By Emily Levesque, University of Washington
The status of Pluto as a planet has been very short-lived. It went from discovery to demotion in less than 80 years. Spotted in 1930, Pluto enjoyed mere decades of fame as our ninth planet before being stripped of its status and downgraded to a dwarf planet in 2006. What happened?
Discovery Of Pluto
At the start of 1930, our solar system had eight planets that we knew about. Neptune had been discovered almost a century earlier, in 1846, and astronomers had long since mapped and tracked the orbits of it and the other planets in great detail. Those orbits were the first sign that our picture of the solar system might not be complete.
In January of 1930, Clyde Tombaugh captured several images that revealed a faint moving object. Blinking between them, the object shifted across the plates, showing the telltale motion expected for a ninth planet. Tombaugh had found Pluto.
Speculations and Findings
Astronomers began wondering whether Pluto was indeed a planet or just a really large asteroid at a surprisingly large distance from the Sun.
Tombaugh himself considered the possibility and continued searching beyond Neptune for years, hunting for other Pluto-like objects. Still, after searching for years and finding nothing else like Pluto, he concluded that it was, in fact, a planet.
The research that led to Pluto’s new classification can be traced back to decades of research on the outer planets and bold new ideas for how planetary systems should work. In fact, Gerard Kuiper wrote a paper in which he speculated that Pluto might be a member of a disk of small bodies circling the Sun at distances well beyond the orbits of the other planets. However, he also assumed that Pluto, which astronomers believed was nearly Earth-sized at the time, would have disrupted and scattered these other small objects, leaving no remnant of that disk behind.
This article comes directly from content in the video series Great Heroes and Discoveries of Astronomy. Watch it now, on Wondrium.
Discovery of the Kuiper Belt and its Members
After years of research by Jim Elliot, evidence was starting to mount that, while Pluto may have an atmosphere, its status as a planet looked a bit more complicated.
The discovery of the Kuiper belt, the ring that begins just past Neptune and stretches to more than 90 billion miles away from the Sun, further raised questions about Pluto.
Julio Angel Fernandez Alves suggested a belt of comets and other rocky and icy objects, sitting precisely where the Kuiper belt lies today. Astronomers David Jewitt and Jane Luu hit pay dirt in August of 1992. They were able to confirm what they’d found: the first official member of the Kuiper belt, an object known today as Albion.
Astronomer Mike Brown, in 2005, he came across a surprisingly bright and slow-moving object. He recognized that his survey had found a new Kuiper belt object and that it was enormous. The object, later dubbed Eris, was the largest Kuiper belt object ever found at the time, at least, officially.
Questions Raised on Pluto
Mike Brown’s discovery of Eris in the Kuiper belt put Pluto in an awkward position. Eris was thought to be practically identical in size to Pluto.
Did this mean that Eris was now our tenth planet? What about Albion, and the heaps of other objects in the Kuiper belt? Were some of those planets too? Pluto certainly seemed to be a member of the Kuiper belt. But, what exactly is a planet to begin with?
Astronomers Attempt to Define a Planet
The definition of a planet was the subject of an infamous vote carried out in 2006 at a meeting of the International Astronomical Union. Astronomers realized that a clear definition of what constituted a planet was long overdue, and a number of experts weighed in on proposed criteria for what we might call a planet.
A proposal drafted in part by Julio Fernandez—the astronomer who originally proposed the Kuiper belt—gained traction, and after the intensive debate, a vote was held to approve an official definition for a planet.
Pluto According to the Proposed Definition of a Planet
A planet, astronomers ultimately agreed, had to meet three criteria: It had to be orbiting a star; it had to be massive enough to hold itself together with its own gravity and assume a roughly round shape as a result; and, finally, it had to have cleared its neighborhood, gravitationally dominating its orbit and kicking out any other objects of comparable size that weren’t tied to the planet itself as companions.
Pluto was orbiting the Sun, and it was plenty round but, critically, it had not cleared its orbit; its orbit was in the now-cluttered Kuiper belt.
Pluto’s Demotion and the Impact
Astronomers voted. The vote passed. And Pluto was demoted. According to the newly approved definition, Pluto was now no longer a full-fledged planet but a dwarf planet, joining Eris and other large members of the Kuiper Belt.
The news made international headlines and prompted a flurry of editing as Pluto was removed from lists and textbooks and solar system dioramas in classrooms across the world. Some astronomers objected—and still object today—to the classification, arguing that the definition was arbitrary or not properly considered.
Still, as of now, the solar system stands with eight planets, from Mercury to Neptune, and Pluto is one of a handful of dwarf planets.
Common Questions about the Demotion of Pluto as a Planet
In January of 1930, Clyde Tombaugh captured several images that revealed a faint moving object. Blinking between them, the object shifted across the plates, showing the telltale motion expected for a ninth planet. Tombaugh had found Pluto.
In 2005, astronomer Mike Brown was examining one dataset when he found a new Kuiper belt object and that it was enormous. The object, later dubbed Eris, was the largest Kuiper belt object ever found at the time, at least, officially.
A planet, astronomers agreed, had to meet three criteria. It had to be orbiting a star. It had to be massive enough to hold itself together with its own gravity and assume a roughly round shape as a result. And finally, it had to have cleared its neighborhood, gravitationally dominating its orbit and kicking out any other objects of comparable size that weren’t tied to the planet itself as companions.
