Do we need a class of "Plutonian" planets?

Abstract:

The physical and chemical characteristics of dwarf planets and large moons of the gas giants have suggested to the authors to question the present differentiation. Instead of classifying by orbits only, the structure of the worlds should be stronger weighted and distinct classes be developed that may include worlds as similar to each other as Pluto, Triton and Titan. The article suggests that the obsolete term secondary planets might be revived for moons in hydrostatic equilibrium.

Keywords:
Pluto, Charon, Titan, Ceres, Triton, dwarf planets, moons, Tholin

Montage by Emily Lakdawalla. Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA,
processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla.

Do we need a new definition of our moons?

The year 2015 has initiated a scientific revolution whose scope we have not fully grasped yet. With Ceres and Pluto, two dwarf planets have been visited by unmanned spacecraft at the same time. Their findings suggest to us to question the present division of solar system objects.

When it is said today that Ceriesi was an intruder to the asteroid belt that has developed farther out in the solar system, should it not to be reckoned then with the icy worlds of the Kuiper Belt [1]? Watching how alike to each other are the chemical, physical and even geographical properties of Pluto and Saturn's moon Titan [2] (they have virtually the same density, to begin with), is it still justified to sort them into two different classes for the sole reason that one of them orbits a larger body and the other, a smaller one?

It might be more sensible to classiy objects in the solar system based on their planetological characteristics instead of merely their orbits. We have made a good beginning with "terrestrial" and "Jovian" planets, a little bit less fortunate we have been with the introduction of the dwarf planets. In our opinion, we need a similar division for our moons that include common classes for objects with similar properties.

space.com

But not "dwarf planets", please! I couldn't call Titan in good conscience a dwarf though it is larger than Mercury.

That's one of the problems with the definition of the International Astronomical Union: Ceres is more or less the tiniest object that may still pass as a dwarf planet, but they have failed to define an upper limit. How big does a dwarf planet have to grow to become a proper planet? Basically we believe that everything should be covered by the umbrella term "planet" that, according to the applying definition, is in hydrostatic equilibrium, regardless of what it orbits around.

In other words, everything that is more or less round.

Though at least Haumea poses a major problem here! But with regard to the big moons we suggest reviving the obsolete expression secondary planets (first used by Newton, I think) for moons in hydrostatic equilibrium that has been in common use for any moons satellites in the solar system in the 18th and 19th centuries. It fell into disuse, it seems, when the moons of Mars were discovered because it felt too awkward to call those two potatoes planets. As a result, we would have five terretesrial planets in the inner solar system: as before, the four primary planets Mercury to Mars, and one secondary planet, the Moon. The class of Jovian planets stays the same. Then, a new category should follow for ice/tholin worlds like Titan, Triton and Pluto.

Or a more general one, for planets with icy crust and liquid water mantle? That would also cover the dwarf planet Ceres or other active secondary planets like Enceladus.

That might be worth consideration. This category could be dubbed Plutonian planets, to cool some moods across the Pond, [3], even though from the historical point of view, Titan was the first discovered, and prototype, of the big ice planets.

What about the Galilean Moons?

We are undecided about whether they should be included. Io is definitely a case apart. Especially Callisto may absolutely belong into the same category, because its density is also comparable to that of Pluto. What do our readers think? We are waiting for your suggestions!

Postscriptum 2016-02-08: Such a proposal seems to actually have been made, see Six New Horizons scientists propose geophysical planet definition by L. Kornfeld. I do not like the term "moon planet" much because it is hard to translate into other languages. Planète lunaire for French is already occupied by the meaning "planet looking like the moon", i.e. Mercury, Mondplanet for German, though better than "Satellitenplanet", is commonly understood as referring to inhabited SF moons (like Endor or Pandora), meaning that verbatim translations will lead to a lot of avoidable confusion. I am sure we can come up with something better than that. French astronomers of the 19th cntury also used the term planètes subalternes, what about that?

[1] Möhn/Klemenčič/Riemann (Codex Regius): „Ceres - Pluto's little sister“, Wiesbaden/Ljubljana 2017 (prep.)

[2] Möhn/Klemenčič/Riemann (Codex Regius): „Titan - Pluto's big brother“, Wiesbaden/Ljubljana 2016

[3] Möhn/Klemenčič/Riemann (Codex Regius): „Pluto & Charon“, Wiesbaden/Ljubljana 2016