Why is Pluto not a planet? Most people will tell you it got kicked out for being too small. That’s the popular answer. It’s also not the real reason.
Where the “too small” story comes from
For 76 years, Pluto was the ninth planet. NASA says it was discovered in 1930, and it stayed on the planet list until 2006.
Then, in 2006, the International Astronomical Union (the IAU, the group of astronomers that names and classifies objects in space) reclassified Pluto as a dwarf planet. The word “dwarf” sounds like a size ruling. Pluto is a small world, about one-fifth as wide as Earth, so “too small” feels like an easy explanation.
It’s a natural guess. But if you read the rules the astronomers actually wrote, none of them sets a size cutoff.
The verdict: Myth
Myth. Pluto wasn’t dropped for being small. The IAU’s test for a planet has three parts, and none of them is a size limit. Pluto passes two of the three. It fails the third, which is about the neighborhood around its orbit.
Mass does matter in a roundabout way, because a planet needs enough gravity to rule its lane. But no rule says “must be bigger than this.” Pluto’s label changed because of where it lives, in a crowded neighborhood, not because of a size cutoff.
The three rules for a planet
NASA explains that the IAU adopted its definition in 2006, in a document called Resolution B5. To count as a planet, an object has to meet all three of these tests:
- It orbits the Sun. Pluto does.
- It has enough gravity to pull itself into a nearly round shape. Pluto does this one, too.
- It has cleared the neighborhood around its orbit. Pluto has not.
An object that passes the first two tests but not the third is a dwarf planet. (A dwarf planet also can’t be a moon of something else.) That’s Pluto’s category.
“Clearing the neighborhood” sounds odd, so here’s a picture. Imagine a path around the Sun, like a running track. A planet is the biggest thing on its track, and its gravity has long since swept the lane. Pluto’s lane is different. It sits in the Kuiper Belt, a doughnut-shaped zone beyond Neptune that NASA says holds thousands of small icy worlds. Pluto is one of the crowd, not the boss of its lane.
Why 2006? Blame Eris
The timing has a story behind it. In 2005, astronomers announced Eris, a distant icy world. NASA says it’s about the same size as Pluto.
That created a puzzle. If Pluto was a planet, what about Eris? And what about the other Pluto-sized worlds that surely were waiting to be found? The Planetary Society explains that the IAU chose not to keep adding planets. Instead, in 2006, it wrote the first formal definition of the word.
NASA dates the decision to August 26, 2006. Pluto, Eris and Ceres became dwarf planets. NASA now lists five official dwarf planets: Eris, Ceres, Pluto, Haumea and Makemake. It adds that there may be about 100 more in our solar system.
Pluto up close
Losing the “planet” label didn’t make Pluto any less interesting. It made it better known. In July 2015, NASA’s New Horizons became the first spacecraft to explore Pluto up close. It flew past on July 14, after launching in January 2006, the same year of the vote.
What it found surprised scientists. Pluto is about 1,470 miles (2,370 km) across. NASA says that is only about half the width of the United States. NASA’s New Horizons page describes a vast heart-shaped glacier of frozen nitrogen on its surface, about 600 miles wide. It also found haze in Pluto’s atmosphere.
NASA says Pluto has five known moons: Charon, Styx, Nix, Kerberos and Hydra. Charon is the big one, about half Pluto’s size. The other four are each less than 100 miles (160 km) wide.
Out there it’s cold. NASA gives an average temperature of about minus 387 degrees Fahrenheit (minus 232 degrees Celsius).
Do scientists still argue about it?
Yes. The Planetary Society says the 2006 decision sparked a debate that continues today. Not everyone accepts the IAU’s definition, and NASA notes the same.
National Geographic lays out both sides. Mike Brown, the Caltech astronomer who found Eris, argues that Pluto doesn’t fit what astronomers now know about planets. On the other side, Philip Metzger, a retired NASA planetary physicist, argues the 2006 vote was invalid because the IAU broke its own bylaws. National Geographic also reports that Brown says a looser definition would add about 200 planets to the solar system.
One thing both sides agree on: the discussion is about words and definitions. Pluto itself hasn’t changed. It’s still a round, icy world with a heart-shaped glacier, five moons and a very long year.
Small wasn’t the problem. Pluto just wasn’t alone in its lane.
