What led to the discovery of Neptune and Pluto?

Answer 1

Both planets were discovered by perturbations (irregularities) in the orbits of other planets.

Uranus' orbit was discovered to be a bit sped up and slowed at a certain point in its orbit. William Herschel figured there must be another planet's gravity out there tugging on it. He looked for it, and found Neptune.

Similarly, the orbits of Uranus and Neptune were still a bit out of whack. Clyde Tombaugh used a blink comparator to study the sky in an area where another planet might be causing those perturbations, and he discovered Pluto.

An old wonderful book called "The Search for Planet X" details these discoveries. I read it as a kid and bought a copy off of ebay. It's been out of print for many years, but you can still get it off ebay from old bookstores for cheap. It's a good, easy read. Try it!

Answer 2

Using Newton's law of gravity and Einstein's General Relativity, the motion of the orbits is fairly well understood. In classical orbital mechanics, usually only the sun and the planet are considered to determine the orbit, however the presence of large bodies (other nearby planets) can perturb the motion slightly so that it does not exactly follow the path we predict from our models. These differences between where we thought the planet should be to where we measured it to be led astronomers to hypothesize that there were additional planets that were affecting the observed motion of the other planets. This led to the discovery of Neptune as they were affecting the orbit of Uranus. Supposedly this also led to the discovery of Pluto, but realistically it is too small to significantly impact the orbits of Neptune and Uranus.

Answer 3

Neptune was discovered because the orbit of Uranus was displaying anomalies that could best be explained by another yet undiscovered planet's gravitational influence. Calculations even allowed to determine where that planet would be, and when astronomers looked there, there it was.

Pluto was a somewhat similar story, as perturbations on Uranus were not allegedly all caused by Neptune, and some claimed that there could be yet another planet, and looking in that direction, Pluto was spotted. It was only later that Pluto was found to be too light to have any measurable effect on Uranus, and the initial perturbations thought to be caused by it were actually due to wrong estimates of the mass of Neptune.

So, discovery of Neptune: good math
Discovery of Pluto: bad estimations and coincidence

Answer 4

Small errors in the orbit of Uranus.
Astronomers though the errors might be caused by the gravity of another, unseen planet. They used gravitational calculations to figure out where the other planet should be and when they pointed a telescope at that area, they saw Neptune. Neptune didn't have enough gravity to account for all the problems with the orbit of Uranus, so they kept looking and found Pluto. Later, after the Voyager space craft visited Uranus and Neptune, they found out they had estimated the mass of those planets incorrectly. When redoing the gravity calculations with the correct mass, Neptune did account for all the problems it the orbit of Uranus. They were just lucky that Pluto was in the area of the sky they were looking at because it is far too small to have enough gravity to make any different to the orbit of Uranus.

Answer 5

Go to http://en.wikipedia.org/wiki/Neptune#Discovery to learn about the discovery of Neptune. For Pluto, go to http://en.wikipedia.org/wiki/Pluto#Discovery. You probably already know this, but Pluto is NOT a planet.

Answer 6

Neptune - i don't know
Pluto - i don't know
actually there's no explanation but i must say that this is a very good question. i'll just star this up.