CeleryKills

Planets, Dwarf Planets, Moons, and the Problem of Classification

On a cool, unexpectedly gray August morning in the Pacific Northwest, the kind where low clouds cling to the Cascades and summer seems to have misplaced itself, I settled into my usual routine with an Americano and a scroll through my science news feed. What began as a quick look at recent astronomy headlines soon turned into a deep dive into one of the oldest and strangest questions in planetary science: what exactly makes a planet a planet? Stories about exomoons, dwarf planets, brown dwarfs, and newly discovered worlds with no obvious place in our cosmic taxonomy revealed a field wrestling not merely with new discoveries, but with the limits of its own definitions. As observations push farther beyond our Solar System, astronomers are finding that the universe often refuses to fit into the categories we have carefully constructed.

For most of human history, the Solar System seemed orderly. Planets were planets, moons were moons, and the stars remained distant points of light. Over the last century, however, astronomy has repeatedly forced us to confront an uncomfortable reality: nature does not care about our categories. Every new generation of discoveries has challenged assumptions about what a planet is, what a moon is, and whether those distinctions are based on an object’s physical nature or simply its relationship to something larger. Recent discoveries involving exomoons, brown dwarfs, and planetary-mass companions suggest that the debate that demoted Pluto in 2006 may be only the beginning (Lecavelier des Etangs & Lissauer, The IAU Working Definition of an Exoplanet, 2022; Grossman, A Newly Found World Blurs the Boundary Between Planet and Moon, 2026).

I find it remarkable that astronomy, often viewed as one of the most objective sciences, keeps returning to philosophical questions. The farther we look from Earth, the more our definitions begin to unravel. If a Jupiter-sized body orbits a brown dwarf that itself orbits a star, is it a planet? A moon? Something entirely new? Increasingly, the answer depends less on what the object is and more on how we choose to classify it (Hoy et al., Planetary-Mass Exosatellite Detected Around the Substellar Companion of a Star, 2026).

The history of planetary classification demonstrates that these disagreements are not new. The ancient Greeks defined planets as wandering stars, objects that moved across the sky relative to the fixed stars. Following the Copernican Revolution, the definition shifted from apparent motion to orbital dynamics. A planet became an object that orbited the Sun. This change removed the Moon and Sun from the planetary roster while adding Earth. The defining feature was not composition, mass, or shape, but motion and hierarchy within a celestial system (Lecavelier des Etangs & Lissauer, 2022).

During the nineteenth century, classification faced its first major crisis. Ceres, discovered in 1801 between Mars and Jupiter, was initially considered a planet. As additional objects such as Pallas, Vesta, and Juno were discovered, astronomers realized that the region contained an entire population of similar bodies. Rather than continually expanding the planetary roster, they created a new category: asteroids or minor planets. Ceres lost its planetary status not because it changed, but because our understanding of its environment changed (Lecavelier des Etangs & Lissauer, 2022).

More than a century later, Pluto underwent a similar transformation. For decades it was accepted as the ninth planet despite growing evidence that it was much smaller than expected. The discovery of the Kuiper Belt and the dwarf planet Eris revealed that Pluto was simply one member of a much larger population of icy worlds beyond Neptune. In 2006, the International Astronomical Union established the modern Solar System definition of a planet, requiring that an object orbit the Sun, possess sufficient gravity to become nearly round, and clear its orbital neighborhood. Pluto failed the third criterion and became the most famous dwarf planet in history. Ceres, ironically, became a dwarf planet as well (Lecavelier des Etangs & Lissauer, 2022).

For those interested the IAU definition for a planet is:

1. Orbits the Sun.
2. Is nearly round due to its own gravity.
3. Has cleared the neighborhood around its orbit.
4. Is not a satellite (moon) of another body.

And that’s a very system specific definition.

What fascinates me about the Pluto controversy is that it exposed a conflict that remains unresolved. Should planets be defined by intrinsic properties such as size, mass, composition, and geology? Or should they be defined by their dynamical relationships to neighboring objects? Pluto did not become less spherical or less geologically active when it was reclassified. Humanity simply decided that context mattered more than physical characteristics. That decision continues to shape astronomical debates today (Lecavelier des Etangs & Lissauer, 2022).

Many planetary scientists therefore prefer a more generalized version:

1. Orbits a star (or stellar remnant).
2. Is large enough to be rounded by self-gravity.
3. Is gravitationally dominant in its orbital zone.

And many are now choosing to see a planet as defined by its intrinsic properties; a moon is defined only by its orbital relationship. So that means as a geophysical definition:

1. The object is not a star.
   It does not sustain hydrogen fusion in its core.
2. The object has sufficient mass for self-gravity to make it approximately round (hydrostatic equilibrium).
   Gravity has overcome the strength of the object’s materials, producing a roughly spherical shape.
3. The object is a coherent planetary body with its own geology and internal evolution.
   It is large enough to be a world in its own right rather than a small irregular fragment such as an asteroid or comet.
4. Its classification does not depend on what it orbits.
   Whether it orbits a star, a giant planet, a brown dwarf, or nothing at all (a rogue planet) is irrelevant. The object’s physical characteristics determine its status.

Under this definition: Earth = Planet, Mars = Planet, Pluto = Planet, Ceres = Planet, Titan = Planet, Europa = Planet, Ganymede = Planet, Rogue planets = Planet. Moons become satellites, and planets become terrestrial, giant, ice giant, dwarf. Other system bodies become rocky and icy bodies, and like moons are classified by orbital characteristics.

So with orbital characteristics:

1. Binary Companion – Orbits a common barycenter with another object.
2. Rogue Body – Not bound to a star.
3. Primary Resonant Body – Locked into a resonance with a star.
4. Secondary Resonant Body – Locked into a resonance with another non-star body.
5. Scattered Body – Strongly perturbed by larger bodies so a dominant resonance is not apparent.

This make Earth = Primary Resonant Planet, Titan = Secondary Resonant Planet, Kuiper Belt = Primary Resonant Planet or Scattered Body, etc.

And finally, the activity of the object could be added, although it’s more like a bonus descriptor. The would be Active Cometary, Inactive, Volcanically Active, Atmosphere-Bearing, Ocean, etc. These are not exclusive like physical or orbital definitions, as Earth = Volcanically Active, Atmosphere-Bearing, Ocean.

Modern exoplanet science has only intensified this disagreement. The IAU’s current working definition for exoplanets incorporates both physical limits and orbital hierarchy, defining planets partly through their relationship to stars, brown dwarfs, or stellar remnants (Lecavelier des Etangs & Lissauer, 2022). Yet recent observations have uncovered systems that challenge those boundaries.

The most compelling example is CD-35 2722. In 2026, astronomers reported evidence for a roughly Jupiter-mass companion orbiting the brown dwarf CD-35 2722 B, which itself orbits a low-mass star. The researchers deliberately described the new object as an “exosatellite” because existing terminology appears inadequate (Hoy et al., 2026). Is it a moon because it orbits a larger substellar object? A planet because of its mass? Or something else entirely? Even the discoverers expressed discomfort with applying familiar labels to an unfamiliar architecture (Grossman, 2026).

The discovery highlights how dependent our definitions are on orbital relationships. Jupiter’s moon Ganymede is larger than Mercury. If Ganymede orbited the Sun directly, many people would almost certainly consider it a planet. If Mercury orbited Jupiter, most would call it a moon. Their classifications derive not from their physical properties but from where they happen to be (Grossman, 2026).

The ambiguity extends beyond moons and planets. Brown dwarfs themselves blur the distinction between planets and stars. They occupy an intermediate category, massive enough to fuse deuterium but not hydrogen. Systems such as CD-35 2722 raise difficult questions about whether formation history, mass, composition, or orbital hierarchy should carry the greatest weight in classification (Hoy et al., 2026; Lecavelier des Etangs & Lissauer, 2022).

Additional evidence suggests that CD-35 2722 may not be unique. Kral and colleagues reported a candidate exomoon around the brown dwarf companion HD 206893B, providing another example of a system that appears to sit at the edge of established categories (Kral et al., Exomoon Search with VLTI/GRAVITY Around the Substellar Companion HD 206893B, 2026).

Looking ahead, the classification problem may become even worse. Science fiction has spent decades imagining worlds that astronomers are only beginning to consider seriously. James Cameron’s Avatar centers on Pandora, a habitable moon orbiting a giant planet. Isaac Asimov’s and Larry Niven’s works frequently feature complex multi-body systems. Star Wars does the same with Endor and presents worlds orbiting binary stars as commonplace. Yet modern astronomy is now revealing systems whose structures can be just as strange as those fictional settings.

Imagine a double-planet system in which two Earth-sized worlds orbit a shared center of gravity while circling a star. Is one a moon and one a planet, or are both planets? In sci-fi that’s Robert L. Forward’s Rocheworld (1982), and in real system that almost Pluto and Charon, because their barycenter lies outside Pluto’s surface. Imagine a rogue planet drifting through interstellar space with its own moon. If no star is involved, what role should orbital hierarchy play? Imagine a moon orbiting a planet orbiting a brown dwarf orbiting a binary star. At what point does a moon cease being a moon?

These questions reveal something profound about science itself. A planet is not a naturally occurring label embedded within the universe. It is a human category, created to organize observations. As our observations become more sophisticated, our categories must evolve as well.

The last century transformed Ceres from planet to asteroid to dwarf planet. Pluto followed a similar path from planet to dwarf planet. The next century may force astronomers to create entirely new classes for exosatellites, binary planets, and hybrid systems that fit neither traditional planetary nor lunar categories. Discoveries such as CD-35 2722 suggest that the future of planetary science may not be about finding new planets. Instead, it may be about realizing that the universe contains more varieties of worlds than our vocabulary can currently describe.

And that leaves us with a dilemma worth considering. If Mercury can become a moon by changing its orbit, and a moon can become a planet by changing its host, are planets and moons truly distinct classes of objects? Or are they merely roles played within a much larger cosmic hierarchy? As astronomy continues to explore strange new systems, we may discover that the universe is far less interested in our definitions than we are.

References

• Grossman, L. (2026). A Newly Found World Blurs the Boundary Between Planet and Moon. Science News. July 22, 2026.
• Hoy, K., Zurlo, A., Peña R., P. A., et al. (2026). Planetary-Mass Exosatellite Detected Around the Substellar Companion of a Star. Nature, 655, 865-869.
• Kral, Q., et al. (2026). Exomoon Search with VLTI/GRAVITY Around the Substellar Companion HD 206893B. Astronomy & Astrophysics, 695, A15.
• Lecavelier des Etangs, A., & Lissauer, J. J. (2022). The IAU Working Definition of an Exoplanet. New Astronomy Reviews, 94, 101641. doi:10.1016/j.newar.2022.101641.