Putting on my amateur astronomer hat…
My 2005 submission to the International Astronomical Union’s planetary Working Group, prior to their issuing of the current definition, suggested using an intermediate term such as planeta (as opposed to other forms such as planemo, planeto, planetism) to denote a planetary body of sufficient mass to be rounded by gravity and rotation to a (more or less) equal-potential surface, and then to define the term planet with a little more subtlety to rule out satellites, and minor planets that are a members of collection of bodies sharing the same orbital region without establishing gravitational dominance.
Under that suggested definition, the category of “planetas” would have been enlarged by not only Ceres, Pluto, and several dozen or so various trans-Neptunian bodies, but also by a variety of natural satellites down to about the size of Enceladus (Saturn II): the lower mass limit for gravitational rounding to work a planetary mass into a rounded shape can be estimated using the Boltzmann constant to be approximately 10²¹ kg. As it transpired, they decided to use a mass criterion and a measure of gravitational dominance to establish a definition for “dwarf planets” that separates them from the eight [major] planets and also excludes natural satellites: candidates must be in solar orbit. There are five bodies which are intrinsically large and bright enough to be more or less automatically regarded as dwarf planets, and have been defined so: Ceres, Pluto, Haumea, Makemake, and Eris. There are arguably many more which could fit the definition if the IAU were inclined to categorise them: about 390 trans-Neptunians
have been listed as meeting the “possible” criteria, the largest of which are 2007 OR10 (an official name is presumably pending), Orcus, Sedna, and Quaoar.
The 19 natural satellites which would have satisfied the criteria for “dwarf planets” provided they had been in solar orbits are the earth’s moon; the four Galilean satellites Io, Europa, Ganymede, and Callisto; seven moons of Saturn (Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Iapetus); the five large moons of Uranus (Miranda, Ariel, Umbriel, Titania, and Oberon); one moon of Neptune (Triton); and the largest moon of Pluto (Charon). Plenty of scope for musical treatment without covering old ground.
Eris seems particularly worthy of musical attention, in the sense of Holst’s suite where the characters of the mythological gods associated astrologically with the planets suggested musical expression to him: in the ancient Greek pantheon, Eris was the goddess of strife and discord. Eris has one small moon named Dysnomia (for Eris’ daughter, who was the goddess of lawlessness) which also references the in-joke that the discoverers’ stand-in names for the two bodies were “Xena” and “Gabrielle” (X standing for tenth planet; and the actress playing Xena was Lucy Lawless).
PML
