There was a time when Pluto was the answer to the question of which planet was the smallest in our solar system. Then, around 2006, something changed. Suddenly Pluto wasn't a planet anymore. What happened? Why was it demoted from a planet to a dwarf planet? Let's find out.
The story of Pluto
Discovered in 1930 by the American astronomer Clyde Tombaugh, Pluto is around 18–19% as large as our planet Earth and sat as the ninth planet in our solar system for generations. Generations of schoolchildren learned its name in the classic planetary lineup, right after Neptune. It was also something of a fan favourite — just look at the outrage that followed its demotion to see how much people loved it. Some fans still refuse to accept the change.
What changed in 2006
Around 2006, the story took a turn. The International Astronomical Union (IAU), an organization of professional astronomers, met in a general assembly on 24th August, 2006, and made the decision to demote the much-loved Pluto from its status as a planet. The union laid out certain rules that a celestial body would need to follow to be considered a planet.
According to those rules, a planet should:
- Orbit around the sun.
- Possess a round or nearly round shape.
- Be able to clear its orbit path around the sun of any other objects. In other words, during its orbit, if it encounters asteroids, dwarf planets, debris, or anything else, the body should be able to push those objects away through its own force. The gravity or force a planet exerts should be enough for the road around the sun to stay clear.
Pluto was able to meet the first two conditions, but it couldn't meet the third — and that's what led to its demotion. It simply couldn't clear its neighbourhood the way the other planets could. A big part of the problem was that Pluto's orbit crossed the path of Neptune as well, and it shared space with plenty of other icy bodies in the Kuiper Belt beyond the gas giants.
Why did scientists create these rules in the first place?
That's a fair question. Before the decision and the rule set came about, scientists had found many celestial bodies that behaved like Pluto but not like the other planets. There was Haumea, MakeMake, and Eris — which, like Pluto, sat beyond Neptune. And there was Ceres, which sat along the asteroid belt between Mars and Jupiter. These discoveries ultimately pushed scientists to define rules for what could count as a planet, and Pluto simply didn't fit the new definition.
Further refinements
Later discoveries led to further resolutions, like 5A and 6A, being added to the existing rules — for example, that the body should not be a satellite of another planet.
Where things stand now
Even now, more than 18 years after the decision, it's still met with outrage. Multiple appeals have been made for Pluto to be reinstated as a planet, and people with strong feelings about it make themselves heard every time space is in the news. As it stands, though, Pluto remains in the realm of dwarf planets — a much smaller club, but one with some very interesting members.
A fun Pluto fact
One thing worth remembering: Pluto is so far from the Sun that it takes around 5 hours for sunlight to reach it. Compare that to Earth, where sunlight arrives in about 8 minutes. That alone tells you just how far out on the edge of our solar system Pluto really is.
Well, that's it for today's curious question. Until next time, remember that the only requirement for a curious mind is simply being curious enough.