Mesoplanets are
planetary-mass objects with sizes smaller than
Mercury but larger than
Ceres. The term was coined by
Isaac Asimov
Isaac Asimov ( ; – April 6, 1992) was an Russian-born American writer and professor of biochemistry at Boston University. During his lifetime, Asimov was considered one of the "Big Three" science fiction writers, along with Robert A. H ...
. 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 (message), letter, a term paper, paper, an article (publishing), article, a pamphlet, and a s ...
"
What's in a Name?", which first appeared in the ''
Los Angeles Times
The ''Los Angeles Times'' is an American Newspaper#Daily, daily newspaper that began publishing in Los Angeles, California, in 1881. Based in the Greater Los Angeles city of El Segundo, California, El Segundo since 2018, it is the List of new ...
'' 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''
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 SystemCapitalization 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 "Sola ...
has many planetary bodies (as opposed to the
Sun and
natural satellites
A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
) 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 ''minor ...
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 Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
, 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.
... 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 around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s (though with some doubt regarding Orcus); other smaller bodies have been proposed, but astronomers disagree about their dwarf planethood.
See also
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Asteroid
An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...
*
Centaur (minor planet)
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Fusor (astronomy)
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Protoplanet
A protoplanet is a large planetary embryo that originated within a protoplanetary disk and has undergone internal melting to produce a differentiated interior. Protoplanets are thought to form out of kilometer-sized planetesimals that gravitatio ...
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Planetesimal
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Brown dwarf
Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main sequence, main-sequence stars. Their mass is approximately 13 to 80 Jupiter mass, times that of Jupiter ()not big en ...
References
{{Exoplanet
Isaac Asimov
Types of planet
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Definition of planet