What must a mechanical wave travel through

A mechanical wave always consists of wiggles in some kind of material stuff. The material stuff may be air, water, rock, wood, steel ... almost anything, as long as it's some material. In general, whatever the waves are moving through is called the "medium". If there isn't any, like the vacuum in space, then mechanical waves can't travel there. For example, sound can't travel in space ... not even an inch.

Neptune is the eighth planet from the sun. it was the first planet to get its existence predicted by  mathematical calculations  before it was actually seen through a telescope on sept. 23, 1846. irregularities in the orbit of  uranus  led french astronomer alexis bouvard to suggest that the gravitational pull from another celestial body might be responsible. german astronomer johann galle then relied on subsequent calculations to   spot neptune via telescope. previously, astronomer galileo galilei sketched the planet, but he mistook it for a star due to its slow motion. in accordance with all the other planets seen in the sky, this new world was given a name from greek and roman mythology — neptune, the roman god of the sea.