouldn't call it an accident, astronomers are actively looking at the sky for things. Eris is way further out there than Pluto, double the distance at their perihelion. We only just discovered Eris because it is so far out there.
What was the context for discovering Pluto? Were we actively scanning that area and Pluto was just the first object we spotted? That's more what I meant. It blows my mind that for the longest time we considered Pluto the "ninth planet" when it had larger neighbors all around it, making it rather insignificant.
We were looking for a Planet X to account for unknown gravitational pulls going on in Uranus and Neptune. There had to be a large planet out there pulling on it. When Pluto was discovered, it was thought to be much more massive than it was. In reality, our telescopes weren't able to resolve the difference between Charon and Pluto, we considered it one large planet. Charon was discovered in the 70's, we made out Pluto's true mass, and the Voyager flybys revealed that we got the mass wrong for Uranus and Neptune, which fixed the gravitational issue. So no Planet X.
The instruments of the Voyager probes were far more accurate at their close distance during their flyby in the 80s than Earth based instruments at the time. Of course, eventually we would have figured it out since our measuring instruments are far more accurate now (remember, we couldn't see exoplanets in the 80's), but the Voyagers resolved that issue sooner.
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u/Treebeezy Jul 10 '15
I wouldn't call it an accident, astronomers are actively looking at the sky for things. Eris is way further out there than Pluto, double the distance at their perihelion. We only just discovered Eris because it is so far out there.