Exploring the Planets (http://explanet.info) is a free online college textbook covering thebasic concepts of planetary science emphasizing the character and evolution of the planetarybodies in the Solar System. The latest edition (3rd edition) was published online in 2007 by EricH Christiansen. Since the release of the third edition, two important planetary missions havebeen completed: MESSENGER (to Mercury) and New Horizons (to Pluto). These missionsprovided new information and fundamental insights into these planetary bodies, which have notyet been included in Exploring the Planets. The modern results based on recent investigations ofMercury and Pluto are critical for our understanding of the nature and history of these bodies andthe Solar System and build upon the previous information on Mercury and Pluto gained fromMariner 10 (1974-1975) and the Hubble Space Telescope, respectively. These two planetarybodies are end members in a spectrum of objects in the Solar System. Mercury is small, hot,dense, and a silicate metal rich end member of the planets, helping scientists understand thethermal and accretionary evolution of the terrestrial planets. Pluto is cold, icy, distant from theSun, and a representative object of the vast Kuiper Belt, and is thus another end member amongplanetary bodies. These two bodies refine models of how different planets will evolve over time,and how our Solar System has evolved. For these reasons, it is important to update Exploring thePlanets to summarize the current understanding of the geology of Mercury and Pluto. This way,students can better understand their formation and evolution and the implications for theevolution of our Solar System.