Thread by @bsgaudi

In my Astronomy 2291 class recently we talked about Pluto and why it was demoted to a ‘dwarf planet’ by Resolution 5A of the 26th General Assembly of the International Astronomical Union on August 24, 2006. Hold on, here we go… (1/20)
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Pluto was discovered on February 18, 1930 by Clyde Tombaugh (in his early 20s at the time) working at Lowell Observatory. The discovery story of Pluto is fascinating, but a topic for another thread. (2/20)

Pluto was immediately classified as a planet, largely because it was thought to be the cause of the (now known to be spurious) observed deviations of Neptune from its expected orbit. (3/20)

The original estimates of Pluto’s mass were somewhere in the neighborhood of 10 times the mass of the Earth; we now know that Pluto’s mass is only 0.00218 times the mass of the Earth. (4/20)

But: Pluto remained the only known denizen of the solar system with an orbit larger than Neptune for over 60 years, until the discovery of 15760 Albion in 1992 by Dave Jewitt and Jane Luu. (5/20)

15760 Albion was never seriously considered a planet – it was far too faint (and thus small) to be a bona fide planet. (6/20)

But what the discovery of 15760 Albion did herald was a new age of discovery in the solar system. With the advent of large format, sensitive optical detectors, it became possible for astronomers to survey large areas of the sky to look for distant solar system objects. (7/20)

And they did. And they found them in droves. We know now of nearly 4000 “Trans-Neptunian Objects (TNOs)” – objects with orbits larger than that of Neptune. (8/20)

But the real plot twist happened late on the Friday afternoon of July 29, 2005, when Mike Brown @plutokiller, Chad Trujillo @chad222, and David Rabinowitz announced the discovery of 136199 Eris. (9/20)

At the time, Eris was thought to be larger than Pluto (it even had its own moon!), and thus should, by all rights, be a planet. (10/20)

Chaos ensued, for many reasons. The discovery of Eris has its own fascinating backstory, again left for another thread. (11/20)

If Eris is larger than Pluto, then it should be a planet, right? Well, then, if Pluto is a planet, then why not the other bodies that we have discovered that are only slightly smaller than Pluto? Why shouldn’t Ceres (the largest asteroid) be a planet as well? (12/20)

Where do we stop? How many planets should we have? (13/20)

It turns out that we have already faced this question. Ceres was discovered on 1 January 1801 by Giuseppe Piazzi at Palermo Astronomical Observatory in Sicily. It was originally labelled as a planet. (14/20)

Well, not long after, on March 28, 1802, Pallas was discovered, and was labelled a planet. Then Juno was discovered (another planet), then Vesta (you get the idea), Asteraea, Hebe, Iris, Iris, Flora, Metis, Hygiea, Parthenope, Victoria, Egeria, Irene, Eunomia …. (15/20)

All planets! When will it stop? Well, after Eunomia (I feel sorry for Eunomia, it was a planet for less than a year!), it was realized that this was all just silly. Ceres, et al., were just the largest members of its class of “asteroids”. (16/20)

Here’s the deal: the same exact story played out with Pluto, Eris and the TNO belt, just over a much longer span of time. (17/20)

The gap between the discovery of Pluto and the discovery of Eris and the thousands of members of the TNO belt was over sixty years, by which time Pluto had cemented itself in our collective brain as a planet, not to mention a fixture in our culture. (18/20)

And thus, the discovery of Eris led to the following showdown with the International Astronomical Union meeting on August 24, 2006. They concluded that Pluto, Eric, Ceres, et al. should all be Dwarf Planets (19/20)

So, that’s the story. What do ya’ll think? Should Pluto have been demoted to a Dwarf Planet, or not? (20/20)