I think there's thirteen planets. Eight big ones, three dwarf ones.
That would mean that there were 11.
]]> A planet has swept all other bodies out of its orbit, either by disrupting their path or colliding with them.
The third point is the one they added, which allowed them to say that Pluto wasn't a planet because its orbit overlaps with Neptune's. But surely if that's the case, then Neptune shouldn't be a planet either?
It's very possible that I'm misremembering something here, so please do correct me if you know better.
No, the difference is that they have different orbits. Pluto is one of thousands of objects in its orbit, and it's smaller than a lot of them.
]]>Eight Planets. There are eight planets.
No, there isn't. There are more than eight. 'planet' has subcategories, one of them being 'Dwarf Planets'. So there are, at least, more than 10 planets.
]]> A planet is sufficiently large such that it assumes uniform shape under its own gravity (ie. it's a sphere).
A planet orbits around a star.
A planet has swept all other bodies out of its orbit, either by disrupting their path or colliding with them.
The third point is the one they added, which allowed them to say that Pluto wasn't a planet because its orbit overlaps with Neptune's. But surely if that's the case, then Neptune shouldn't be a planet either?
It's very possible that I'm misremembering something here, so please do correct me if you know better.
]]>After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.