Click image to open full size
Astronomy
Why Pluto Is Not a Planet
The 2006 IAU Reclassification
Related Tools
Related Worksheets
Related Cheat Sheets
Pluto was discovered in 1930 and was long taught as the ninth planet, but better observations showed that it is part of a much larger population of icy worlds beyond Neptune. In 2006, the International Astronomical Union created a formal definition of a planet that changed Pluto’s classification. This matters because scientific categories are based on evidence, not tradition, and they help astronomers describe how objects form and move in the Solar System. Pluto is now classified as a dwarf planet, not because it is unimportant, but because it does not meet all the criteria for a planet.
Key Facts
- IAU planet rule 1: the object must orbit the Sun.
- IAU planet rule 2: the object must be nearly round because of its own gravity, called hydrostatic equilibrium.
- IAU planet rule 3: the object must have cleared its orbital neighborhood.
- Pluto satisfies the first two planet rules but fails the third because it shares its region with many Kuiper Belt objects.
- Orbital period relation: T^2 proportional to a^3, where T is orbital period and a is average distance from the Sun.
- Pluto’s orbital period is about 248 Earth years, and its average distance from the Sun is about 39.5 AU.
Vocabulary
- Dwarf planet
- A dwarf planet orbits the Sun and is nearly round, but it has not cleared its orbital neighborhood.
- Kuiper Belt
- The Kuiper Belt is a region beyond Neptune filled with icy bodies, small worlds, and dwarf planets.
- Hydrostatic equilibrium
- Hydrostatic equilibrium is the state in which an object’s gravity pulls it into a nearly round shape.
- Cleared neighborhood
- An object has cleared its neighborhood if it is gravitationally dominant near its orbit and has removed, captured, or controlled most nearby objects.
- IAU
- The International Astronomical Union is the organization that sets official naming and classification standards for astronomical objects.
Common Mistakes to Avoid
- Saying Pluto is not a planet because it is too small is wrong because size alone is not the IAU criterion. Pluto is smaller than Earth’s Moon, but its classification depends mainly on whether it has cleared its orbital neighborhood.
- Thinking Pluto was demoted because its orbit crosses Neptune’s orbit is wrong because the key issue is not simple orbit crossing. Pluto fails the planet definition because it shares its orbital region with many Kuiper Belt objects.
- Calling Pluto a moon or asteroid is wrong because Pluto orbits the Sun directly and is nearly round. It is best classified as a dwarf planet.
- Assuming dwarf planets are unimportant is wrong because they preserve clues about the early Solar System. Pluto, Eris, Ceres, Makemake, and Haumea are scientifically valuable objects.
Practice Questions
- 1 Pluto’s average distance from the Sun is about 39.5 AU. Using T^2 = a^3 with T in Earth years and a in AU, estimate Pluto’s orbital period.
- 2 A newly discovered object orbits the Sun and has an average distance of 100 AU. Using T^2 = a^3, estimate its orbital period in Earth years.
- 3 An icy object beyond Neptune orbits the Sun, is nearly round, and shares its orbit with many similar objects. Explain whether it should be classified as a planet or a dwarf planet using the 2006 IAU definition.