In 2006, the International Astronomical Union (IAU) voted to define the planet. famous, Pluto It no longer met the criteria and was demoted to a dwarf planet. Things have been a bit of a mess since then, so is it time to redefine the planet?
To be fair, Pluto was coming. The word “planet“There has never been a formal definition, and astronomers have always played around with its use very quickly and loosely. To the ancient Greeks, a planet was any ‘wandering star’, including stars.” sun and moon. With the Copernican Revolution, the definition changed: land It was considered a planet in its own right, the Moon was demoted to a satellite, and the Sun was promoted.
This worked for more than 200 years, until William Herschel discovered it Uranus Discover Giuseppe Piazzi CeresIt is the largest body in the main asteroid belt between Mars and Jupiter. Initially, both Uranus and Ceres were classified as planets. But once more objects were found that shared similar orbits with Ceres, astronomers had to rethink things — planets certainly live on their own, after all. Herschel proposed the term “AsteroidsFor small bodies between Mars and Jupiter, while Uranus remained a planet (a situation that certainly benefited Herschel’s legacy).
Astronomers were comfortable with these classifications even when Clyde Tombaugh discovered Pluto in 1930. But this new planet was a stretch — it had a really wonky orbit and was much smaller than the other planets, so it didn’t take long for astronomers to start wondering if it should They need to start reconsidering how they classify things.
Starting in the 1990s, astronomers began finding more objects that shared similar orbits with Pluto. But the real nail in the planets’ coffin came in 2005, when astronomer Mike Brown discovered them IrisIt is an object the same size as Pluto and orbits behind Neptune.
So, in 2006, when astronomers gathered at the International Astronomical Union meeting in Prague, a large group moved to have this object define what a planet should look like. There were two camps: the geophysicists who argued that the planets should be defined by their appearance, and the dynamicists who believed that the planets should be defined by their properties.
In essence, geophysicists argued that a planet must be anything large enough that its own gravity pulls it into a roughly spherical shape. Dynamicists responded that the planet must be anything that can dominate and clear its orbit of any debris. The first identification will allow Pluto, along with Ceres and all of Pluto’s friends in orbit, to become planets. The latter definition would exclude all such smaller bodies.
In the end, the settlement included both definitions, which means that the dynamists actually won. Pluto has not cleared its orbit – it simply has too many neighbors – hence He was famously demoted.
Related to: What exactly is a planet? Astronomers want to modify the definition
But not everyone agrees with the new rules, and there are plenty of arguments and counter-arguments to be addressed. First, the definitions of “aspheric” and “mostly clear orbit” are not entirely clear.
But supporters of the rules dispute that all specific planets, from mercury to Neptuneare exceptionally round, while the vast majority of non-planets are not. As for orbital clearance, Mars 5,000 times larger than the second largest object in its orbit. In contrast, Pluto only has about 7% of the total mass in its orbit. So there is a clear and wide contradiction between these two worlds.
But the biggest confusion comes with the non-planetary subspecies. If an object is large enough to make itself round but not clearly visible in its orbit, such as Pluto or Ceres, it is called a Dwarf planet. Astronomers often use the term “dwarf” to refer to smaller versions of the same type, such as dwarf stars and dwarf galaxies. But dwarf stars are still stars, and dwarf galaxies are still galaxies; They’re just smaller. This is not the case for dwarf planets, as they are definitely not planets.
If an object is too small to be round, it is classified as a “small solar system body.” Asteroids are not officially defined as such, but astronomers generally take the word to mean small solar system bodies living in the inner solar system. Comets They are also considered small bodies in the solar system.
Then there are the “minor planets,” which include dwarf planets and all small bodies in the solar system that are not comets. And don’t forget the dwarf planets outside the orbit of Pluto, and the trans-Neptunian objects, which include minor planets and other small solar system bodies that orbit beyond Neptune.
It’s a bit of a mess, but it’s an attempt to match the complex nature of our unfolding knowledge of the solar system. We live in an active, dynamic system, where many types of organisms interact and interact. We struggle to come up with clear definitions because the universe itself refuses to be neatly ordered.
For now, perhaps we should stick with the definitions we have, even if they are imperfect. As we make new discoveries and gain new understanding, we can update our definitions accordingly and, hopefully, simplify things a little.
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