Samantha Lawler (opens in a new tab)Assistant Professor, Astronomy, University of Regina
“Why does it matter if Pluto is a planet or a dwarf planet? Because to me it makes things more confusing in our solar system. I know some things in space are planets and some are stars and others have other names like moons or comets. The dwarf planet is a more different name and I think that 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.
Our solar system consists of the sun, planets (which orbit around the sun) and small bodies (which orbit around the sun or planets). The category “small bodies” is divided into even categories smaller categories (opens in a new tab)mainly depending on the shape and size of the orbits.
In 1801, astronomers discovered Ceres, which was originally classified as a “planet”. (opens in a new tab) Astronomers have measured it to be 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 we have since discovered hundreds of thousands of them in the asteroid belt (opens in a new tab).
Pluto flyby anniversary: The most amazing photos from NASA’s New Horizons
A similar process of discovery and re-categorization has occurred for smaller bodies further out in the solar system.
Pluto has been discovered in 1930 (opens in a new tab) and has been called the ninth planet in our solar system for many decades. But astronomers quickly realized that Pluto was quite different from the other eight planets: it’s in an inclined orbit and it’s much smaller than the other planets.
Over the years, astronomers have discovered more and more small planet-like objects crossing Pluto’s orbit. These are now classified as “Kuiper Belt Objects (opens in a new tab).” It looked increasingly like Pluto might fit the category of Kuiper Belt Objects better than 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 has led astronomers to wonder whether Eris and Pluto are planets or not. Astronomers thought it was a big enough decision that the International Astronomical Union voted in 2006 (opens in a new tab). Astronomers decided that instead of downgrading Pluto to just a Kuiper Belt Object, they would create a new category of small bodies called “dwarf planet (opens in a new tab)Both Pluto and Eris would fall into this new category.
How do planets form?
Solar systems like ours form from 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 watch carefully (opens in a new tab) to form very distant solar systems, but they are so far away 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 a new tab). The planetesimals then grab pebbles, dust, and sometimes even smaller planetesimals with their gravity, which gets stronger as they get bigger. 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).
Measuring small bodies in our solar system, including dwarf planets, and comparing them with 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 tab).
Ceres, in the asteroid belt, and Pluto, Eris and about a dozen other Kuiper Belt objects (opens in a new tab) are large enough to be in the category of dwarf planets. This means that even though these are planetesimals that have grown large enough to be round, they have not developed gravity strong enough to catch all the other planetesimals near their orbit.
Other solar systems
Astronomers have now measured more than 5,000 exoplanets (opens in a new tab), planets of 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 tell us how planets form everywhere.
Follow all Expert Voices issues and debates — and join the discussion — on Facebook and Twitter. The opinions expressed are those of the author and do not necessarily reflect the views of the publisher.
#Curious #Kids #matter #Pluto #planet #dwarf #planet