Happy 90th Birthday Pluto! Hope you’re having a lovely time in the Kuiper belt! ✨
It was 90 years ago on this date that the world learned there was a ninth major planet in our solar system, discovered by Clyde Tombaugh, a 23-year-old researcher at Lowell Observatory in Flagstaff, Arizona. But in 2006, following the recent discovery of many more Pluto-sized bodies orbiting in the same region of our solar system, the International Astronomical Union (IAU) decided to reconsider the way “Major Planets” are defined, and Pluto was reclassified to now belong to a new designation, the “Dwarf Planets”.
Note that a similar reassessment was made back in the 1850s following the discovery of Ceres in 1801, which was the first-known ninth planet to be discovered in our solar system. But within a few years, many other planets had been subsequently discovered all orbiting in the same region between Mars and Jupiter. And so it was decided that Ceres and all these other planets would be given a new designation: they are now all called “Asteroids” or Minor Planets. — Today Ceres also meets the criteria and therefore is also classified as being a dwarf planet for the same reason that Pluto is. Ceres is the first dominant object to be discovered orbiting in the Asteroid Belt; and Pluto is the first dominant object to be discovered orbiting in the Kuiper Belt.
To learn more about Pluto, have a look at the following: https://www.sciencemag.org/news/2017/02/birthday-pluto-s-discovery-science-takes-look-back-dwarf-planet-s-long-strange-history
And to learn more about Ceres, have a look at: https://solarsystem.nasa.gov/planets/dwarf-planets/ceres/overview/
-DDO Defenders 🙂