Following the general meeting of the IAU in 2006, a new classification scheme was released for the various members of the solar system and which was inspired by the 2003 discovery of Eris, a Kuiper belt object beyond the orbit of Pluto and which happened to be also larger than Pluto by about 100 km. To be more specific, the three categories adopted where (1) planets, (2) dwarf planets and (3) small solar system bodies.

Dwarf planets are similar to planets in the sense they orbit the sun and have sufficient mass so that gravitational forces lead to a generally round shape but which have failed to clear their orbit of debris due to a lack of a dominating gravity. Using this definition, the solar system is now characterized with eight (and not nine) planets, five dwarf planets (Pluto, Ceres, Iris, Makemake and Haumea) and various small solar system bodies excluding satellites.

Three members of the asteroid belt (Vesta, Pallas, Hygieia) as well as over 40 trans-Neptunian objects (Orcus, Ixion, Huya, Varuna, Quaoar, Sedna etc) are believed to eventually be classified as dwarf planets.

Note: The fourth dwarf planet of the solar system was discovered by Brown et al in the spring of 2005 and officially named Makemake by the IAU in the summer of 2008 and after a deity worshipped by the Rapa Nui peoples of Easter Island. Makemake is second only to Pluto with respect to brightness amongst dwarf planets and is the only transneptunian object discovered so far not to have a satellite. As is the case with Haumea, Makemake is characterized with an orbit inclination of over 28°, thus placing it well above the ecliptic for observation and, hence, for its recent discovery. Spectra of Makemake suggest the presence of methane and which explains its red colouration in the visible portion of the spectrum. For an ephemeris of Makemake using the facilities of the Jet Propulsion Laboratory (NASA), click here. Similarly, for Makemake's orbital elements, click here.