Why Pluto is no longer a planet: the rise and fall of a Kuiper Belt Object

In the past two centuries, Pluto has gone from an orbital anomaly to a planet to the gatekeeper of the Kuiper Belt.

Once the 9th planet in the Solar System, Pluto is now classified as a Kuiper Belt Object. Credit: NASA/JHUAPL/SwRI

Pluto’s journey from its discovery as the 9th planet in our Solar System to its reclassification as a dwarf planet and Kuiper Belt Object is a fascinating tale. What started off as a curious orbital anomaly in the 18th century turned into the discovery of an object that forced us to rethink our ideas of what a planet should be, and ultimately how our Solar System began.


By the mid 17th Century, a scientific revolution was in full flow. Kepler’s laws of planetary motion, combined with Newton’s law of gravity, provided astronomers with a powerful tool.

The motions of comets and planets could be determined with absolute certainty. The prevailing view was of a Solar System that ran like clockwork.

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By 1821 a curious anomaly had arisen: the recently discovered planet Uranus was not behaving as it should. French astronomer Alexis Bouvard published tables of Uranus’s position, but found that the planet frequently strayed from its predicted path.

He realised these discrepancies implied the presence of a large unseen body, tugging Uranus off course.

Both John Couch Adams and Urbain Le Verrier predicted where this new planet might be. But it was Johann Galle and Heinrich d’Arrest who claimed victory, and the discovery of Neptune on the night of 23 September 1846 went to them.

Mathematicians set to work finalising the details of Neptune’s orbit, but they found the discrepancies in Uranus’s path around the Sun didn’t completely vanish as expected.

Something was still causing Uranus to wander. The problem was of keen interest to Percival Lowell, who had established a large observatory in the frontier town of Flagstaff, Arizona, in 1894.

American astronomer Percival Lowell was one of the prominent figures arguing for a planet beyond Neptune that was affecting the orbit of Uranus. Credit: Mondadori via Getty Images
American astronomer Percival Lowell was a prominent figure arguing for a planet beyond Neptune that could explain the odd orbit of Uranus. Credit: Mondadori via Getty Images

Lowell concluded that Uranus and Neptune were being drawn off course by another more distant planet, which he named ‘Planet X’.

In 1906 he began searching for it, but after a decade Planet X had eluded him and he died in 1916. The new director of the observatory, Vesto Slipher, continued the search.

In 1929, the task of searching for Planet X was given to a recent addition to the Lowell Observatory: Clyde Tombaugh. The work was hard – at night careful photographs were taken of the sky where Lowell had predicted the new planet should be.

During the day, the plates were analysed using a ‘blink comparator’ – a device that switched back and forth between two photographs taken at different times. Looking through the comparator, stars remained fixed in position but a planet moving in its orbit would appear in a different location in each plate.

On 18 February 1930, after months of searching, Tombaugh made a discovery: he noticed a tiny dot moving among the background stars of Gemini. The long search was over – Planet X had been found at last.

Clyde Tombaugh pictured at a Zeiss Blink Comparator, a machine used in the discovery of Pluto. Credit: Lowell Observatory Archives
Clyde Tombaugh pictured at a Zeiss Blink Comparator, a machine used in the discovery of Pluto. Credit: Lowell Observatory Archives

Planet X was named Pluto after the classical god of the underworld, but almost immediately there were doubts as to whether Pluto could really be Planet X. It was very faint, and its disc remained unresolved even in the world’s largest telescopes.

Pluto was obviously small, perhaps only the size of Earth or even Mars. It seemed unlikely it could have much influence over Uranus and Neptune. Observing the planet was extremely difficult, but a handful of dedicated astronomers continued the work.

Gradually, larger telescopes were constructed and observations showed that Pluto was probably covered with methane-ice. In 1978, astronomer James Christy discovered Charon, Pluto’s largest moon.

Charon enabled astronomers to better estimate the mass of Pluto, which was much lower than expected – smaller even than Earth’s own Moon.

Astronomer James Christy pictured with the photographic plate on which he and Robert Harrington discovered Pluto’s largest moon Charon, in 1978. Credit: U.S. Naval Observatory
Astronomer James Christy pictured with the photographic plate on which he and Robert Harrington discovered Pluto’s largest moon Charon, in 1978. Credit: U.S. Naval Observatory

The small size, combined with its unusual orbit, began to cast serious doubt as to whether Pluto could really be a planet at all.

The 1990s saw the arrival of CCD cameras, which were much more sensitive to light than photographic film. They allowed astronomers to see fainter objects and peer deeper into the Solar System. Using CCDs it became apparent that there were many Pluto-sized bodies out beyond the orbit of Neptune.

These objects belong to the Kuiper Belt, a vast field of debris left over from the formation of the Solar System, and Pluto was its largest member. Kuiper Belt objects are like fossils: their ancient surfaces are like postcards from the past.

From 2002 a host of large bodies was found: Quaoar, Sedna, Makemake and Eris.

Loud astronomical voices began to debate Pluto’s status. It was argued that if Pluto was a planet, were these other objects planets also? In 2006, the International Astronomical Union (IAU) settled the matter and defined what was required for an object to be classed as a planet.

One of the conditions is that all material must be cleared from the object’s orbit, something Pluto hasn’t done due to its small size. Pluto received a new classification: it was a dwarf planet, but this decision is not without its critics. Pluto may have been demoted, but it plays a vital role in understanding the conditions of the early Solar System.

IAU members cast their votes at the 2006 General Assembly. Credit: Credit: IAU/Robert Hurt (SSC)
IAU members cast their votes at the 2006 General Assembly, and Pluto’s fate is sealed. Credit: IAU/Robert Hurt (SSC)

Should Pluto be reinstated as a planet?


The official act that took planetary classification from Pluto was IAU Resolution 5A, ‘Definition of a Planet in the Solar System’, on 24 August 2006.

This document states that for an object to be considered a planet must satisfy three conditions.

“A planet is a celestial body that:

  1. is in orbit around the Sun
  2. has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape
  3. has cleared the neighbourhood around its orbit.”

Since Pluto is not massive enough to clear its orbit of debris, it fails to meet the third condition.

There’s also the problem that if Pluto were to be classed as a planet then so too would many other objects in the Kuiper Belt, and the planet count in the Solar System would rise dramatically.


Some have argued to keep Pluto as a planet for sentimental reasons: for much of its history, it has been thought of as a planet and therefore should continue to be so.

Some astronomers point out that there are inconsistencies in the wording of Resolution 5A – while it is true that Pluto hasn’t cleared its orbit, neither has Earth or Jupiter. Earth orbits with 10,000 near-Earth asteroids, while 100,000 Trojan asteroids lie within Jupiter’s orbit.

One could argue that both Jupiter and Earth also fail the IAU definition.

Those who hold this view say that a better definition of what a planet is might be to say that any object orbiting the Sun that has a surface area greater than 1,000km can be called a planet.

If this definition were to be adopted both Pluto and Eris would be classed as planets.


Dr Paul Abel is an astronomer based at the University of Leicester. This article originally appeared in the August 2015 issue of BBC Sky at Night Magazine.