Mesoplanets are planetary bodies with sizes smaller than Mercury but larger than Ceres. The term was coined by Isaac Asimov. Assuming "size" is defined by linear dimension (or by volume), mesoplanets should be approximately 1,000 km to 5,000 km in diameter. The classification would include Eris, Pluto, Makemake, 2007 OR10, Haumea and probably also Quaoar and Sedna.
The term was coined in Asimov's essay "What's in a Name?", which first appeared in The Los Angeles Times in the late 1980s and was reprinted in his 1991 book Frontiers; the term was later revisited in his essay, "The Incredible Shrinking Planet" which appeared first in the Magazine of Fantasy and Science Fiction and then in the anthology The relativity of wrong (1988).
Asimov noted that the Solar System has a large number of planetary bodies (as opposed to the Sun and natural satellites) 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, since mesos in the Greek language means "middle".
Since the year 2000, several trans-Neptunian objects have been discovered that lie in size between Mercury and Ceres, and would thus be mesoplanets by Asimov's definition. In 2006 the IAU ratified the classification "dwarf planet" which includes Ceres, the "mesoplanets" Pluto, Haumea, Makemake, Eris, and a not-yet-determined number of other objects.
The extrasolar planet Kepler-37b is also smaller than Mercury and would be an extrasolar mesoplanet.
- Asimov, Isaac (1998). The relativity of wrong. p. 121. "...my own suggestion is that everything from Mercury up be called a major planet; everything from Ceres down be called a minor planet; and everything between Mercury and Ceres be called a "mesoplanet" (from a Greek word for "intermediate"). At the moment, Pluto is the only mesoplanet known."