This article was originally published in The Conversation. (opens in a new section) The publication contributed to the article in Expert Voices on Space.com: Op-Ed & Insights.
Samantha Lawler (opens in a new section), Assistant Professor of Astronomy, University of Regina
“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, others are stars, and others are names like moons or comets. The dwarf planet is a different name and I think it just makes it more confusing. “- Timmy, 11, Kitchener, Ont.
“Comet”, “star” and “planet” are names of categories that immediately tell you something important about what they describe.
Our solar system consists of the sun, planets (which orbit the sun) and small bodies (which either orbit the sun or planets). The “small bodies” category is divided into even smaller categories (opens in a new section), mostly depending on the shape and size of the orbits.
In 1801, astronomers discovered Ceres, which was originally categorized as a “planet.” (opens in a new section) Astronomers measure that it is much smaller than other known planets. Much smaller objects were soon discovered in orbits very close to Ceres. These small bodies were categorized as “asteroids” and we have since found hundreds of thousands of them in the asteroid belt (opens in a new section).
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New discoveries
A similar process of discovery and re-categorization occurred for small bodies further in the solar system.
Pluto was discovered in 1930 (opens in a new section) and has been called the ninth planet in our solar system for many decades. But astronomers soon learned that Pluto is quite different from the other eight planets: it is in an inclined orbit and is much smaller than the other planets.
Over the years, astronomers have discovered more and more small, planet-like objects that cross Pluto’s orbit. They are already categorized as “Kuiper Belt objects (opens in a new section)”. It seemed more and more that Pluto could fit better into the category of objects in the Kuiper Belt than with the planets.
In 2005, a new object was discovered in the outer solar system, Eris (opened in a new section), which is even heavier than Pluto. This has led astronomers to consider whether both Eris and Pluto are planets or not. Astronomers thought it was an important enough decision for the International Astronomical Union to vote for it in 2006 (opens in a new section). Astronomers have decided that instead of lowering Pluto to a simple old object from the Kuiper Belt, they will create a new category of small bodies called the dwarf planet (opens in a new section). Pluto and Eris will be part of this new category.
Ceres, seen by NASA’s Dawn spacecraft. (Image Credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA)
How planets form
Solar systems like ours are formed by large clouds of dust and gas that collapse into disks around young stars, but astronomers are still learning exactly how this process works. We use telescopes to look closely (a new section opens) at the formation of solar systems far away, but they are so far away that it is really difficult to see how planets form directly.
The planetesimal – baby planet – is first formed by lumps of dust in a disk orbiting a young star (opens in a new section). Then the planetesimals grab the nearby pebbles, dust and sometimes even smaller planetesimals with their gravity, which becomes stronger as they get bigger. When they reach several hundred kilometers in diameter, they have enough gravity to pull in a round shape, which is the definition of a dwarf planet (opens in a new section).
The two images above reveal disks of debris around young stars found in archival images made by NASA’s Hubble Space Telescope. The illustration below each image depicts the orientation of the waste discs. (Image Credit: (NASA / ESA, R. Soummer, Ann Feild (STScI))
Measuring small bodies 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 have been many dwarf planets that formed in our solar system (opens in a new section).
Ceres, in the asteroid belt, and Pluto, Eris, and about a dozen other objects in the Kuiper belt (opens in a new section) are large enough to be in the category of dwarf planets. This means that although they are planetesimals that have grown large enough to be round, they have not developed gravity strong enough to catch all other planetesimals close to their orbit.
Other solar systems
Astronomers have already measured more than 5,000 exoplanets (opens in a new section), planets in other solar systems. We won’t be able to measure dwarf planets there for very long, but those we’ve found in our own solar system can teach us how planets form everywhere.
This article has been republished by The Conversation (opens in a new section) under a Creative Commons license. Read the original article (opens in a new section).
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