Things that surprised me (and might surprise you!) about space

As I've mentioned before, I really like space. Even so, until a few years ago, I really didn't know that much about it. I knew about the nine planets I'd learned about in school (more about that later); I knew the Sun was a star; and I had heard something about scientists discovering some sort of outer asteroid belt out by Pluto. I'd seen some of the headlines about newly discovered would-be planets like Quaoar and Sedna, but I didn't really know much about them.

For some reason, in the last year or two, space has become one of my major interests. It might have something to do with last year (2015) being such a great year for space—after all, we had spacecraft visit two of the five official dwarf planets last year, including poor old Pluto. Like a lot of other people, I decided to learn a little bit more about Pluto's “demotion”, and in the process, I started learning a lot more about the whole solar system. A lot of the things I learned surprised me, and I continue to be surprised by new things I learn, and by new correlations I hadn't put together before.

Here are a few of the things that surprised me the most:

There are moons that are bigger than planets

That's right. When I was learning more about Pluto, I was surprised to find out that Pluto is actually smaller than the Moon. I was even more surprised to find out that our Solar System actually has seven moons that are larger than Pluto. But what surprised me the most was that there are two moons that are actually bigger than the planet Mercury. Jupiter and Saturn each have dozens of moons, and most of those moons are nothing to write home about. But Jupiter's largest moon, Ganymede, and Saturn's largest moon, Titan, are both bigger than the planet Mercury. Ganymede has a thin oxygen atmosphere and Titan has a fully formed atmosphere, clouds, rivers, seas, sand dunes, and a fully functional water cycle (except with methane instead of H₂0). On top of that, Jupiter's second largest moon, Callisto, is only one percent smaller than Mercury. If these moons were orbiting the Sun instead of Jupiter and Saturn, they would be full-blown planets. So would our Moon. Go figure.

Not all planets are created equal

This might seem obvious after the last section, but bear with me. I'm pretty sure that I knew that there were different sizes of planets, but I didn't realize that they were entirely different kinds of planets. What do I mean by this? Well, Earth, Venus Mars and Mercury are all rocky inner planets. They have solid surfaces that you can land a robot on, and maybe even land people on. Jupiter, Saturn, Uranus, and Neptune are all giant planets, and don't have solid surfaces. Even among the giant planets there are two different kinds: Jupiter and Saturn are both gas giants, while many scientists describe Uranus and Neptune as ice giants instead. But none of them have solid surfaces. This means that there's really no way to actually go to Jupiter. You can fly there, you can orbit it, but you can't land on it. It's not that landing there would kill you (even though it would); it's that there is literally nothing to land on. The closest you could get would be landing on one of its (planet-sized) moons.

The reason the giant planets don't have a solid surface is because they are made mostly out of clouds and gas. But what does it mean to say that the rocky planets and the giant planets are all “planets” if half of them have solid surfaces and half of them don't? It really hit me as I was learning more and more about this that my understanding of planets was severely lacking. This, along with the fact that all four of the giant planets have planet-sized moons, really brings home the fact that the giant planets are essentially and fundamentally different from the inner planets. Understanding this helps me to understand how it is the dwarf planets can be thought of as so different from the rest of planets, since the planets themselves are so different from each other.

The giant planets only kind of orbit the Sun

In school we learn that the planets orbit the Sun, and that a long time ago people thought everything orbited the Earth, but later on they realized they were wrong. Well the truth is, they were wrong, but it's still more complicated than simply saying that the planets orbit the Sun. The inner planets definitely orbit the Sun directly, meaning that the center of their orbits is actually inside the Sun. But the outer planets actually orbit a point in space outside of the sun, somewhere between the Sun and Mercury. How can this be? Well, the reality is that Jupiter is so massive that its gravity actually pulls on the Sun, making its own rotation wobble back and forth, similar to the way Pluto and its moon Charon orbit each other. Although the Sun is unquestionably the center of the solar system, the pull of Jupiter makes it so Jupiter and everything beyond it actually orbit an empty point in space, somewhere above the surface of the Sun (referred to as the solar system's barycenter).

In other solar systems there are sometimes two stars that orbit each other, with planets orbiting around both of them together or around each of them individually. When it comes down to it, our solar system is actually more like this than we might think, with the majority of objects orbiting the sun, but a number of objects ranging from tiny asteroids to planet-sized moons orbiting Jupiter and the other giant planets as well.

The asteroid belt is a sparse, lonely place

In just about every space-themed TV show or movie, at some point, the characters end up flying into an asteroid field, where they have to carefully dodge and sometimes even blow up the hundreds of asteroids that are all just rolling around crashing together in space. The reality is that most of the asteroids in the asteroid belt are actually so far apart that if you were standing on one, you wouldn't even be able to see any others without a telescope. Even asteroids whose orbits are relatively close together are hundreds of thousands of kilometers apart, and much of the time, they're at opposite ends of their orbits, so they're even further than that!

On top of all that, asteroids just aren't that big. Asteroids fly by Earth every single day (there's even an official online newspaper with daily asteroid headlines) but most of them are so small that we can only see them with professional telescopes. The biggest asteroid, Ceres, is less than one-tenth the size of the Earth, and just over a fourth the size of the moon. Even if you smashed the entire asteroid belt together in one place, it would still only make up a mass 4% that of the moon; it would actually still be smaller than Pluto.

Over 97% of the objects in the solar system were discovered after the year 2000

When I was born, there were around 10,000 known objects (planets, moons, asteroids, etc.) in the solar system. Because our technology gets better and better every year, we discover more and more solar-system objects every year. By the end of December, 1999, we had more than doubled the number of known objects, with around 20,700 objects discovered at the end of 1999. As of the time of writing, there are over 700,000 known objects in the solar system. (717,768 to be exact. You can actually check the exact current number in the right-side menu on this page; look under Minor Planets Discovered, All Time.) This means that literally, over ninety-seven percent of everything we know about in the solar system was discovered after the year 2000

The take-home

I could say a lot more, but I'll stop there. With new discoveries being made every year, the theories and understanding that were held by the best scientists just a few years ago are already largely obsolete. Just this last year, compelling evidence was presented indicating that there's another full sized planet out beyond Pluto, and possibly even an Earth-like planet orbiting the nearest star. There are undoubtedly more things out there that we haven't even begun to imagine yet, both in our own solar system and beyond.

What space-related facts surprised you? Share in the comments!