Online Astronomy eText: The Planets
The Discovery of Pluto Link for sharing this page on Facebook
(also see Pictures of Pluto / The Rotation of Pluto / The Satellites (Moons) of Pluto)
     The controversial decision by the International Astronomical Union to strip Pluto of its status as a major planet was a surprise to many, but seemed long overdue to others. The purpose of this page is to discuss how Pluto was discovered, why it was given status as a major planet, and why it may or may not be worthy of that status. However, for now it is primarily a place-holder.


The redefined Solar System. Major planets above, "dwarf planets" below. (IAU, apod060828)

The Search for Planet X
     (original very brief post moved to the end of The Discovery of Neptune and Uranus)


Discovery photos of Pluto (Jan 23, left, and Jan 29, right, 1930) (Lowell Observatory Archives; used by permission)
The arrows point to the position of Pluto in each image.

Alternating between the discovery images, as if in a blink comparator, makes it easier to see the change in position of the previously unknown planet.