A graphical timeline of humanities exploration of the solar system, showing the dates that each planet (and dwarf planet) had a close encounter with a probe from Earth.
Our first probe encounter with a planet was our own. Sputnik was placed in orbit in 1957, followed a couple of years later by a successful probing of our Moon. It has taken 58 years for us to send a probe to each individual planet (when this whole space exploration thing began, Pluto was still classed as a planet.
Originally we looked inward; probes from the USSR were sent to Venus but failed along the way. In 1962, the Mariner 2 probe managed to make it.
Mars quickly followed and soon after the Pioneer and Voyager probes were sent out on a tour of the outer planets; Mariner 10 was also sent to neglected Mercury.
And now, New Horizons has completed its flyby of the dwarf planet Pluto (following asteroid and comet encounters).