Size and density of the planets
The planets appear to be divided according to size and density.
The inner planets are small and rocky (the terrestrial planets):
The outer planets are 'gas giants':
Nearly every planet has an atmosphere.
Note: Pluto (formerly regarded as a planet) has been re-classified as a dwarf planet in the Kuiper meteorite belt. It may be further declassified to a binary dwarf planet, as it is only slightly larger than its moon. (International Astronomical Union