File:Mesoplanet.jpg

Asimov noted that the Solar System has many planetary bodies and stated that lines dividing "major planets" from minor planets were necessarily arbitrary. Asimov then pointed out that there was a large gap in size between Mercury, the smallest planetary body that was considered to be undoubtedly a major planet, and Ceres, the largest planetary body that was considered to be undoubtedly a minor planet. Only one planetary body known at the time, Pluto, fell within the gap. Rather than arbitrarily decide whether Pluto belonged with the major planets or the minor planets, Asimov suggested that any planetary body that fell within the size gap between Mercury and Ceres be called a mesoplanet, because mesos means "middle" in Greek.

Today, the known objects that would be included by this definition are Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, probably Sedna, and perhaps Orcus. These eight, together with Ceres, are the objects astronomers generally agree are dwarf planets; other smaller bodies have been proposed, but astronomers disagree about their dwarf planethood.