The History of Planets in our Solar System

A brief history of the 30 or so objects in our solar system that have been considered planets at some point.

While the planets themselves may seem constant and timeless, the way we define the term “planet” has changed drastically over the centuries.

Here’s my timeline of all the objects considered planets in our solar system, followed by a quick description of each time-period.

The Classical Planets

The word ‘planet’ comes from the Ancient Greek term “planḗtēs”, which means “wanderer”. This term was used to describe 7 celestial bodies that moved among the many fixed stars in the sky. These 7 classical planets include the 5 planets that can be seen with the naked eye (Mercury, Venus, Mars, Jupiter, and Saturn), as well as the Sun and the Moon, which also appear to wander in our sky. It was believed that these 7 planets revolved around the Earth, which was considered the centre of the Universe. The Earth itself was not considered a planet at all. This geocentric view of the solar system dominated astronomy until the 1500s.

The Copernican Revolution

While the idea of the sun being the centre of the universe had been suggested by some ancient astronomers, it was never widely accepted, and was actively repressed by the catholic church. This changed in 1543, when the German-Polish astronomer Nicolaus Copernicus published his book “On the Revolutions of the Heavenly Spheres“. A well respected astronomer, he put off publishing his book until he was on his deathbed. He was reluctant to publish sooner as his book presented the heretical idea that the 6 known planets orbited the Sun, with only the Moon orbiting the Earth. He included the Earth among the planets. This heliocentric system was directly in opposition to the view pushed by the catholic church. This idea gradually spread among European astronomers and scientists.

New rule: The Sun is not a planet. Planets orbit the Sun, not the Earth.

The Age of Satellite Planets

The Galilean Planets

In 1610 the Italian Galileo Galilei became the first telescope astronomer, and upon turning his telescope to Jupiter he discovered 4 objects orbiting Jupiter, now called the Galilean Moons: Io, Europa, Ganymede, and Callisto. He described these objects as “four planets flying around the star of Jupiter at unequal intervals and periods with wonderful swiftness”. This created a precedent of referring to all objects in the solar system as planets. Galileo used this discovery, and his other telescope discoveries, as evidence for the heliocentric theory. He was imprisoned for most of his life for his beliefs, but the idea of heliocentrism became increasingly popular across Europe.

New Planets Around Saturn

Galileo’s discovery was followed by the Dutch astronomer Christiaan Huygens, who in 1655 used a telescope to discover Saturn’s moon Titan. Like the Galilean moons of Jupiter, Titan is a planet-sized moon (even larger than Mercury). Huygens variously called it a “planeta” (planet), “stella” (star), “luna” (moon), and “satellite”. This last comes from a now outdated term meaning “attendant”, reflecting the idea that these objects are part of the retinue of a planet, rather than true planets in their own right.

Next, the Italian astronomer Giovanni Cassini turned his own telescope to the sky, and from 1671-1681 he discovered 4 more objects in orbit around Saturn: Iapetus, Rhea, Tethys, and Dione. He referred to them as Nouvelles Planetes autour de Saturne, or “New planets around Saturn”. With this growing number of new planets, astronomers began to use terms like “satellite planet” or “secondary planet” to distinguish them from the “primary planets” that directly orbit the Sun.

Uranus

In 1781, the English astronomer William Herschel, making observations with his telescope in his garden in Bath, recorded a fuzzy object that he originally believed to be a comet or nebula. Upon subsequent observations he realised the object was orbiting the Sun in a circular orbit, and suggested it may be a planet. This caused a huge stir in the astronomical community, and this new world was soon confirmed as the 7th primary planet, or 17th planet in total. He named this planet Georgium Sidus (“Georges Star“), after his king. This patriotic name was widely disregarded outside Britain, and the German astronomer Johann Bode suggested “Uranus” instead, after the Greek god of the sky. There is a nice pattern to this, as Uranus was the father of Cronos (whose Roman counterpart is Saturn), who in turn is the father of Zeus (Jupiter), who is the father of Ares (Mars).

Planets around Uranus

Six years later Herschel discovered two moons orbiting Uranus, now named Oberon and Titania. He called them “satellites” and “secondary planets”. This is one of the last times a major astronomer referred to satellites as “planets”. With the discovery of a real, “primary planet”, and with so many “secondary planets” being discovered, astronomers began using the term “planets” only for objects that directly orbit the Sun. By 1800, objects orbiting planets were now called “satellites” or simply “moons”.

New rule: Planets must directly orbit the Sun.

The Age of Asteroid Planets

After the satellite planets were firmly reclassified as moons, there was a brief time where the solar system had a measly 7 planets. However, with the discovery of Uranus, astronomers were inspired to begin searching for more. Their search focused on the gap between Mars and Jupiter, which was seen as suspiciously wide, and the ideal place for an 8th planet to be lurking.

Ceres

In 1801 this race to find the 8th planet was won by Italian astronomer Giuseppe Piazzi, who found a tiny planet between Mars and Jupiter. He named it Ceres, after the Roman god of the harvest. (*actually he named it “Ceres Ferdinandea”, to include the surname of his patron family, but this didn’t stick). Ceres became by far the smallest planet, being about 6000 times less massive than the Earth.

Pallas, Juno, and Vesta

Over the next decade, 3 more planets were discovered between Mars and Jupiter. They are even smaller than Ceres (Juno is only 3% the mass of Ceres). Herschel suggested these worlds between Mars and Jupiter be given the collective name “asteroids“, meaning “star-like” to reflect their tiny, star-like appearance in telescopes, but they were still considered a type of planet. And so for the next 38 years, the number of planets in the solar system stayed at 11. However, a 3 year period in the 1840s changed things rapidly, beginning in 1845 when a 5th asteroid planet was discovered, named Astraea.

Neptune

Since the discovery of Uranus, astronomers had been frantically searching the skies for a planet beyond it. Irregularities in Uranus’ orbit had been noticed in 1821, and these were used to narrow down the region of space where a planet could be. In 1846, French astronomer and mathematician Urbain Le Verrier managed to calculate the location of this new planet to within 1 degree. He sent a letter to the Berlin Observatory, and German astronomer Johann Galle used that data to quickly locate the new planet. This is the first example of a planetary discovery being a real team effort, and Neptune is often said to have been discovered with mathematics before it was ever spotted through a telescope.

However, there is an interesting asterisk to this: Galle was far from the first astronomer to record the location of Neptune. There are examples of astronomers recording a ‘star’ in the exact position of Neptune going all the way back to Galileo. If Galileo had noticed that Neptune moved slowly across the background stars over the course of weeks, he could have discovered this new planet 2 centuries earlier.

With 5 asteroid planets already discovered, Neptune was viewed not as the 8th planet, but as the 13th.

More and more asteroid planets

Just a year after Neptune was discovered, two more planets, named Hebe and Iris, were found between Mars and Jupiter. This increased the total number of planets to 15. However, soon astronomers made yet more discoveries, and so many stopped including these new findings in the planet lists. By 1851, an additional 15 planets were discovered, and astronomers started numbering them based on the order of its discovery. Ceres became (1) Ceres, Pallas became (2) Pallas, and so on. This solidified the idea that these smaller planets were distinct from the 8 large planets. In 1867, a widely respected astronomical almanac officially classified these smaller objects as “asteroids” or “minor planets,” terms that are still used today. These asteroids turned out to be part of an Asteroid Belt that contains millions of asteroids of various sizes. The number of planets was back down to 8, and it would stay that way for the next 6 decades.

New rule: Planets must orbit the Sun, and be relatively big. Small objects that orbit the sun are called ‘asteroids’ or ‘minor planets’.

Planet 9

Like all good scientists, astronomers are never happy to just sit back and accept that they’re finished: there’s always something left to discover, and science doesn’t wait. So soon after Neptune was discovered, scientists began speculating about another planet even further out. Wobbles in the orbits of Uranus and Neptune could not be explained by the 8 known planets, and so scientists searched frantically for a 9th planet (referred to as “planet X”).
In 1930, 22-year-old American astronomer Clyde Tombaugh used a new technique to hunt for this missing planet. By using a series of photographs of one region of the sky, and quickly transitioning between them, you can create a sort of basic video, allowing you to spot anything that moves across the background stars. Low and behold, Tombaugh noticed a tiny shape that moved between images.

It was an incredibly subtle difference, but it revealed the newest planet in the solar system.

Growing up I had a very old atlas that included images of all the planets, and curiously it showed Pluto as similar in size to the Earth. However, as time went on, better imaging gradually revealed that Pluto was much smaller than first thought. When it’s moon Charon was discovered, an accurate mass could be calculated: 0.22% that of the Earth. This put Pluto in an unusual position: 25 times lighter than Mercury (the previous smallest planet), but 14 times heavier than Ceres (the largest asteroid).
It also had an unusual orbit: it is bound by resonance to the orbit of Neptune, making 3 orbits for ever 2 of Neptune’s, and it crosses the orbit of Neptune for part of that orbit.
As the only known object in its region of space (unlike the asteroids in their belt) and being much bigger than any of them, Pluto was firmly classed as a planet; but it was a strange planet.

And so began the era of astronomical peace that most humans alive today grew up in: we learnt the 9 planets at school, and that was all there was to it. But in science, such peace is often fragile: always there are more answers waiting out there to rock the boat of our understanding.

Planet 10

In 1992 planet Smiley was discovered, and the newspapers were gripped with new-planet-fever. Smiley (a nickname, later replaced with the more mythical “15760 Albion”) was the first new object to be discovered beyond Neptune since Pluto and its moon Charon. You will notice it is not in my image, and this is because it was never accepted as a planet by anyone other than the tabloids. A twentieth the diameter of Pluto and dozens of times less massive, Smiley was clearly too small to be a planet.

But Smiley was just the beginning. Over the next decade, hundreds of other objects were discovered beyond Neptune, the largest of which were approaching the diameter of Pluto. As these discoveries piled up astronomers began to realise the Pluto, far from being a unique object at the edge of our solar system, was simply the largest body of a belt of minor planets beyond Neptune, which was named the Kuiper Belt. Astronomers who knew their history began to feel increasingly uneasy with how similar this situation was to the debacle with Ceres and the asteroids. But as long as none of the objects were actually as big as Pluto, the status quo could be held. And then, in 2005, they found Eris.

Eris

Eris was discovered in 2005 by a team lead by Michael Brown, after they analysed images taken in 2003. They announced their discovery the same week as two other large bodies beyond Neptune, Makemake and Huemea. Makemake and Huemea are both around two thirds the mass of Pluto, and they alone would have been a threat to Pluto’s planethood. But Eris was as big as Pluto, and even more massive. Any list of planets that included Pluto, surely had to include Eris, and so Eris was considered a planet by many. But what about Makemake, Huemea, and any other large objects to be discovered? What if there were 20, 30, or 100 such objects out there? Just as with the asteroids 150 years earlier, the definition of planet had to be set. Either we would have 12+ planets, or we would go back down to 8. And it was the job of the International Astronomical Union (IAU) to set the rules.

The Age of Dwarf Planets

The IAU‘s Decision

In 2006 the IAU finally set the definitions of a planet.

The first was easy, as this rule had been around for a long time:

1st IAU rule: A planet must orbit the Sun

This seems pretty obvious but interestingly it is worded to refer to “The Sun”. This implies this definition is specific to our solar system, and so it doesn’t necessarily apply to exoplanets (planetary bodies around other stars), or rogue planets (planetary bodies without stars).

2nd IAU rule: A planet must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.

This basically just means an object must be massive enough to pull itself into something close to a sphere. This is an attempt to make the older “Planets must be pretty big I guess, not too small ok?” rule into something a bit more quantifiable and objective. The smallest bodies able to achieve this are about 0.0006% the mass of the Earth, or about 4% the mass of Ceres. Oh, and different compositions (e.g. rock vs ice) can affect how likely a body is to pull itself into a sphere.

Using just these two rules, Ceres, Pluto, Eris, and potentially a bunch of other smaller objects beyond Neptune would be added to the planet list. To prevent the number of planets rising indefinitely, a final, controversial rule was added.

3rd IAU rule: A planet must have cleared the neighbourhood around its orbit.

This means a planet should have its own orbit, not be part of a belt of small objects.

Ceres fails this rule by being in the Asteroid Belt. Pluto fails by being in the Kuiper Belt and crossing the orbit of Neptune. Eris fails by being in the scattered disc, a diffuse disc of bodies beyond the Kuiper Belt.

But they also introduced another category to include objects that fulfil the first two rules: dwarf planets. Anything big enough to be round and in orbit of the Sun without dominating its orbit falls into this group. This includes Ceres, Pluto, and Eris.

Everything that only achieves the first rule (such as all other asteroids and smaller objects beyond Neptune) is grouped as a “small solar system body” (SSSB).

As time went on and the masses of other bodies beyond Neptune could be accurately calculated, the number of dwarf planets gradually increased. Huamea and Makemake were added to the list in 2008. Quaoar, Sedna, Orcus, and Gonggong were all accepted as dwarf planets by many astronomers but have not yet been added to the IAU list, and many other objects are listed as “probable/possible dwarf planets”. Here’s an image of the 10 largest dwarf planets or probable dwarf planets beyond Neptune, compared to the Earth and Moon:

The Future

More dwarf planets

There are currently over 100 objects beyond Neptune that are listed as at least “Probably dwarf planets” by Mike Brown (Source), so as time goes on it is likely that the number of confirmed dwarf planets will continue to grow.

More planets

As for future planets, some astronomers now suspect there may be a planetary-mass object somewhere well beyond Pluto. Irregularities in the orbits of many of the most distant dwarf planets and trans-Neptunian objects may be explained by a world larger than Earth drifting in the space beyond the Kuiper Belt and Scattered Disc.

If this world exists, and can be shown to have “cleared the neighbourhood” of its orbit of other objects, it will no doubt be listed as our solar system’s 9th planet.

Meanwhile, if it exists in an orbit cluttered with other objects, we could end up with a dwarf planet larger than the Earth. If that is the case, it may result in future astronomers redefining “planet” one more time.

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Oh, and if anyone is wondering where the names of all these planets and dwarf planets come from, I have a post about that too:

The Etymology of Planets and Dwarf Planets in English