By Natural Philosopher Mike Prestwood

How many planets does our solar system have: 8, 9, or 12?

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Cosmology < Science
or Planet Definitions < Philosophy of Science < Critical Thinking

How many planets does our solar system have?

The solar system has 8 planets, not 9, because Pluto is classified as a dwarf planet, and we currently have 4 dwarf planets for a total of 12. Let me explain.

Before Neil deGrasse Tyson, our personal astrophysicist, decided to “hate on” Pluto, we had 9 planets. That’s how many we had my whole life, and we were just waiting to see if the tenth, Planet X, would ever be spotted. Well, in 2006, in a fit of power, Neil independently decided Pluto was no longer a planet! Well, not really, but his fame as a great communicator did play a role, and he does get too much of the blame. What actually happened was that the International Astronomical Union (IAU), responsible for classifying celestial bodies, decided in 2006 to redefine the criteria for what constitutes a planet.

To be considered a planet, a celestial body must meet three criteria: it must orbit the Sun, be spherical due to its own gravity, and have cleared its orbit of other debris. Pluto meets the first two criteria but fails the third because it shares its orbit with other objects in the Kuiper Belt. Interestingly, Pluto actually has 5 moons! 

If the IAU didn’t reclassify Pluto, we’d likely be talking about 12 planets right now. In the years leading up to 2006, no less than three more planets were discovered: Haumea, Eris, and Makemake. Haumea even has two moons, and Eris and Makemake have one moon each. So, for the sake of a future trivia game, remember that we “currently” have 8 planets, and 4 dwarf planets in our solar system.

Image: New Horizons captured Charon and Pluto in 2015. The background image was captured in 2012.

Bonus 1-minute Hot Topic FAQ: Trivia: Did Einstein or Galileo discover the Relativity Principle?

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4 Minutes with Mike Prestwood: Weekly Wisdom Builder
June 16, 2024 Edition
Time Left: 
Wisdom at the crossroads of knowledge.

Wisdom emerges from the consistent exploration of the intersections of philosophy, science, critical thinking, and history.

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