Are some moons actually planets just orbiting the wrong center?
In short: The line between a moon and a planet is often a matter of perspective rather than physics. Some of our largest moons are larger than the planet Mercury, leading scientists to debate whether they should be classified as 'satellite planets' based on their geology rather than their orbits.
The classification of celestial bodies is currently caught in a conceptual tug-of-war. While the International Astronomical Union (IAU) maintains a strict distinction between planets and satellites, some planetary scientists, including Alan Stern, argue for a 'geophysical' definition. This view suggests that if a body possesses planetary geology—meaning it is large enough to achieve hydrostatic equilibrium—it should be considered a planet, regardless of whether it orbits a star or another non-stellar object.
Hydrostatic equilibrium occurs when a body's mass is sufficient to overcome rigid-body forces, causing it to relax into an ellipsoidal or spherical shape under its own gravity. This is often driven by internal partial melting, differentiation, tidal heating, or radiogenic heating. For example, seven moons—including Ganymede, Titan, and Triton—are more massive than the dwarf planet Pluto. Notably, Ganymede and Titan are larger than Mercury, though they possess less than half of Mercury's mass.
The distinction is nuanced by composition. Ice has less tensile strength than rock and deforms at lower pressures, which influences which bodies reach equilibrium. Unlike our Moon, which lacks hydrostatic equilibrium, Ganymede likely possesses it, as evidenced by a magnetic field suggesting internal liquid flow. Conversely, some moons like Mimas and Tethys are ellipsoidal but no longer in equilibrium due to cooling, while others like Proteus are too irregular to qualify.
Historically, the terms were fluid. Early observers like Galileo and Huygens often used 'planet' to describe moons, with some even referring to them as 'secondary planets.' It wasn't until the late 18th century that 'satellite' became the exclusive term in discovery reports. Today, the debate centers on whether we should prioritize the 'how' of a body's formation and structure over the 'where' of its orbital path.
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- Summary by Intellect · source: Planetary-mass moon
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