Many people were understandably shaken when the International Astronomical Union rocked the world—err, solar system—by reclassifying Pluto as a dwarf planet in 2006. There were all sorts of opinions about it, even among scientists, but ultimately what the scientific community all agreed on was that our understanding of planets had changed drastically in the nearly eighty years since Pluto had been discovered, and our classifications needed to reflect that.
Some pushed for classifications that would include Pluto as a planet, while others (perhaps tongue-in-cheek) argued that worlds like Earth, Mercury, Venus, and Mars, aren't really planets either. The long and short of it is that any informed opinion recognizes that the concept of a planet is, in the mind of the public at least, very loosely constructed, and largely misinformed.
We now understand that there are multiple types of planet, including rocky inner planets, gas giants, ice giants, and—depending on how you classify things—dwarf planets. Beginning with the most familiar, we have Earth, a rocky, inner planet, and the only place in the universe confirmed to sustain life. Saying that the Earth is a rocky planet really just means that it has a solid surface to land on. Perhaps surprisingly for some, this isn't a fact that is common to all planets: Mercury, Venus, and Mars also have solid surfaces, as do Pluto, Eris, and the other dwarf planets, but the giant planets Jupiter, Saturn, Uranus, and Neptune do not.
In fact, the closest you could come to landing on any of the giant planets would be to land on one of their planet-sized moons. Jupiter's moon Ganymede is larger than the planet Mercury, as is Saturn's moon Titan, which has clouds, rivers, seas, sand dunes, and a fully formed atmosphere. All said, there are over a dozen moons in the solar system—including our own—that are large enough to be either planets or dwarf planets, and all of them have solid surfaces. (Two of them are larger than Mercury, a total of seven are larger than Pluto, and in all sixteen are larger than the dwarf planet Ceres.)
| Earth | 1 | “Luna”¹ |
| Jupiter | 4 | Europa¹, Ganymede², Io¹, Callisto¹ |
| Saturn | 5 | Tethys, Dione, Rhea, Titan², Iapetus |
| Uranus | 4 | Ariel, Umbriel, Titania, Oberon |
| Neptune | 1 | Triton¹ |
| Pluto | 1 | Charon |
| Total | 16 |
In many ways, these planet-sized moons fit the common idea of what a planet is better than the giant planets do. They are all large enough to be round. They have solid surfaces that we can land on. They have enough gravity to keep us from flying off into space, but not enough to kill us. A few of them even have vast oceans of liquid water, which would allow us to build underwater colonies, and perhaps even find life outside of our home world.
For a bit of contrast, Jupiter is made almost entirely of thick clouds of poisonous gas. At some point, the gas may transition to liquid, but the pressure and gravitational forces of the giant planet would crush any spacecraft long before it got that far. On top of that, it produces extreme levels of radiation, so much so that Jupiter actually puts out more heat than it gets from the Sun, scorching the faces of its nearby moons. Jupiter's gravitational pull is so strong that from nearly half a billion miles away (about 780 million kilometers), it pulls on the Sun itself, causing the gravitational center of the solar system to actually be an empty point in space somewhere between the Sun and Mercury. On top of this, Jupiter has 67 known moons orbiting around it—almost like its own mini solar system—complete with four that are planet-sized. The stories for the other giant planets are similar, with the key take-home message being that these are places to orbit around, not places to visit and land on. In this way, although they are clearly not stars, they are in some ways much more like stars than like the concept of planets that most people have.
In the coming decades and centuries, as we venture out further into the solar system and begin more in-depth exploration—and even colonization—of the planet-sized moons, these contrasts between the giant planets and the rest of the worlds out there will become even more apparent. As such, I propose that in the coming centuries, we will likely again adopt a new definition of a planet: A round object with a solid surface that humans can land on.
This new definition would bring the number of (known) planets up to around seventy, including four rocky planets³; one asteroid planet⁴; eighteen satellite planets⁵; and at least 50 icy planets out past Neptune⁶. Part of the debate at the time of Pluto's reclassification centered on the idea that allowing dwarf planets to be called planets would lead to an unwieldy number of planets—too many for school children to learn. Frankly, this argument is absurd. (Continued below.)
With over seventy planets, the difficulty of learning them all could indeed become an issue, but as we come to explore and colonize these worlds, they will each develop their own unique histories and cultures, much like the countries of our own world, of which there are many more than seventy. Just as children learn a few facts about major countries and regions of the world today in their geography classes, they would learn the principle facts about the major planets and groupings of planets in their classes on solar system “geography”. Planetary populations—such as the planets orbiting Jupiter, or those orbiting Saturn or those orbiting within the Kuiper belt—would become the new continents, and interplanetary and other regional alliances would be studied and understood just as well as other countries, regions, and continents are understood today.
- Inner system
- Mercury
- Venus
- Earth & Luna
- Mars
- Near-Earth objects
- Asteroid belt
- Ceres
- Vesta, Pallas, and the other asteroids
- Jupiter system
- Europa
- Ganymede
- Io
- Callisto
- minor moons and trojans of Jupiter
- Saturn system
- Mimas
- Enceladus
- Tethys
- Dione
- Rhea
- Titan
- Iapetus
- rings and minor moons of Saturn
- Uranus system
- Miranda
- Ariel
- Umbriel
- Titania
- Oberon
- rings, minor moons, and trojans of Uranus
- Neptune system
- Triton
- minor moons and trojans of Neptune
- Outer system
- Pluto & Charon
- Haumea
- Quaoar
- Makemake
- OR10
- Eris & Dysnomia
- Sedna
- many others
To be perfectly clear, I'm not proposing a new definition of planet for right now, or for a decade or even a century from now. I'm simply making the observation that as we step out into the solar system, our current idea of what is and isn't a planet will likely naturally shift as we gain specific experience with the various worlds of the solar system. As this happens, it will become more and more clear that the giant planets are more things to orbit around than things to land on, and as such, it is likely that we will begin to think of them as essentially different from the solid bodies of the solar system.
