Pluto was not the first: A short story about the forgotten planets of our solar system

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

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

So while the argument about planetarity might seem like a modern astronomical debate, 19th century astronomers were confused by the question of how to define what actually counts as a planet.

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And, as mentioned, Ceres goes before Pluto in his controversy. The asteroid belt, which lies roughly between Mars and Jupiter, is filled with smaller planets and asteroids. One of these celestial bodies, Ceres, has a surface covered with minerals such as clay and carbonates as well as water ice. It’s a strange world, to be sure: because it’s not completely frozen and covered in salt water, scientists believe Ceres may contain microbial life. This place Ceres in stark contrast to Pluto, which is on the other side of the solar system and has a completely frozen surface. Additionally, while Ceres is a matte monochromatic gray, Pluto’s colors range from white and black to vibrant orange.

Yet Ceres and Pluto have one very important thing in common: Astronomers at one time 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 does matter.

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, even though no one could be 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 of Taurus. Scientists quickly concluded that this must be the missing planet and assumed that the matter was resolved.


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Then another “planet” was discovered. On March 28, 1802, the German physician and astronomer Heinrich Olbers discovered Pallas; this was quickly followed by Juno in 1804 and Vesta in 1807. Each was duly designated as a planet, though astronomers began to doubt that this increasingly cumbersome system was functioning. Although scientists were given a respite for a few decades, an abundance of new discoveries between 1845 and 1852 left the astronomical community with 15 asteroids to take into account. None of the new ones were branded as planets, but it became more and more clear that reforms would be needed. In 1867 it became clear that Ceres was too small to be grouped in a body like Earth, and therefore it was given a new name: Minor planet. And instead of being given clever 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 7918 miles in diameter), Pluto has a relatively more powerful diameter of 1477 miles. Yet this did not save Pluto from having the ax as a planet when the International Astronomical Union met in 2006. The reason was simply that astronomers had decided that there were three criteria to be considered a planet:

So the IAU’s three criteria for a full-size planet are:

It is in orbit around the Sun.

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

It has “cleared the neighborhood” around its circuit.

Because Pluto did not meet the third requirement – it has not “cleared the neighborhood” around its orbit – it lost its status as a planet. Clearing the neighborhood means that the area of ​​space where it orbits the sun is deprived of larger bodies that have been absorbed into the planet. Ceres, like Pluto, clearly does not meet this criterion: the asteroid belt in which Ceres resides is evidence of a “failed” planet that did not clear its neighborhood. In fact, there are several other relatively massive bodies – Vesta, Pallas and Hygiea – also in the vicinity of Ceres.

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

“We found that during the 19th century, the non-scientific public in the Latin West developed its own folk taxonomy on planets that reflected the concerns of astrology and theology, and that this folk taxonomy eventually influenced scientists,” the researchers explained. They later concluded that “using the geophysical planetary concept with subcategories of the individual traits (including gravitational dominance) makes the planetary concept both useful and deeply insightful for communicating with the public.” This did not happen in 2006, they argue, because “because not enough time was taken to sort out these issues,” with the resulting vote leading to “a deeper division in society.”

Ironically, even when Pluto was demoted, Ceres almost received 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 almost round and to orbit a star without being a planet for 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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