For a very long time, we humans knew about only one planet, which is ours. The universe, for us, consisted basically of the Sun, the Earth and the Moon. Nobody had much of a clue about any of them, so they generally made up a lot of stuff. About 2000 BCE, Mercury, Venus, Mars, Saturn and Jupiter were discovered by the Babylonians. Those planets are all big enough to see with the naked eye, but the Babylonians were the first to determine that they weren’t the same as all the rest of the bright spots in the sky. Most everyone at the time thought the Earth was the center of it all, since it’s our planet and we’re really important and all, but that’s about as far as our understanding went for the next 1500 or so years, until the Greeks more or less invented science. It turns out that the Greeks were wrong about a great many things, but they deserve a lot of credit for developing the systematic study of the natural world. It was a Greek who, somewhere around 250 BCE, came up with the first (recorded) theory that all the planets were orbiting the Sun, instead of everything orbiting the Earth. This heliocentric theory wasn’t really accepted, even by most of the scientific community, until Copernicus published his findings about 1800 years later, in 1543.
It wasn’t until 1610 that Galileo found the next big members of the Solar System. It says much about the ability of the ancient Babylonians that it took over 3600 years, and the invention of the telescope, before anyone found any other members of our Solar System. It would be even longer before anyone discovered any new planets. What Galileo discovered were the four huge moons of Jupiter. Over time, we’ve discovered more. It turns out that Jupiter has at least 79 moons. During the rest of the seventeenth century, the Dutch astronomer, Huygens, and the Italian, Cassini, discovered five of what are now known to be at least 250 moons of Saturn. It’s probably pretty hard to determine what is a moon for Saturn and what is just a chunk of something in Saturn’s rings. So, from about 2000 BCE to almost 1800, the Solar System was limited to 6 planets, 10 really big moons, and the Sun.
Frederick Herschel, a German-English astronomer, discovered Uranus, some of her moons (out of a total of 28 known today) and a couple more big moons of Saturn in the 1780s. Uranus had actually been observed much earlier, since it is (barely) visible to the naked eye, but because of how far away it is (about 3 billion kilometers [about 1.9 billion miles]), it was always thought to be a star. In 1801, Piazzi, an Italian, discovered what he originally thought to be the eighth planet, named Ceres, located in between the orbits of Mars and Jupiter. It was later decided that Ceres was actually a very large asteroid, as it was located in the asteroid belt. In 2006, Ceres was reclassified as a dwarf planet by the International Astronomical Union, which is the scientific body responsible for the official names of celestial bodies. In 1846 Neptune was observed for the first time by a German astronomer, named Galle. Neptune is so far away from Earth that it is not visible at all to the naked eye, and it was only discovered using a telescope after someone noticed that Uranus had an odd “wobble” in its orbital path. Deducing that the “wobble” was caused by the presence of another planet close by, a French mathematician named Le Verrier calculated the likely location of the unknown planet. He sent a letter to Galle with his prediction and when Galle pointed his telescope to that position, he observed Neptune for the first time. That is some amazing math skill. Neptune is 4.5 billion kilometers (about 3 billion miles) away and it was found within one degree of where Le Verrier predicted it would be. That’s like calculating the location of a tennis ball 200 miles away and then going to find it within about an inch of where you thought it would be.
We were now up to 8 planets and an increasing number of moons, as more and more moons were discovered, usually based on size—biggest ones first. We stayed at eight planets until 1930, when Pluto was first sighted by American astronomer, Clyde Tombaugh, using the telescope at the Lowell Observatory in Flagstaff, Arizona, which is sort of interesting in its own right. The observatory was built by a member of the wealthy Lowell family of Boston in 1894, and it is still in use today. The observatory has been overseen by a single trustee, always a member of the Lowell family, ever since it was built. Uranus and Neptune are both very large planets—the third and fourth largest in the Solar System, after Saturn and Jupiter—so seeing them with a telescope is not hard, if you know where to look. Pluto, on the other hand, is not large (about 1/20 the size of Uranus and Neptune and 1/5 the diameter of Earth) and it is quite a lot further away than Neptune, at 7.4 billion kilometers (4.6 billion miles), so finding it was a challenge. It turns out that the same “wobble” in Uranus’ orbit that was used to find Neptune was also used to postulate the existence of yet another planet out there causing Uranus’ orbital perturbations. The result was Pluto.
Pluto brought us up to nine planets, and that is where we were when I was a little science nerd growing up. However, we are now back to eight. How, you might ask, did we lose a planet? One would think that would be a pretty big deal, but many people probably didn’t even notice. It turns out that Pluto is still there, but because it’s quite a lot different from the other planets in its orbit and composition, it was re-classified, along with Ceres, as a dwarf planet in 2006. One big reason Pluto was re-classified is that it is located in a region of the Solar System called the Kuiper Belt, which is composed of many similar bodies which, like Pluto, are composed mostly of frozen gasses and bit of rock and are much smaller than any of the other rocky planets in the Solar System. People being people, there has been a lot of fuss over the “demotion” of Pluto from planet to dwarf planet.
So, as far as anyone knew, there were nine (ish) planets, not just in the Solar System, but in the entire Milky Way galaxy, and that’s the way things stood from 1930 up until 1992, when the first exoplanet (meaning a planet outside the Solar System) was discovered. This planet was found around a pulsar, which is a star that has run out of fuel, exploded in a supernova and then collapsed in on itself, forming a very dense neutron star. The first exoplanet found around a “main sequence” star, which is a star that is still “burning”, more-or-less like our sun, was found in 1995. Over time, more and more exoplanets were discovered. The Hubble Space telescope helped to accelerate the process, but discovery really took off with the launch of the Kepler space telescope by NASA in 2009. Kepler was designed primarily with the goal of finding exoplanets. It was a great success, operating for about four years and helping to observe almost 3000 new exoplanets. As of 2025, NASA has catalogued a total of over 5900 exoplanets.
What’s all the fuss, you ask? I dare say that whether or not Pluto is defined as a planet or a dwarf planet really isn’t a fundamentally life-altering issue. You are, of course, free to form your own opinion on the matter. My son, who wasn’t even in kindergarten in 2006, was indignant with the re-classification at the time and now that he is in college, he still thinks it’s a miscarriage of justice. No, most of the fuss is about the exoplanets. As we’ve discovered more and more of them, more and more of them are found that could, potentially, support life. These are rocky, Earth-like, planets that exist in what is called the “Goldilocks Zone”. I first mentioned the Goldilocks Zone in one of my articles on water. It’s where the planet is just close enough to its star to be warm enough to have liquid water without being too warm so the water would evaporate. Life, as far as we know, requires liquid water, so if an exoplanet could have liquid water, it is possible that they could support life. This is, by far, the best opportunity we, as a species, have ever had to possibly observe the evidence of life on other planets, or if there isn’t any, why there isn’t. You should find that exciting. We are reaching a new point in our technological development.
Since the first time our earliest ancestors looked up at the night sky and wondered what all those specks of light were, humans have wondered about the universe and where we fit into it. It is the greatest and most fundamental of questions and our ability to wonder about it is a big part of what it is to be human. Now, perhaps as never before, it is important that we explore our common humanity and our tiny place in the vastness of the universe. With the realization of who we are as a species and how insignificant our tiny little world is in the infinity of space and time, perhaps we can stop fussing so much over who voted for whom, how much skin pigment is optimal or whose deity is the best.