Samantha Lawler (opens in new tab)Assistant Professor, Astronomy, University of Regina
“Why does it matter if Pluto is a planet or a dwarf planet? For me it just makes it more confusing in our solar system. I know some things in space are planets and some are stars and others are other names like moons or comets “The dwarf planet is a more different name and I think that only makes it more confusing.” – Timmy, 11, Kitchener, Ont.
“Comet”, “star” and “planet” are category names that immediately tell something important about what they describe.
Spring Solar system consists of the sun, planets (which orbit the sun) and small bodies (which either orbit the sun or planets). The category “small bodies” is divided into even smaller categories (opens in new tab)mostly depending on the shape and size of the tracks.
In 1801, astronomers discovered Ceres, which was originally categorized as a “planet”. (opens in new tab) Astronomers measured that it was much smaller than the other known planets. Soon many smaller objects were discovered in 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 (opens in new tab).
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A similar process of discovery and recategorization happened for small bodies further out in the solar system.
Pluto where was discovered in 1930 (opens in new tab) and was called the ninth planet in our solar system for many decades. But astronomers soon learned that Pluto was quite different from the other eight planets: it is on an inclined orbit and is much 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 “The Kuiper Belt objects (opens in new tab)“It looked more and more like Pluto would fit better into the Kuiper Belt object category than with planets.
In 2005, a new object was discovered in the outer solar system, Eris (opens in new tab), it’s even heavier than Pluto. This prompted astronomers to consider whether or not both Eris and Pluto are planets. Astronomers thought this was a more important decision than the International Astronomical Union voted on it in 2006 (opens in new tab). Astronomers decided that instead of degrading Pluto to an ordinary old Kuiper belt object, they would make a new category of small bodies called a “the dwarf plane (opens in new tab)“Pluto and Eris would both fall into this new category.
How planets are formed
Solar systems like ours are formed from large 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 closely (opens in new tab) on forming solar systems far away, but they are so far that it is really difficult to see the planets form directly.
A planetesimal – a baby planet – is first formed by lumps of dust in a disk orbiting a young star (opens in new tab). Planetary decimals then take hold of nearby pebbles, dust and sometimes even smaller planetesimals with their gravity, which becomes stronger as they get larger. When they are a few hundred kilometers wide, they have enough gravity to pull themselves into a round shape, which is definition of a dwarf planet (opens in new tab).
Measuring small bodies in our solar system, including dwarf planets, and comparing them to computer simulations is another way of seeing how our solar system was formed. Our current theory is that it must have existed a lot of dwarf planets formed in our solar system (opens in new tab).
Ceres, in the asteroid belt, and Pluto, Eris and a dozen other Kuiper belt objects (opens in new tab) are large enough to be in the dwarf planet category. This means that while they are planetesimals that grew large enough to be round, they did not develop a gravity that was strong enough to grip all the other planetesimals near their orbit.
Other solar systems
Astronomers have now measured more than 5,000 exoplanets (opens in new tab), planets in other solar systems. We will not be able to measure dwarf planets there for a very long time, but those we have found in our own solar system can learn about how planets form everywhere.
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