The Planet Number Dance

The definition of a planet has evolved over the course of history, from the divine wandering stars of antiquity, to the modern scientific understanding of our solar system and the discovery of hundreds of other planets orbiting distant stars. As our knowledge of what a planet is has expanded, the attempt to define it has generated an ongoing conversation that has yet to reach consensus.

Here we take a look at how the number of planets in our solar system has changed throughout history.

The Planets in Antiquity

In ancient times, anything that moved across the sky in relation to the fixed stars was called a planet. The term planet derives from the ancient Greek word for wanderer. The Sun, Moon, and the five naked-eye wanderers—Mercury, Venus, Mars, Jupiter, and Saturn—were all considered planets.

In antiquity, our solar system had 7 planets: Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn.

The Renaissance Planets

In 1543, Polish astronomer, mathematician, and church cleric Nicolaus Copernicus (1473-1543) published his book De revolutionibus orbium coelestium (On the Revolutions of the Celestial Spheres), where he proposed a heliocentric model in which the Earth and the other planets revolve uniformly around the Sun in circular orbits. This was a different view from the generally accepted geocentric model of the time, which placed the Earth at the center.

The heliocentric model was later further expanded upon by German astronomer Johannes Kepler (1571 -1630) to include elliptical orbits, and corroborated by Italian astronomer Galileo Galilei (1564-1642) who made observations using a telescope.

As a result of the shift from a geocentric to a heliocentric view of our solar system, the Earth was included in the list of planets, whereas the Sun and Moon were now excluded. So, from 1543 to 1781 our solar system had 6 planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn.

First Planet Discovered with a Telescope

Uranus is visible to the naked eye but it was not recognized as a planet by ancient astronomers because it moved slowly across the sky and was just at the threshold of visibility. It was mistaken as a star until it was observed with a telescope by German-born British astronomer Sir William Herschel (1738-1822) on March 13, 1781 and shortly after correctly identified as a planet.

The discovery of Uranus greatly expanded the boundaries of our solar system and added a new planet. By the late 18th century our solar system had 7 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, and Uranus.

New Planets In the Asteroid Belt

In 1772, German astronomer Johann Elert Bode (1747-1826) predicted the existence of another planet between the orbits of Mars and Jupiter.

Bode made his prediction by noticing that the orbits of the planets around the Sun are not equally spaced, but they do form a pattern. Noticing this pattern, Bode and German astronomer Johann Daniel Titius (1729-1796) developed a rule that predicted the spacing of the planets in our solar system. The spacing is roughly doubled from planet to planet.

The pattern predicted that the missing planet ought to have an orbit with an average distance from the Sun of about 2.8 astronomical units (AU).

On January 1, 1801, Italian astronomer and catholic priest Giuseppe Piazzi (1746-1826) discovered Ceres while looking for a planet between the orbits of Mars and Jupiter. Ceres was quickly categorized as a planet, in part, because it was originally believed to be larger than the Earth.

Shortly after, other objects including Pallas, Juno, and Vesta, were discovered in the neighborhood of Ceres and were categorized as planets.

By 1845, our solar system had 11 planets: Mercury, Venus, Earth, Mars, Vesta, Juno, Ceres, Pallas, Jupiter, Saturn, and Uranus.

The Discovery of Neptune

As more objects were discovered in the region between Mars and Jupiter, it became clear by the mid-19th century that these objects were much smaller and behaved much differently than the other planets. As a result, these objects, including Ceres, Pallas, Juno, and Vesta, were reclassified as asteroids, a term originally suggested by Herschel.

Around the same time, unexpected changes in the orbit of Uranus led French astronomer Alexis Bouvard (1767-1843) to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently discovered and observed with a telescope on September 23, 1846 by German astronomer Johann Galle (1812-1910) within a degree of the position predicted by French mathematician Urbain Le Verrier (1811-1877). A triumph for our understanding of celestial mechanics, Le Verrier predicted the existence and position of Neptune using only mathematics.

Because the size difference between the asteroids and planets was so significant, and the likelihood of finding another large planet was so remote, there was no formal need to come up with a definition for a planet beyond “a large body that orbits the Sun.”

By 1930, our solar system had shrunk to 8 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The Discovery of Pluto

On February 18, 1930, Pluto was discovered by American astronomer Clyde Tombaugh (1906–1987). As with Ceres, Pluto was initially believed to be larger than the Earth and was quickly accepted as a planet.

Until early 2006, our solar system had 9 planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

21st Century Planets

Late twentieth-century science revealed that Pluto was rather unlike the other eight large objects orbiting the Sun. Pluto is much smaller than Mercury, and only two-thirds the size of the Moon. Its orbit is tilted and eccentric, crossing Neptune's. No other planet acted like this.

At the beginning of the 21st century, astronomers found other objects orbiting the Sun in the outer solar system, with qualities very much like Pluto's. They were given names like Sedna, Quaoar, Ixion, Varuna, Makemake, and Haumea. Many were close (but not quite equal) to Pluto in size. All of them had tilted, eccentric orbits; quite a few of those orbits crossed Neptune’s.

A change came in 2005. California Institute of Technology (CalTech) astronomer Mike Brown, along with Chad Trujillo and David Rabinowitz, discovered an object believed to be 27% larger than Pluto. Brown first named it Xena (after the protagonist of the eponymous TV show, with a sneaky Planet X reference). While the name didn't stick, the object was later officially—and aptly—christened Eris by the International Astronomical Union (IAU), after the Greek goddess of chaos and discord.

It seemed quite clear that if Pluto was our solar system's 9th planet, then Eris should be its 10th. And if Eris and Pluto were planets, why not also Makemake and Haumea? Why shouldn't the solar system have fifteen planets? Or forty? An equally adamant camp insisted that none of these objects deserved to be called a planet, including Pluto. They posited that our solar system contains only eight objects worthy of planet status, ending at Neptune.

The Reclassification of Pluto

Recognizing the problem, the IAU produced a revised definition of planet in 2006. The number of planets in our solar system was reduced to 8: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. They also created a new class called dwarf planets, though they are not really planets at all, which included Pluto, Ceres, Eris, Haumea, and Makemake.

The IAU’s definition of a planet and the subsequent reclassification of Pluto from planet to dwarf planet status has generated an unprecedented level of public interest and discord. Even among the scientific community, there is a general agreement that the current definition of a planet is poor and as a result there is still no consensus on what defines a planet.