Today in History: February 18



In 1930

Tombaugh discovers Pluto.

Once a planet,

Now a pseudo.*


*"According to the IAU, a celestial body is a planet if it meets three key criteria: It must orbit the sun; it must be large enough that it pulls itself into a round shape by its own gravity; and it must clear its orbit of other objects — meaning that it casts out other bodies in its orbit and is gravitationally dominant in its region of space.It’s the third criterion that the IAU used to ding Pluto. While the other planets have their orbits pretty much to themselves — except for their moons — Pluto is one of thousands of celestial objects in the Kuiper Belt, the region of space beyond the orbit of Neptune. With so many other objects in the region exerting gravitational fields of their own, Pluto can’t be considered gravitationally dominant."   []

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  • Thanks for another fun fact. I remember seeing that the Adler Planetarium didn't have to change its list of the planets on one wall, since it was carved there before this date in 1930. Pluto wasn't there to be taken off!

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