How would you explain the planets if you didn’t know what they were; if you didn’t know if you lived on one? Long before written history, people noticed that five “stars” seemed to wander among all the others from night to night, wondered about their meaning, and recorded their movements—later learning to predict them. These points of light became known as Mercury, Venus, Mars, Jupiter, and Saturn. In time, telescopes and spaceflight would profoundly change our understanding of these planets.
Our exploration and understanding of the planets has changed over time and continues to change. How did we go from noticing these celestial bodies as points of light in the sky to calling them planets? The word “planet” can be traced back to the ancient Greeks. Because their movements were different and seemingly less predictable than the other stars, the ancient Greeks called the asteres planetai, or “wandering stars.”
As astronomical tools improved, our views of the planets sharpened and our understanding deepened. The focus of astronomers shifted from how planets moved to what they were like as individual worlds.
So, what are some of the ways we have explored our Solar System’s planets?
1. With our eyes.
While we got our word for planet from the Greeks, it was the ancient Babylonians who created the oldest known written records of the planets. They recorded their observations on clay tablets, such as astronomical calculations made more than 2,000 years ago related to the rising and setting of the Moon and planets.
2. By telescope.
Galileo Galilei was the first to focus a telescope on the planets in 1609. What he saw completely changed humanity’s understanding of them. Through his telescope, stars remained points of light—but planets looked like spheres with surfaces and features. He drew sketches of what he saw—even drawing Saturn’s rings although he did not know it at the time. Galileo also discovered four moons around Jupiter and saw the Venus had phases like our Moon.
Since Galileo’s time, many people have used telescopes to explore the planets and expand humanity's knowledge of them. In 1877, Asaph Hall, an astronomer at the U.S. Naval Observatory in Washington, DC, discovered two small moons around Mars that he named Phobos and Deimos. The fog from the Potomac River made it hard to detect the moons. But Hall was persistent. He searched the edge of Mars with his telescope eyepiece until he found them.
On the night of August 16, 1898, Harvard University astronomer William H. Pickering discovered a new moon of Saturn. Looking through a 24-inch telescope in Arequipa, Peru, he recorded his observations in his notebook. Pickering named the moon Phoebe.
The Kepler Space telescope revolutionized our knowledge of the planets yet again. Rather than viewing the planets from Earth, this telescope was sent into space to explore the planets. Scientists using Kepler have discovered that many of the stars we see in the sky have planets of their own. Comparing our solar system to others discovered by Kepler is teaching us about how planets evolve.
3. By uncrewed spacecraft.
Ideas about the planets based on observations from Earth are have been improved over time with data collected by robotic spacecraft. By the 1980s, uncrewed spacecraft had traveled to all seven of the other planets and viewed them close up. This allowed scientists to study the atmosphere and geology of the planets. By comparing the similarities and differences among them, they have learned more about our own world. Spacecraft missions have revealed planets as dynamic worlds, each with its own distinct physical nature and history.
The first successful mission to Mars took place in 1965, when Mariner 4 flew by the planet and returned 21 images. As spacecraft imaging capabilities have improved, we have been able to view the Martian surface with even more detail.
Mariner 9 studied Mars from orbit for nearly a year in 1971–1972. It sent back more than 7,000 television images that covered 85% of the planet’s surface. It discovered huge volcanoes and a vast canyon system. None of this had been seen before, despite hundreds of years of looking at Mars through telescopes.
This globe of Mars was produced from Mariner 9 images. Image courtesy of the Smithsonian Institution.
4. By lander and rover.
Despite amazing progress in space travel, humans have been unable to travel to other planets. We have, however, been able to send rovers to the surface of one of our planetary neighbors: Mars.
Sojourner—named for the 19th century abolitionist who fought for women’s rights—was the first successful Mars rover. It landed aboard the spacecraft Pathfinder on July 4, 1997, and operated for 85 days. The Pathfinder mission tested the landing system, cameras, rover mobility, and other technology used for later Mars rovers. The solar powered Sojourner weighed about 25 pounds (22.5 kilograms) and drove about 330 feet (100 meters). It measured the composition of rocks near the lander.
The rover in this image is named Marie Curie, after the famous scientist who pioneered the study of radioactivity. Marie Curie was Sojourner's flight spare. Image courtesy of the Smithsonian Institution.
Multiple rovers have been sent to Mars since Sojourner's successful landing. In 2021, the rover Perseverance landed on Mars. Perseverance’s job is to collect rock samples, among other things—but it also introduced an entirely new way to explore the planets: a helicopter named Ingenuity. Ingenuity was deployed from Perseverance and carried out the first powered spacecraft flight on another planet.
Planets were once thought of as simply lights in the sky that moved mysteriously from night to night. When people first saw planets through telescopes in the 1600s, they discovered they were not just lights, but places. Using increasingly powerful tools, we came to know them as actual worlds with unique characteristics.