Online Astronomy eText: The Planets
The Discovery of Pluto
(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.

Very Brief Arguments For and Against Pluto Being a Planet
(added March 14, 2022 as a result of a question from a reader)
 Right from the start, the fact that Pluto is so faint meant that it could not be the planet being looked for (that is, something comparable to Uranus and Neptune); but the excitement caused by its discovery led to a not entirely universally accepted decision to call it a planet, based on the idea that it might be as large and massive as the Earth, and simply very dark. However, over the decades following its discovery it became obvious that it had to be much smaller than the Earth. By the time I started my career (in 1970), textbooks listed it as being as massive as the Earth but about the size of Mars, which was ridiculous, because that would have made it denser than any known material, and using the assumption that like other small bodies in the outer Solar System it must be made of rocks and ices, I taught my students that it was probably no more than 1/5 the diameter of the Earth, and 1% of the Earth's mass (which would have made it smaller than our Moon, and proved remarkably prescient, as shown by the version of the Planetary Data Table that I was passing out nearly 30 years later).
 The belated realization by astronomers in general, and Neil deGrasse Tyson in particular, that Pluto was much smaller than the other planets led to arguments that it should no longer be called a planet. (Tyson's argument was partially based on the fact that while the director of the former Hayden Planetarium in New York, he helped obtain a large grant that led to renaming the facility the Rose Center for Earth and Space, and among the changes made to the Center was a very large scale model of the Solar System, which if Pluto were considered a planet, would have been twice as big as the building or else only half the proposed scale, making all the smaller planets too small to see; so he had a personal interest in demoting Pluto, as well as an astronomical one.)
 Even without Tyson's very vocal involvement in trying to delist Pluto as a planet, the discovery of other large bodies far out in the Solar System (in the Kuiper Belt), and in particular Eris, which though much too far out to be Planet X, turned out to be comparable in size to Pluto (and for a while was thought to be considerably larger than Pluto), several other astronomers (among them, the discoverer of Eris) argued that Pluto should be dropped from the list of planets. The end result of several years' discussion was that at a meeting of the IAU attended by about 2500 astronomers, about 250 of them deigned to attend a meeting about Pluto's status (most astronomers have little or no interest in anything closer than other stars and galaxies), and a slight majority of those present voted to call Pluto, Eris, and Ceres "dwarf planets", and only the eight planets known prior to the discovery of Pluto "major planets" (although if Mercury hadn't been observed since ancient times, it might have been demoted as well, leaving only seven "major planets").
 However, a few years before Pluto was demoted to "dwarf" status the New Horizons spacecraft was launched, and more than a decade later it successfully sent back images and data (see Pictures of Pluto) that showed that Pluto was a far more interesting and geologically active object than most of the "major" planets. As a result, some members of the astronomical community and most members of the public want Pluto to be reinstated as the ninth planet; but whether and when that might happen is something that only the future will reveal.