If you grew up in the 80's or earlier, and had to learn the mnemonic "my very educated mother just showed us nine planets", you were probably disconcerted to hear that in 2006, Pluto got demoted from planet status.
When Pluto was first discovered in 1930, it was thought to be larger than Mercury (the smallest planet in our solar system). In 1978, however, the discovery of Pluto's moon Charon enabled astronomers to accurately calculate Pluto's mass for the first time. They found Pluto's mass to be less than one twenty fifth that of Mercury's making it even smaller than the earth's moon.
Then, in the 1990's, astronomers began finding objects known as Kuiper Belt objects many of which shared Pluto's key orbital characteristics and began to be called Plutinos. At this point, Pluto came to be seen as the largest member of a new class of celestial bodies.
Starting in 2000, with the discovery of at Quaoar, Sedna and Eris, all comparable to Pluto in terms of size and orbit, it became clear that either all three would have to be named as planets, or Pluto would have to be reclassified. Astronomers were concerned about the large number of celestial objects that would be discovered in the future increasing the number of "planets" in our planetary system.
Therefore, in 2006, the International Astronomical Union (IAU) defined what it meant to be a "planet" in the solar system. Per the IAU, a planet is a celestial body that:
The reason Pluto doesn't satisfy criteria # 3 is because it shares its orbit with the plutinos.
I sincerely hope this helps answer any questions you had as to why the mnemonic you had to learn growing up now has changed to "My Very Eager Mother Just Served Us Nachos".
What mnemonics did you learn growing up?
When Pluto was first discovered in 1930, it was thought to be larger than Mercury (the smallest planet in our solar system). In 1978, however, the discovery of Pluto's moon Charon enabled astronomers to accurately calculate Pluto's mass for the first time. They found Pluto's mass to be less than one twenty fifth that of Mercury's making it even smaller than the earth's moon.
Then, in the 1990's, astronomers began finding objects known as Kuiper Belt objects many of which shared Pluto's key orbital characteristics and began to be called Plutinos. At this point, Pluto came to be seen as the largest member of a new class of celestial bodies.
Starting in 2000, with the discovery of at Quaoar, Sedna and Eris, all comparable to Pluto in terms of size and orbit, it became clear that either all three would have to be named as planets, or Pluto would have to be reclassified. Astronomers were concerned about the large number of celestial objects that would be discovered in the future increasing the number of "planets" in our planetary system.
Therefore, in 2006, the International Astronomical Union (IAU) defined what it meant to be a "planet" in the solar system. Per the IAU, a planet is a celestial body that:
- is in orbit around the sun,
- has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
- has "cleared the neighbourhood" around its orbit. (is gravitationally dominant, and has no other objects other than its own satellites in its orbital zone)
The reason Pluto doesn't satisfy criteria # 3 is because it shares its orbit with the plutinos.
I sincerely hope this helps answer any questions you had as to why the mnemonic you had to learn growing up now has changed to "My Very Eager Mother Just Served Us Nachos".
What mnemonics did you learn growing up?
Never*Eat*Sour*Weenies
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