Scientists and astronomers have spent centuries performing rigorous scientific research and analysis to understand our solar system. Here are some of the significant discoveries that contributed to our modern solar system knowledge:

• Around 400 BCE—Greek astronomers identified five planets. As far back as Ancient Greece, astronomers observed celestial bodies that, unlike the stars, move across the night sky. The Ancient Greeks named these objects “planets,” meaning “wanderers.” They were able to identify five planets with the naked eye: Mercury, Venus, Mars, Jupiter, and Saturn.
• 1543—Copernicus proposes the heliocentric model. Greek astronomer Aristarchus of Samos was the first person to suggest that the Earth revolves around the sun. Centuries later, an astronomer named Nicolaus Copernicus validated his theory, proposing that the sun was a fixed point which Earth and other planets orbited around. While Copernicus postulated that these orbits were perfect circles, a few decades later, a scientist named Johannes Kepler theorized that the orbits were elliptical rather than circular. The heliocentric model (that planets orbit the sun) was hotly debated during this time frame. Galileo Galilei was famously put on trial and sentenced to house arrest for his advocacy of heliocentrism.
• 1669—Newton theorizes the laws of gravity. Until the mid-1600s, astronomers struggled to determine why the planets orbit the sun or the rules they followed when doing so. In 1669, Sir Isaac Newton discovered the mathematical equation that could chart precisely how the planets moved.
• 1781—Herschel discovers Uranus. In 1781, an astronomer named William Herschel discovered through a telescope what he thought was a new comet. But after observing the comet’s orbit, Herschel discovered that it was a new planet, which was later named Uranus. This was the first planet discovered in our solar system since ancient times, as every other planet had been observable by the naked eye.
• 1801—Piazzi discovers the asteroid belt. Astronomer Giuseppe Piazzi discovered an object between Mars and Jupiter that he announced as a new planet named Ceres. However, with later scrutiny, astronomers discovered thousands of other similarly sized small objects in Ceres’s vicinity, leading to the classification of an asteroid belt between the inner planets and outer planets.
• 1846—Galle discovers Neptune. The discovery of Neptune, the last known planet in our solar system, was a historical moment that drew upon many of the astronomy community’s previous findings. After the discovery of Uranus by William Herschel, a scientist named Alexis Bouvard charted Uranus’s path and discovered that something wasn’t quite right—its orbit wasn’t following Newton’s laws of gravity. Rather than throw Newton’s laws out, he postulated that there was something out in space interfering with Uranus’s orbit. Two astronomers, John Couch Adams and Urbain Le Verrier, began crunching numbers and published findings with what they believed was the exact location of the interfering celestial body. An astronomer at an observatory, Johann Gottfried Galle, looked up at the night sky through their large telescope—and was the first person to see the new planet, Neptune, bringing the total number of planets in our solar system to eight.
• 1930—Tombaugh discovers Pluto. Astronomer Percival Lowell noticed minor inconsistencies in Uranus and Neptune’s orbits, postulating that another planet (which he called “Planet X”) was out there. This led to the discovery in 1930 of Pluto by astronomer Clyde Tombaugh. Pluto was later classified by the International Astronomical Union (IAU) as a dwarf planet in 2006, and not a true planet in the solar system.
• 1971—Black holes confirmed. In the 1960s, astronomers began a new form of research called X-ray astronomy. Researchers would send rockets and satellites equipped with X-ray technology outside the Earth’s atmosphere to detect X-ray sources in the nearby universe. Researchers discovered several extremely bright X-ray sources that weren’t giving off any optical light; in 1971, they identified the first black hole, confirming their existence.
• 1992—Jewitt and Luu discover the Kuiper belt. In the early 1990s, astronomers David C. Jewitt and Jane Luu were doing a new study observing objects beyond Neptune when they discovered a large population of distant objects (similar to the asteroid belt). They named this field of icy asteroids “the Kuiper belt.”
• 2002—Eris discovery. In 2002, a group of scientists led by Mike Brown discovered a large object orbiting the sun along an elliptical path that, for the most part, extended much further than Neptune or Pluto. Further research showed that the object was slightly more massive than Pluto, which has slightly more volume. The large object was officially categorized as a dwarf planet and eventually named Eris.
• 2008—Water discovery on the moon. During a moon expedition, India’s Chandrayaan-1 spacecraft deployed a probe that impacted the moon’s Shackleton Crater and released subsurface debris. The research team analyzed the debris and detected the first direct evidence of water in the cold, shadowed poles of the moon’s surface.
• 2011—Possibility of water on Mars. In 2011, NASA scientists observed what appeared to be water making dark paths down certain hills during Mars’s warmer months, suggesting that other planets in our solar system may have water.
• 2020—Water discovery on the moon’s sunlit surface. In 2020, NASA researchers discovered that lunar water was much more abundant than previously thought. While earlier researchers only found evidence of water in the cold, shadowy craters of the moon, NASA found evidence of water even in sunny areas, suggesting that water may be distributed across much of the moon’s surface.