Pluto, once believed to be the ninth planet, was discovered on today's date, February 18 in 1930 by astronomer Clyde W. Tombaugh working at the Lowell Observatory at Flagstaff, Arizona. Now I will readily admit that in this subject of the study of planetary bodies I am taking on a subject about which I know almost nothing. But this seemed like an important subject. So please forgive in advance for all of the mistakes I may end up making!
Planet out There?
First of a all planets are way too faraway to be discovered by simply going out and just taking a picture of it like the one above of Pluto and saying "there it is." In this case of Pluto, just as in the case of the many planets which have been discovered since way back in the 1930's a
after the Roman god of the underworld, the equivalent of Hades in Greek mythology. The discovery was confirmed by other astronomers and announced in March of that year.
But Why Was Pluto "Demoted" From Planetary Status?
Pluto exists within the Kuiper belt, a ring of bodies beyond the orbit of Neptune. And there had been debate for some time on whether it should have been declassified to just a minor dwarf planet within the Kuiper belt. The discussion came to a conclusion within the International Astronomical Union with an official definition of a "Planet" quoted directly from Wikipedia:
(Above: Pluto as photographed from the Hubble Space Telescope in 1998.)
1.) The object must be in orbit around the Sun.
2.) The object must be massive enough to be rounded by its own gravity. More specifically, its own gravity should pull it into a shape defined by hydrostatic equilibrium.
3.) It must have cleared the neighborhood around its orbit.
Again quoting directly from Wikipedia which is far beyond my level of understanding:
"Pluto fails to meet the third condition. Its mass is substantially less than the combined mass of the other objects in its orbit: 0.07 times, in contrast to Earth, which is 1.7 million times the remaining mass in its orbit (excluding the moon).
The IAU further decided that bodies that, like Pluto, meet criteria 1 and 2, but do not meet criterion 3 would be called dwarf planets."
There was considerable disagreement within the scientific community about this announcement by the IAU. The portion which said Pluto would no longer be considered a planet, due to new rules that said planets must “clear the neighborhood around its orbit.” Since Pluto’s oblong orbit overlaps that of Neptune, it was disqualified. Alan Stern, principal investigator with NASA's New Horizons mission to Pluto dismissed the reclassification out of hand: ""the definition stinks, for technical reasons". Stern has said that under IAU's new rules, Earth, Mars, Jupiter, and Neptune would not qualify as planets because they all share their orbits with asteroids. Stern also stated that fewer than 5% voted for it and thus the kick Pluto out of the planets decision was not representative of the many of the astronomical scholars. Again as I said at the start this is way outside my field, so if I've made some glaring errors please do feel free to write in on the response box below, and as long as you don't get TOO nasty I will publish it right here.
Sources =
https://en.wikipedia.org/wiki/Pluto
https://www.history.com/this-day-in-history/pluto-discovered