Mesoplanet
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Mesoplanet is a term coined by Isaac Asimov to refer to planetary bodies with sizes smaller than Mercury but larger than Ceres. Assuming "size" is defined by linear dimension (or by volume), mesoplanets should be approximately 1000 km to 5000 km in diameter. The classification would include Eris, Pluto, (136472) 2005 FY9, (136108) 2003 EL61, 90377 Sedna, 90482 Orcus and 50000 Quaoar; it might also include (84522) 2002 TC302 and 20000 Varuna if they are towards the larger end of current estimates.
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. In the essay, 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."
In 2006 the IAU ratified the classification "dwarf planet" which includes Ceres, Pluto, Eris, and a not yet determined number of smaller objects.
Since the year 2000, several Trans-Neptunian objects have been discovered that lie in size between Mercury and Ceres, which arguably strengthens the case for a special classification such as mesoplanet.
