This is a list of the tallest mountains in the Solar System. The tallest peak or peaks on worlds where significant mountains have been measured are given. For some worlds, the tallest peaks of different classes are also listed. At 21.9 km, the enormous shield volcano Olympus Mons on Mars is the tallest mountain on any planet in the Solar System. For 40 years, following its discovery in 1971, it was the tallest mountain known in the Solar System. However, in 2011, the central peak of the crater Rheasilvia on the asteroid and protoplanet Vesta was found to be of comparable height. Due to limitations in the data and the definition problem described below, it is difficult to determine which of the two is taller.