How Did We Discover the Planets?

Our night sky is full of wonders. When you look up you can see the stars, the Moon, and sometimes even other planets! We know what these otherworldly objects are because of centuries of research carried out by curious stargazers before us.  

Ancient peoples from across the Earth only knew about five of the planets: Mercury, Venus, Mars, Jupiter, and Saturn. To the people of some ancient civilizations, the planets were thought to be representations of deities, for others they were a means through which gods communicated to humans. The Romans named the planets based qualities they displayed that resembled certain deities. For example, Venus was bright and beautiful, so it was named after the goddess of beauty. Mars was red and ominous, so it was named after the god of war. We still use the Roman names for the planets today.

Ancient civilizations did not have the powerful telescopes we use today. To their unaided eyes, these planets looked like the other stars that dotted the skies. But there was a difference in the way some of these stars moved. While all the stars in the sky move east to west as our Earth rotates, planets also move within their own orbit. This causes them to appear to wander throughout the night sky when compared to other stars, which move in a fixed position each day. For this reason, the Greeks referred to the planets as wandering stars. Our word "planet" comes from the Greek word planetes, meaning "wanderer." 

What’s at the Center of Our Solar System: The Earth or the Sun? 

The Greco-Roman astronomer Ptolemy came up with a theory for the solar system in the second century C.E. This theory survived for next 1,400 years. The ancient Greek philosophers Plato and Aristotle had previously introduced the idea that the heavens move in perfect circles. However, their theories were unable to account for some of the behaviors of the planets, like retrograde movement—when planets seemingly moved backward. 

In Ptolemy’s book, Almagest, he made their theory work by offsetting the center of the planetary spheres and then adding additional smaller circles for the planets to move in on top of the circles they moved in around the Earth. This theory was accurate to a point, describing the apparent motions of the planet from the perspective of Earth, and accounted for changes in the planets’ brightness. Ptolemy’s theory, while ultimately incorrect, was the dominant view until the Renaissance. However, that does not mean it was not questioned and modified by others. As early as the 10th century, Islamic astronomers saw problems with Ptolemy's planetary hypothesis. They developed new mathematical tools that made it work even better. And these tools were later adopted by European astronomers.

Diagrams illustrating Ptolemy’s planetary system.

In the 1500s, Nicolaus Copernicus, borrowing the mathematical tools developed by Islamic astronomers, broke with centuries of tradition when he argued that the Sun, not the Earth, was at the center of our Solar System. Copernicus was correct about the Sun, but he was not the first one to have this idea. About 1100 years earlier, the Greek astronomer Aristarchus came up with the same theory, but Ptolemy rejected the theory and Aristarchus was ignored. Copernicus was also still tied to the idea that the planets moved in a perfectly circular motion. 

Copernicus’s theory arranged the Earth and the five planets that were known in the correct order. The planets were shown to revolve around the Sun in circular orbits. It was not until 1609 that the German astronomer, Johannes Kepler, described planetary paths correctly as ellipses, or oval shaped.  However, there were many competing views of the arrangement of the universe during this period, and Copernicus himself did not release his manuscript for publication until he was on his deathbed.   

Telescopes Put Things into Focus 

Until the development of the telescope in the early 1600s, all astronomical observations were made with the naked eye. When Galileo Galilei turned his telescope to the sky in 1609, planetary astronomy became a new and different science.  

Telescopes were in use for many years before it was known how exactly they worked. The first theory of the telescope was written by noted astronomer Johannes Kepler in his 1610 book Dioptrice. One challenge astronomers faced was the distortion of the lenses used by telescopes. To correct for this distortion, telescopes became longer and longer. Telescopes longer than 30 meters (100 feet) were not uncommon.  

Sir Isaac Newton designed a reflecting telescope to overcome the color distortions produced by lenses. His telescope focused light with a concave mirror rather than by lenses. The telescope no longer needed to be a long and unwieldy tool.  

To the unaided eye, Venus appears to be just a point of light. Galileo observed Venus through a telescope and found the planet to have phases similar to the Moon. Early in 1610, Galileo observed four "stars" near Jupiter with his telescope. The motions of the stars relative to Jupiter led Galileo to the conclusion that they were moons of the giant planet. These moons — Io, Europa, Ganymede, and Callisto — are known today as the Galilean satellites in honor of their discoverer. 

Discovering the Rest of our Solar System 

Before the 1700s, humans had only identified five planets besides our own in the night sky. The planet Uranus was discovered by a British astronomer named Sir William Herschel on March 13, 1781. Uranus had been observed many times previously, but it was always thought to be another star.  

William Herschel built his own telescopes, including one which had a focal length of 6 meters (20 feet) and a larger telescope of the same design with a 12 meter (40 foot) focal length. Uranus was discovered to be a planet accidentally while Herschel was surveying all stars down to magnitude eight. These were stars that are about ten times dimmer than can be seen by the naked eye. One of these “stars” looked different than the others. Within a year it was shown to have a planetary orbit 18 times father from the Sun than Earth. The new planet was named Uranus after the father of Saturn in Roman mythology.  

Uranus and the dwarf planet Ceres were discovered by accident. Neptune, however, was not. Astronomers proposed that a planet beyond Uranus could account for irregularities in its orbit. Independently two astronomers calculated the position of this yet unknown planet. The two astronomers were John Couch Adams in England and Urbain-Jean-Joseph Le Verrier in France. 

The search began. A British astronomer James Challis, using Adams' predictions, observed the planet on the night of August 4, 1846, but failed to compare his observations with those of the previous night and did not recognize the planet. On September 23, 1846, the planet was finally found on the first try by the German astronomer Johann Galle using Le Verrier's predictions. 

Several astronomers interpreted the irregularities in the orbits of Uranus and Neptune as being caused by a more distant planet. Among these astronomers was the American Percival Lowell. Lowell started the search for this planet. He purchased a Carl Zeiss blink comparator, used to find differences between two photographs of the night sky, in 1911. In 1930, one of Lowell’s successors used that same instrument to finally find the planet, which would later be named Pluto. While Clyde Tombaugh was the person who eventually found Pluto, Lowell is credited with initiating the search. Today we define Pluto as a dwarf planet rather than a planet like Earth or Jupiter. 

What of the Moon? Do other planets have moons like Earth does? In 1900, the planets in our solar system were known to have 22 natural satellites, or moons. The number of known moons has now quadrupled and is still increasing. Likewise, astronomers also discovered new features of some planets. The rings of planets, such as those of Saturn, are a system of countless small satellites. In 1977, it was discovered that Uranus was encircled by a system of rings. Uranus' rings were discovered because they blocked a star from view as Uranus passed between the Earth and the star. The discovery of Uranus' rings occurred through observations made from the Kuiper Airborne Observatory. 

Today astronomers continue to research our solar system and beyond. While our ancestors may have been focused on identifying the objects in our celestial neighborhood, we now have the technology to look even further. In the 1990s, the first exoplanets were discovered. Exoplanets are planets that are in orbit around a star that is different than our own (the Sun). Since then, astronomers have identified the location of over 5,000 other exoplanets and predict there are billions more!