This article was originally published on: The conversation. (opens in new tab) The publication contributed the article to that of Space.com Expert Voices: Opinion and Insights†
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? Because for me it just makes it more confusing in our solar system. I know some things in space are planets and some stars and some other names like moons or comets. Dwarf Planet is a 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.
U.S solar system consists of the sun, planets (which revolve around the sun), and small bodies (which either revolve around the sun or planets). The “small bodies” category is divided into even smaller categories (opens in new tab)usually depending on the shape and size of orbits.
In 1801, astronomers discovered Ceres, which is a initially categorized as a ‘planet’. (opens in new tab) Astronomers have measured that it was much smaller than the other known planets. Soon, much smaller objects were discovered in orbits very close to Ceres. These small bodies were categorized as “asteroidsand since then we have 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 took place for small bodies further out in the solar system.
Pluto used to be discovered in 1930 (opens in new tab) and was called the ninth planet in our solar system for many decades. But astronomers soon found out that Pluto was quite different from the other eight planets: It is in a tilted orbit and is much, much smaller than the other planets.
Over the years, astronomers discovered more and more small, planet-like objects intersecting Pluto’s orbit. These are now categorized as “Kuiper Belt Objects (opens in new tab)It increasingly seemed that Pluto might fit the category of Kuiper belt objects better than planets.
In 2005, a new object was discovered in the outer solar system, There is (opens in new tab), which is even heavier than Pluto. This led astronomers to consider whether both Eris and Pluto are planets or not. Astronomers thought this was a sufficiently important decision that the International Astronomical Union voted in 2006 (opens in new tab)† Astronomers decided that instead of relegating Pluto to a plain old Kuiper Belt object, they would create a new category of small bodies, a “dwarf planet (opens in new tab)Pluto and Eris would both be part of this new category.
How planets form
Solar systems like ours form from large clouds of dust and gas that collapse in disks around young stars, but astronomers are still learning exactly how that process works. We use telescopes to look carefully (opens in new tab) in the formation of solar systems far away, but they are so far that it is really difficult to see the planets forming directly.
A planetesimal – a baby planet – first forms from clumps of dust in a disk orbiting a young star (opens in new tab)† Planetesimals then grab nearby pebbles, dust, and sometimes even smaller planetesimals with their gravitational pull, which gets stronger as they get bigger. When they get a few hundred kilometers wide, they have enough gravity to pull themselves into a round shape, which is the 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 came to be. Our current theory is that there must have been many dwarf planets that formed in our solar system (opens in new tab)†
Ceres, in the asteroid belt, and Pluto, Eris and about a dozen other Kuiper Belt objects (opens in new tab) are large enough to fall into the dwarf planet category. This means that although they are planetesimals that grew large enough to be round, they did not develop a gravitational force strong enough to grab all other planetesimals near their orbits.
Other Solar Systems
Astronomers have now measured more than 5,000 exoplanets (opens in new tab), 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 how planets form everywhere.
This article was republished from The conversation (opens in new tab) under a Creative Commons license. Read the original article (opens in new tab)†
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