Mesoplanet

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 1,000 km to 5,000 km in diameter. The classification would include Eris, Pluto, Makemake, Haumea, 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.

Size comparison between Earth's Moon, Neptune's (probably captured) moon Triton and several of the largest minor planets, including the mesoplanets.

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^{[citation needed]} and was reprinted in his 1991 book Frontiers;^{[citation needed]} the term was later revisited in his essay, "The Incredible Shrinking Planet" which appeared first in the Magazine of Fantasy and Science Fiction^{[citation needed]} and then in the anthology The relativity of wrong (1988).^[1]

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".^[1]

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.

In 2006 the IAU ratified the classification "dwarf planet" which includes Ceres, Pluto, Haumea, Makemake, Eris, and a not yet determined number of smaller objects.

References

1. ^ ^{a b} 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." 

See also

• Asteroid
• Centaur (planetoid)
• Fusor (astronomy)
• Protoplanet
• Planetesimal
• Planetar (astronomy)