Space & the universe · Science & ideas

The Sun and the Moon were once planets. Pluto no longer is.

What counts as a "planet" turns out to be a surprisingly slippery question that has shifted meaning for over two thousand years. The ancient Greeks coined the word for the handful of starlike lights that wandered against the fixed stars, originally lumping in the Sun and Moon alongside Mercury, Venus, Mars, Jupiter and Saturn. Copernicus's heliocentric model forced a conceptual overhaul, demoting the Sun and Moon while promoting Earth to planetary status. Herschel's discovery of Uranus, followed by Neptune and then Pluto, kept expanding the list, while newly found bodies like Ceres were briefly hailed as planets before being reclassified once more objects turned up alongside them in the same orbital zone. Even moons caused confusion early on, with astronomers like Galileo and Huygens unsure whether to call their newly spotted satellites planets in their own right.

The real crisis came in 2005 with the discovery of Eris, a distant icy body bigger than Pluto, which pushed the International Astronomical Union to finally legislate a definition in 2006. Their rule says a planet must orbit the Sun, be round under its own gravity, and have cleared its orbital neighborhood of other debris — a test Pluto fails, landing it in a new "dwarf planet" category alongside Ceres and Eris. Supporters like Mike Brown argue the clearing criterion reflects real physics, since true planets gravitationally dominate everything near them by enormous margins, while Pluto controls only a small fraction of the material in its zone. Critics, especially Alan Stern, call the standard arbitrary and point out that even Earth, Mars and Neptune haven't fully swept their orbits clean, and that only a tiny fraction of astronomers actually voted on the decision. There's also no crisp line for how round is round enough, since even Mercury isn't in perfect hydrostatic equilibrium, which undercuts the IAU's own stated logic.

The argument spreads well beyond Pluto. Large moons like Titan and Ganymede are bigger than Mercury and show earthlike geology, leading some scientists to suggest that round satellites deserve to be called planets too, under a rival "geophysical" definition based on shape rather than orbital dominance. Beyond the Solar System, the boundary between a giant planet and a small failed star is equally blurry, since gas giants and brown dwarfs share similar compositions and the usual dividing line — the ability to briefly fuse deuterium at around thirteen Jupiter masses — has no sharp physical basis and keeps getting challenged by newly discovered objects that straddle it. Some researchers now propose sorting these bodies by how they formed, or by internal pressure physics, rather than by mass cutoffs alone. Even the vocabulary is contested, since calling something a "dwarf planet" while insisting it isn't a planet strikes many linguists and astronomers as self-contradictory. None of these disputes have been resolved, leaving the definition of planet still an open and genuinely unsettled question.

The IAU's official definition applies only to the Solar System — exoplanets had been handled separately, three years earlier.

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