How were the Planets of our Solar System Discovered?

Venus in night sky
Image Credit: Aditya Chache on Unsplash

In ancient times, astronomers were able to distinguish between those stars which seemed to maintain a fixed position relative to one another, and the so-called wandering stars which changed their positions in the sky slightly each night. It was also noticed that these ‘planetes’ did not twinkle like other stars, varied in brightness throughout the year, and appeared to move along the same path traveled by the Sun and Moon called the ecliptic.

They were naturally ascribed special status in the celestial heavens, and ancient civilizations named the five planets easily seen with the naked eye after their most important deities. We know them today as Mercury, Mars, Venus, Jupiter, and Saturn, while our solar system‘s other two planets (not including the Earth), Uranus and Neptune, were not formally discovered until 1781 and 1846 respectively using telescopes.

That said, each of the planets has its own interesting discovery story, so let’s take a look at them:

Discovery of Mercury

  • Diameter: 3,031 miles
  • Orbit: 29-43 million miles
  • One Year: 88 Earth days

Mercury is visible to the naked eye and has been known to ancient people for millenia, with one of its earliest recordings found in a Babylonian star catalog called the Mul.Apin tablets compiled around 1000 BC . The planet Mercury was first seen through a telescope by Galileo in 1631, although his instrument was too small to observe its phases. That task was accomplished a couple of years later by Giovanni Zupi, who was also able to determine that Mercury was orbiting the Sun.

In 1962, Soviets scientists subsequently discovered that Mercury rotated, and in 1965 the notion that the planet was tidally locked to the Sun was dispelled after its orbit was accurately calculated. In 1974, Mercury was visited for the first time by the spacecraft Mariner 10.

Discovery of Venus

  • Diameter: 7,521 miles
  • Orbit: 66-68 million miles
  • One Year: 224 Earth days

Our closest planetary neighbor, Venus is the brightest planet and has been studied for thousands of years. The Babylonian Venus tablet of Ammisaduqa, for instance, is dated to the 7th century BC with its observations stretching back to the second millennium BC. In 650 BC, the Mayans also used observations of Venus to produce a highly accurate calendar.

In 1610, Galileo observed Venus using his telescope, and after noting its phases realized that it orbited the Sun like the Earth, although the planet’s thick carbon dioxide cloud covering meant astronomers were unable to get a clear view of its surface until NASA’s Mariner 2 passed by in 1962. On December 15, 1970, the Soviet Union’s Venera 7 probe became the first spacecraft to land on another planet. Up until that time, it was thought that Venus might be home to life like Earth.

Who discovered the Earth is a planet?

  • Diameter: 7,926 miles
  • Orbit: 91-94 million miles

In Ancient Greece, astronomer Aristarchus of Samos (310-230 BC) went against the accepted geocentric model by proposing that the Earth revolved around the Sun, with an extract from Archimedes book The Sand Reckoner stating:

“His hypotheses are that the fixed stars and the sun remain motion less, that the earth revolves about the sun in the circumference of a circle, the sun lying in the middle of the orbit”

The views of Aristarchus were largely dismissed, however, and it wasn’t until Polish astronomer Nicolaus Copernicus formulated a heliocentric model of the universe in the early 16th century, which was later championed by Galileo Galilei, that attitudes slowly began to change and the Earth began to be viewed as a planet, and not the center of the universe.

Discovery of Mars

  • Diameter: 4,222 miles
  • Orbit: 127-155 million miles
  • One Year: 687 Earth days

Mars has been known since ancient times, and its red coloration can be viewed with relative ease by the naked eye. In 1877, Giovanni Schiaparelli described some features that he saw on the planet as “Canali” leading people to believe that there were canals on Mars built by some type of life form.

This is how the idea of Martians came into being, and in 1898 English author H. G. Wells published his science fiction novel ‘The War of the Worlds’ about a Martian invasion. In 1965, NASA’s Mariner 4 sent back close-up photos of the planet, and by 1976 the Viking 1 and 2 probes had landed on its surface.


Discovery of Jupiter

  • Diameter: 88,846 miles
  • Orbit: 460-508 million miles
  • One Year: 11.86 Earth years

Jupiter, the largest planet in our solar system, is one of the brightest objects in the night sky and held great importance to everyone from the Ancient Chinese to the Greeks. In 1610, Galileo was the first person to make detailed observations of the planet and notice its four largest moons; Io, Europa, Ganymede and Callisto.

In 1664, English scientist Robert Hooke was the first to see the red spot swirling on Jupiter’s surface, and his observation was confirmed by Giovanni Cassini in 1665. During the 20th century, much was learned about the planet, and in 1973 Pioneer 10 flew past Jupiter, while in 1979 NASA’s Voyager 1 and 2 discovered its rings, and several unknown moons.

Discovery of Saturn

  • Diameter: 74,900 miles
  • Orbit: 839-938 million miles
  • One Year: 29 Earth years

As the last classical planet, Saturn has been known since antiquity with its first written record dating back to the Assyrians around 700 BCE. Detailed observations of the planet became possible with the invention of telescopes, and in 1610 Galileo saw its rings for the first time, although he mistakenly believed them to be moons.

In 1659, Christiaan Huygens using a more powerful telescope realized what was really being seen, and soon after discovered the planet’s first moon, Titan. In 1671, Giovanni Cassini found four more moons (Iapetus, Rhea, Tethys, Dione), as well as a space between the planet’s A and B rings, known today as the Cassini Division. The first time Saturn was photographed was in 1979 when Pioneer 11 passed within 20,000 km of the ringed planet, and in 1980/81 more close-up images were taken by Voyager 1 and 2.

Discovery of Uranus

  • Diameter: 31,763 miles
  • Orbit: 1.17-1.86 billion miles
  • One Year: 84 Earth years

Uranus can be seen with the naked eye, but is difficult to spot due to its dimness and the fact it takes 84.3 years to complete one orbit around the Sun. Uranus has been mistaken as a star throughout history, possibly as far back as Hipparchos in 128 BC, but certainly by John Flamsteed in 1690.

In 1781 William Herschel then reported it as a comet, before later that year realizing the object was in fact a planet, and naming it Georgium Sidus in honor of George III. German astronomer Johann Elert Bode suggested the name Uranus, instead, and by 1850 the name had been widely accepted. Only one probe has ever photographed the planet–Voyager 2, which in 1986 passed within 50,600 miles (81,500 kms) of the planet’s top clouds.

Discovery of Neptune

  • Diameter: 30,779 miles
  • Orbit: 2.77-2.83 billion miles
  • One Year: 164.8 Earth years

Neptune cannot be observed without a telescope, and while astronomers such as Galileo and John Herschel did observe what they believed to be a star, it wasn’t until 1846 that Johann Gottfried Galle observed and recognized Neptune as a planet for the first time.

Interestingly, Galle and his assistant Heinrich Louis d’Arrest used a calculation that had been sent to them the very same day by French mathematician Urbain Le Verrier, who realized that the unusual orbit of Uranus must be affected by the gravity from another more distant planet.

Around the same time, British and French mathematicians John Couch Adams and Urbain Jean Joseph Leverrier independently asked astronomers George Airy and James Challis to locate Neptune based on their own calculations, and after their success, both Adams and Le Verreir were subsequently credited for having discovered the planet.

On August 25, 1989, Voyage 2 became the only probe ever to photograph the planet after passing 2,983 miles (4,800 km) above its north pole.

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