[Sticks]

A few notes here

We pretty much know about everything in our solar system of that size (500 miles?) and they are no where near collision course with Earth - This is not going to happen.

Planets

on 24 August 2006 at the meeting of the International Astronomical Union in Prague, Pluto was down graded from planet status to a Dwarf Planet. One of the reasons was the discovery of 2003 UB313 and it's satellite Dysnomia. 2003 UB313 was later renamed 136199 Eris, and is 27% more massive than 134340 Pluto, as it is now designated, which was probably what lead to it's downfall. Eris is also classed as a Dwarf Planet

The other Dwarf Planet is 1 Ceres, which was the largest asteroid It was discovered on January 1, 1801, by Giuseppe Piazzi.

I hope this helps