Curious Kids is a series for children of all ages. Have a question you’d like an expert to answer? Send it to .
Why does it matter if Pluto is a planet or a dwarf planet? Because for me it just makes it more confusing in our solar system. I know that some things in space are planets and some are stars and some are other names like moons or comets. Dwarf planet is a more different name and I think it just makes it more confusing. — Timmy, 11, Kitchener, Ont.
“Comet,” “star” and “planet” are category names that immediately tell you something important about what they describe.
Our solar system consists of the sun, planets (which orbit around the sun) and small bodies (which either orbit around the sun or planets). The “small bodies” category is divided up into even smaller categories, mostly depending on the shape and size of orbits.
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In 1801, astronomers discovered Ceres, which was initially categorized as a “planet.” Astronomers measured that it was much smaller than the other known planets. Soon, a lot of smaller objects were discovered on orbits very close to Ceres. These small bodies were categorized as “asteroids” and we have since discovered hundreds of thousands of these in the asteroid belt.
New discoveries
A similar process of discovery and re-categorization happened for small bodies further out in the solar system.
Pluto was discovered in 1930 and was called the ninth planet in our solar system for many decades. But astronomers soon learned that Pluto was pretty different from the other eight planets: it’s on a tilted orbit and it’s way, way smaller than the other planets.
Over the years, astronomers discovered more and more small, planet-like objects that crossed Pluto’s orbit. These are now categorized as “Kuiper Belt objects.” It was looking more and more like Pluto might fit in better with the category of Kuiper Belt objects than with planets.
In 2005, a new object was discovered in the outer solar system, Eris, that is even heavier than Pluto. This led astronomers to consider if both Eris and Pluto are planets or not. Astronomers thought this was an important enough decision that the International Astronomical Union voted on it in 2006. Astronomers decided that rather than demoting Pluto to a plain old Kuiper Belt object, they would make a new category of small body called a “dwarf planet.” Pluto and Eris would both be part of this new category.
How planets form
Solar systems like ours form from big clouds of dust and gas that collapse into disks around young stars, but astronomers are still learning exactly how that process works. We use telescopes to look carefully at forming solar systems far away, but they are so far that it’s really hard to see the planets forming directly.
A planetesimal — a baby planet — first forms from clumps of dust in a disc orbiting a young star. Planetesimals then grab nearby pebbles, dust and sometimes even smaller planetesimals with their gravity, which gets stronger as they get bigger. When they get to be a few hundred kilometres across, they have enough gravity to pull themselves into a round shape, which is the definition of a dwarf planet.
Measuring small bodies in our solar system, including dwarf planets, and comparing them with computer simulations is another way to see how our solar system formed. Our current theory is that there must have been a lot of dwarf planets that formed in our solar system.
Ceres, in the asteroid belt, and Pluto, Eris and about a dozen other Kuiper Belt objects are big enough to be in the dwarf planet category. This means that while they are planetesimals that grew big enough to be round, they did not develop a gravity strong enough to grab all the other planetesimals near their orbit.
Other solar systems
Astronomers have now measured more than 5,000 exoplanets, planets in other solar systems. We won’t be able to measure dwarf planets there for a very long time, but the ones we’ve found in our own solar system can teach us about how planets form everywhere.
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Pluto used to be known as the smallest planet in our solar system, and the furthest planet from the sun. Pluto was discovered in 1930 by Clyde Tombaugh, an American astronomer. Pluto is over 5.8 billion km (3.6 billion miles) away, and it takes Pluto 248 years to orbit the sun. While many of the other planets orbit the sun in an elliptical orbit that is nearly circular, Pluto’s is very eccentric. This means that sometimes Pluto is closer to the sun than Neptune! Pluto is a little bit smaller than Earth’s moon.
Pluto is in a far away region of space called the Kuiper belt. The Kuiper belt is part of the solar system past Neptune, similar to an asteroid belt. It is filled with asteroids made of rock and metal, as well as “ices” (frozen gases).
Pluto was named by an eleven-year-old girl from England, who chose the name of the mythological guardian of the underworld. Pluto also has three moons of its own: Charon, Nix, and Hydra. Charon is about half the size of Pluto, but Nix and Hydra are very small.
Pluto was originally classified as a planet, but is now known as a “dwarf planet”. In 2005, an American astronomer called Mike Brown thought that he had found a planet further out in the solar system than Pluto. This new planet was thought to be the tenth planet in the solar system, and was named Eris.
However, this prompted the the International Astronomical Union (IAU) to decide that “planet” actually needed to be defined, to see if Eris would fit into the definition. So, in 2006 the IAU decided that to be a planet, the object must:
- Orbit around the sun
- Be massive enough to be pulled into a the shape of a sphere by its own gravitational force; and
- Have ‘cleared the neighbourhood’ around its orbit. This means there must be no other bodies in its orbit that are not under its influence (like a moon).
Eris didn’t meet the criteria, and neither did Pluto! Namely, Pluto didn’t meet the third criteria, and so was downgraded from a planet to a dwarf planet in 2006. This dwarf planet definition was created by the IAU at the same time as the planet definition.
In the same year, NASA launched a mission to Pluto called New Horizons. New Horizons is going to the very edge of the solar system, and it will take approximately 9 years to reach Pluto. Astronomers now think that there may be thousands of dwarf planets in the solar system, but that they are too far away for us to be able to see them. They hope that if we can explore the Kuiper belt further with missions like New Horizons, more dwarf planets may be found.