This article was originally published in The Conversation. (opens in a new tab) The post contributed to Space.com’s Expert Voices: Op-Ed & Insights article.
Samantha Lawler (opens in a new tab), Assistant Professor, Astronomy, Regina University
“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. “Dwarf planet is a different name and I think it just makes it more confusing.” – Timmy, 11, Kitchener, Ontario.
“Comet”, “star” and “planet” are category names that immediately tell you something important about what they describe.
Our solar system is made up of the sun, planets (orbiting the sun) and small bodies (orbiting the sun or planets). The “small bodies” category is divided into even smaller categories (opens in a new tab), mainly depending on the shape and size of the orbits.
In 1801, astronomers discovered Ceres, which was initially classified as a “planet”. (opens new tab) Astronomers measured that it was much smaller than other known planets. Soon, many smaller objects were discovered in orbits very close to Ceres. These small bodies were classified as “asteroids” and since then we have discovered hundreds of thousands of them in the asteroid belt (opens in a new tab).
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New discoveries
A similar process of discovery and re-categorization occurred for smaller bodies further away from the solar system.
Pluto was discovered in 1930 (opens in a new tab) and was named 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 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 have crossed Pluto’s orbit. They are now classified as “Kuiper Belt Objects (opens in a new tab).” It seemed more and more that Pluto could fit better with the Kuiper Belt object category than with the planets.
In 2005, a new object was discovered in the outer solar system, Eris (opens in a new tab), which is even heavier than Pluto. This led astronomers to consider whether Eris and Pluto are planets or not. Astronomers thought this was an important enough decision that the International Astronomical Union voted on in 2006 (opens in a new tab). Astronomers decided that instead of degrading Pluto to a simple object in the Kuiper belt, they would create a new small body category called “dwarf planet (opens in a new tab)”. Pluto and Eris would be part of this new category.
Ceres, seen from NASA’s Dawn spacecraft. (Image credit: NASA / JPL-Caltech / UCLA / MPS / DLR / IDA)
How planets form
Solar systems like ours are formed from large clouds of dust and gas collapsing into disks around young stars, but astronomers are still learning exactly how this process works. We use telescopes to look closely (opens in a new tab) at the formation of distant solar systems, but they are so far away that it is very difficult to see the planets forming directly.
A planetesimal, a baby planet, is first formed from groups of dust on a disk orbiting a young star (opens in a new tab). Then the planetesimals pick up nearby pebbles, dust, and sometimes even smaller planetesimals with their gravity, which gets stronger as they get older. When they reach a few hundred kilometers in diameter, they have enough gravity to take on a round shape, which is the definition of a dwarf planet (opens in a new tab).
The two images at the top reveal discs of debris around young stars discovered in archive images taken by NASA’s Hubble Space Telescope. The illustration below each image shows the orientation of the garbage disks. (Image credit: (NASA / ESA, R. Soummer, Ann Feild (STScI))
Measuring small bodies in our solar system, including the dwarf planets, and comparing them to computer simulations is another way of looking at 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 tab).
Ceres, in the asteroid belt, and Pluto, Eris, and a dozen other objects in the Kuiper belt (opens in a new tab) are large enough to be in the dwarf planet category. This means that although they are planetesimals that grew large enough to be round, they did not develop a gravity strong enough to hold all the other planetesimals close to their orbit.
Other solar systems
Astronomers have now measured more than 5,000 exoplanets (opens in a new tab), planets from other solar systems. We won’t be able to measure dwarf planets there for long, but those we’ve found in our own solar system can teach us how planets form everywhere.
This article is republished from The Conversation (opens in a new tab) under a Creative Commons license. Read the original article (opens in a new tab).
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