Pluto Wasn’t the First: A Brief History of the Forgotten Planets of Our Solar System

A kindergartener in 2005 and a kindergartener in 2006 would have learned very different facts about the number of planets in the solar system. 2006, of course, was the year Pluto was reclassified as a dwarf planet – a move that sparked outrage from a public that tends to romanticize our solar system.

But long before the “controversy” of Pluto, other objects moved in and out of the official list of planets in the solar system. Indeed, a kindergarten child in the early 1800s would have learned that Ceres was a planet.

So while the planet argument may seem like a modern astronomical debate, astronomers in the 19th century were plagued with this question of how to define what actually counts as a planet.

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And, as mentioned, Ceres predates Pluto in its controversy. The asteroid belt, which lies roughly between Mars and Jupiter, is filled with planets and minor asteroids. One of these celestial bodies, Ceres, has a surface covered with minerals like clay and carbonates, as well as water ice. It’s a strange world, of course: because it’s not completely frozen and is covered in salt water, scientists think Ceres could harbor microbial life. This puts Ceres in stark contrast to Pluto, which sits on the other side of the solar system and has an entirely frozen surface. Additionally, while Ceres is a dull monochromatic gray, Pluto’s colors range from white and black to bright orange.

Yet Ceres and Pluto have one very important thing in common: astronomers once thought they should be classified as planets, but then changed their minds. It all comes down to size, which in the case of planetary science really matters.

Flashback to the early 19th century. An Italian priest and astronomer named Giuseppe Piazzi at the Palermo Observatory had answered a nearly three-decade-old question: why did the orbits of Mars and Jupiter indicate that a planet existed between them when neither could have been found? On January 1, 1801, Piazzi seemed to answer this question by announcing that he had found a “star” that had moved from its position in the constellation Taurus. Scientists quickly concluded that it must be the missing planet and assumed the problem was solved.


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Then another “planet” was discovered. On March 28, 1802, the German physician and astronomer Heinrich Olbers discovered Pallas; this was soon followed by Juno in 1804 and Vesta in 1807. Each was duly designated as a planet, although astronomers began to doubt that this increasingly cumbersome system worked. Although scientists had a break for a few decades, a plethora of new discoveries between 1845 and 1852 left the astronomical community with 15 asteroids to consider. None of the news was labeled as a planet, but it was becoming increasingly clear that reforms would be needed. By 1867, it was clear that Ceres was too small to be grouped with a body like Earth, and so it was given a new designation: minor planet. And instead of being given fancy names and symbols, they would be labeled with numbers based on when they were discovered or their orbital determination.

This brings us to Pluto. While Ceres has a diameter of 588 miles (compared to Earth’s diameter of 7918 miles), Pluto has a comparatively heavier diameter of 1477 miles. Still, that didn’t stop Pluto from getting the ax as a planet when the International Astronomical Union met in 2006. The reason was, quite simply, that astronomers had decided there was three criteria to be considered a planet:

Thus, the IAU’s three criteria for a life-size planet are:

It is in orbit around the Sun.

It has sufficient mass to assume hydrostatic equilibrium (an almost round shape).

It “cleaned up the neighborhood” around its orbit.

Because Pluto failed to meet the third requirement – it did not “clean up the neighborhood” around its orbit – it lost its planetary status. Clearing the neighborhood means that the region of space near which it orbits the sun is devoid of larger bodies, having been absorbed by the planet. Ceres, like Pluto, clearly does not meet this criterion: the asteroid belt in which Ceres resides is evidence of a “stranded” planet that has not cleared its vicinity. Indeed, there are several other relatively massive bodies – Vesta, Pallas and Hygiea – also near Ceres.

Pluto had held this planetary distinction for 76 years, beginning with its discovery in 1930 by American astronomer Clyde W. Tombaugh. The demotion of Pluto into a dwarf planet remains controversial, and not just among lay astronomers. A team of American scientists published an article in December in the scientific journal Icarus arguing that a “planet” should be defined as any geologically active celestial body. A co-author argued that we should say that there are “probably more than 150 planets in our solar system”; the article asserted that the need to distinguish planets from moons is cultural, not scientific, and hinders a proper understanding of astronomy.

“We found that during the 1800s the non-scientific public in the Latin West developed its own popular taxonomy on the planets reflecting the concerns of astrology and theology, and that this popular taxonomy eventually affected scientists,” the scientists explained. They then concluded that “using the geophysical planet concept with subcategories for individual characteristics (including gravitational dominance) makes the planet concept both useful and deeply insightful for communicating with the public”. This did not happen in 2006, they claim, because “because the necessary time was not taken to sort out these issues”, the resulting vote led to “a deeper division in the community”.

Ironically, even as Pluto was retrograde, Ceres almost got a promotion. An earlier 21st century proposal to define a planet would have done so by describing a planet as having enough mass to be nearly round and to orbit a star without being a satellite of a planet or a star itself. If this definition had been accepted, Ceres would have become the fifth planet from the Sun.

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