Why is it important if we know whether Pluto is a planet or a dwarf planet? Because to me it just makes it more confusing in our solar system. I know that some things in outer space are planets and some are stars and some are other names like moon or comet. Dwarf planets seem like different names and I think that just makes it more confusing. — Timmy, 11, Kitchener, Ontario, Canada.
“Comets,” “stars” and “planets” are category names that immediately tell something important about what they are trying to describe.
Our solar system consists of the sun, the planets (which orbit around the sun) and small objects (which orbit around the sun or planets). The “small things” category is divided into smaller categorylargely depends on the shape and size of the orbit.
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What is a dwarf planet?
In 1801, astronomers discovered Ceres, which originally categorized as a “planet.” Astronomers measure that it is much smaller than any other known planet. Soon after, many smaller objects were discovered in orbits very close to Ceres. These small objects are categorized as “asteroids” and we have since discovered hundreds of thousands of asteroids in the asteroid belt.
–(NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)
New invention
A similar process of discovery and re-categorization occurs in small objects outside the solar system.
Pluto discovered in 1930 and was referred to as the ninth planet in our solar system for decades. But astronomers soon learned that Pluto is very different from the other eight planets in that its orbit is tilted and much smaller than the other planets.
Over the years, astronomy discovered more and more small objects such as planets walking in the orbit of Pluto. Currently they are categorized as kuiper belt object. It is increasingly clear why Pluto might fall into the Kuiper Belt object category more than a planet.
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How we discovered 840 minor planets beyond Neptune – and what they can tell us
In 2005, a new object was discovered in the outer solar system, namely Eris, which is even heavier than Pluto. This makes astronomers consider whether Eris and Pluto are planets or not. Astronomers consider this a decision important enough that the International Astronomical Union decided Pluto as a dwarf planet in 2006. Astronomers decided that instead of downgrading Pluto into ordinary old Kuiper Belt objects, they would create a new category of small objects called “dwarf planet.” Pluto and Eris will be part of this new category.
How planets are formed
Solar systems like ours formed from huge clouds of dust and gas that collapsed into disks around young stars, but astronomers are still studying how that process works. We use telescopes to look carefully in forming the distant solar system, but the distance is so far that it is very difficult to see the formation of planets directly.
Planetesimals which are baby planets — the first to form from clumps of dust on a disk orbiting a young star. The planetesimals then pick up gravel, dust, and sometimes even smaller planetesimals nearby by their gravity, which get stronger as they get bigger. When they reach several hundred kilometers, they have enough gravity to pull themselves into a spherical shape, which is dwarf planet definition.
–(NASA/ESA, R. Soummer, Ann Feild (STScI))
Measuring small objects in our solar system, including dwarf planets, and comparing them to computer simulations is another way to see how our solar system formed. Our current theory is that there must be many dwarf planets formed in our solar system.
Ceres, in the asteroid belt, and Pluto, Eris, and about a dozen other Kuiper Belt objects large enough to fit into the dwarf planet category. This means that although they are planetesimals that grow large enough to be spherical, they do not develop strong enough gravity to capture all the other planetesimals in close proximity to their orbits.
Another solar system
Astronomers have now measured more than 5,000 exoplanets, a planet in another solar system. We won’t be able to measure dwarf planets there for long, but the planets we find in our own solar system can teach us about how planets form everywhere.
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Arina Apsarini from Binus University translated this article from English.
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