The Planets in Our Solar System – A Timeline

This is a tricky one for the innermost planets. Humankind has known about them for thousands of years. Mercury, Venus, Mars, Jupiter and Saturn are all visible in the night sky with the unaided eye. As such, these planets have been known for thousands of years and there is no one person who can be credited with discovering these planets. These planets appear as bright objects that do not shimmer, like stars do.

Ancient Discoveries

Though many ancient civilizations mention these five planets, I’ll just highlight a few.

Mercury: Mentioned by the Sumerians as early as the second millennium BC. The Babylonians called Mercury Nabu. Check out this paper by Prockter and Bedini for much more detailed information.

Venus: One of the brightest objects in the sky (next to the Sun and the Moon). Owing to its brightness, many ancient civilizations knew of Venus, including those in Mesopotamia, Egypt, Greece and Rome, India, etc. For the Mayans in Mesoamerica, Venus was the most important celestial body. We mostly see Venus on the horizon, either in the morning or evening, and it is often referred to as the Morning or Evening Star. Because of its position in the sky, either in the morning or evening, many cultures identified Venus as two separate objects.

Mars: Appears as a small red dot in the night sky. It is notably different in color when you look at it with the unaided eye. Ancient Egyptian astronomers recorded the motion of Mars across the sky, and it was depicted on the ceiling of the tomb of Senenmut in a star map as an empty boat. The boat meant to represent Mars is hypothesized to be empty because of Mars’ apparent retrograde motion—as Earth passes Mars in its orbit Mars will temporarily appear to reverse direction across the night sky.

Jupiter: Jupiter is the next brightest object in the sky, after Venus. The Babylonians called it Marduk, the Greeks referred to the celestial body as Zeus and the Romans gave it the name we use today.

Saturn: Ancient Chinese and Japanese cultures observed Saturn and associated it with “earth”, which is based on classifying everything into five natural elements: wood, water, fire, earth and metal.

Galileo Galilei (Italian) was the first person credited to observe Mercury, Venus, Mars, Jupiter and Saturn with a telescope. Thomas Harriot (English) and Simon Marius (German) are two other early astronomers that used the newly invented telescope to observe celestial objects in the sky.

Now on to more modern-day planet discoveries using telescopes.

Modern Discoveries

Uranus: March 13, 1781. William Herschel “discovered” Uranus. He was doing a survey of stars in the night sky when he noticed that one particular dot of light was behaving differently from the others. Upon closer inspection he realized the object orbited the Sun. Herschel called it a comet, though other astronomers suspected it was a planet. In 1783 Herschel formally acknowledges it as a planet. I put discovered in quotation marks because Uranus was observed well before 1781 but was generally interpreted to be a star. In 128 BC, it is speculated that Hipparchus included it in his star catalog. Other astronomers, like John Flamsteed in 1690 and Pierre Charles Le Monnier in the 1750s and 1760s, also identified Uranus as a star.

Neptune: Mathematics found Neptune on September 23, 1846. The irregularities in the orbit of Uranus suggested there was a large object perturbing its orbit. Urbain Le Verrier did the math to predict where this 8th planet would be in the night sky and Johann Gottfried Galle used a telescope to prove Le Verrier’s calculations. Separately and at the same time as Le Verrier, John Couch Adams also calculated the likely position of an 8th planet. Like Uranus, Neptune was observed many times before it was identified as a planet by Galileo Galilei (1613), Jérôme Lalande (1795) and John Herschel, son of William Herschel (1830).

Should we talk about Pluto? ...Okay, we can. But we are not going to talk other dwarf planets—there are so many! Ceres, Makemake, Quaoar, and Sedna are some of the larger dwarf planets, just to name a few. But back to Pluto, since it was considered a planet at the time of its discovery.

Pluto: Discovered by Clyde Tombaugh at the Lowell Observatory in Arizona on February 18, 1930. Clyde Tombaugh was working at Lowell Observatory taking pictures of the night sky to search for a “trans-Neptunian object”, which was code for “a planet beyond Neptune”. A planet beyond Neptune had been predicted based on perceived perturbations in Uranus’ orbit. Percival Lowell initiated a search for this “missing” planet that proved unsuccessful. After his death, Lowell Observatory continued the search and employed Clyde Tombaugh to lead the search.

As you move away from the Earth, the planets appear as smaller and smaller, and dimmer and dimmer objects in the sky. Thus, those planets closest to us were discovered first by ancient civilizations. The Ice Giants were only discovered well after the invention of the telescope once technology caught up enough to detect these small dots of light in our sky.