Mesoplanets are

planetary-mass object A planetary-mass object (PMO), planemo, or planetary body is by geophysical definition of celestial objects any celestial object massive enough to achieve hydrostatic equilibrium (to be rounded under its own gravity), but not enough to sustain ...

s with sizes smaller than Mercury but larger than

Ceres Ceres most commonly refers to: * Ceres (dwarf planet), the largest asteroid * Ceres (mythology), the Roman goddess of agriculture Ceres may also refer to: Places Brazil * Ceres, Goiás, Brazil * Ceres Microregion, in north-central Goiás ...

. The term was coined by Isaac Asimov. Assuming size is defined in relation to equatorial radius, mesoplanets should be approximately 500 km to 2,500 km in radius.

History

The term was coined in Asimov's

essay An essay is, generally, a piece of writing that gives the author's own argument, but the definition is vague, overlapping with those of a letter, a paper, an article, a pamphlet, and a short story. Essays have been sub-classified as formal a ...

" What's in a Name?", which first appeared in The ''

Los Angeles Times The ''Los Angeles Times'' (abbreviated as ''LA Times'') is a daily newspaper that started publishing in Los Angeles in 1881. Based in the LA-adjacent suburb of El Segundo since 2018, it is the sixth-largest newspaper by circulation in the U ...

'' in the late 1980s and was reprinted in his 1990 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 ''The Magazine of Fantasy & Science Fiction'' (usually referred to as ''F&SF'') is a U.S. fantasy and science fiction magazine first published in 1949 by Mystery House, a subsidiary of Lawrence Spivak's Mercury Press. Editors Anthony Boucher ...

'' and then in the anthology ''

The Relativity of Wrong ''The Relativity of Wrong'' is a 1988 collection of seventeen essays on science by American writer and scientist Isaac Asimov. The book explores and contrasts the viewpoint that "all theories are proven wrong in time", arguing that there exist deg ...

'' (1988). Asimov noted that the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

has many planetary bodies (as opposed to the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

and natural satellites) and stated that lines dividing "major planets" from

minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''mino ...

s 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 Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...

, 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

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... 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. — I. Asimov (1988)

Today, the known objects that would be included by this definition are Pluto, , , , , , probably , and perhaps . These eight, together with Ceres, are the objects astronomers generally agree are

dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun, smaller than any of the eight classical planets but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of dwarf planets to ...

s; other smaller bodies have been proposed, but astronomers disagree about their dwarf planethood.

References

{{Exoplanet Isaac Asimov Types of planet * Definition of planet

History

See also

References