Before we begin
Hello. I’m glad you could make it. If you’re reading this, chances are you quite like space, just not enough to dive into a university level textbook. The aim of this course is to furnish you with lots of interesting astronomy knowledge,
totally almost maths-free. As a result, many explanations will be simplified, some more than others. Each installment there’ll be an “extra credit” section at the end, in case you fancy getting a bit more technical, or would like to explore some slightly more complex topics, but if you’re after a more rigorous treatment of anything covered here I’d advise you to google a more academically-minded source; these are firmly intended as “casual” reading. Additionally, any definitions I think you might need shall be provided via clickable footnotes.1 I thought it might be nice to start this series with an exploration of the part of space with which we are most familiar: our solar system.
You are here
As you may already know, our solar system contains eight planets, all orbiting the Sun; however, something less commonly considered is that they show a surprising amount of order. They all orbit in the same direction (which is also the direction the Sun rotates), and all more or less sit in the same plane in space.2 The four inner planets are small and rocky, while the four outer ones are giant, icy and have large gaseous envelopes3. What we’re going to consider in this post is why, and to do that we’re going to have to look at how it all formed.
In the beginning…
Our solar system started as a spinning cloud of dust and gas4. As the cloud began to collapse under gravity, it span faster, due to conservation of angular momentum.5 The spinning caused it to collapse into a disk, because the existing angular momentum stops the cloud collapsing inwards, but there’s nothing stopping it collapsing onto the plane of rotation6. Imagine spinning holding a ball on a string – it’s easy to move it up or down, but harder to pull it towards you.
Much of the mass in the cloud is concentrated at the centre of the disk, where collisions between particles cause them to heat up. Once the temperature hits about 10 million degrees kelvin7, the collisions between gas particles become sufficiently energetic for nuclear fusion8 to begin, and the host star is born. This “spinning disk” origin explains why all the planets orbit in the same direction and the same plane.9
What happens next?
So far we’ve arrived at a spinning disk of hot gas and dust, and a (proto)star10 in the centre. Over time, the gas cools and molecules begin to condense into small solid grains. These then collide to form larger clusters, which eventually form clumps of rock called planetesimals.11 Planetesimals are large enough to attract each other under gravity, rather than pure chance causing collisions, and these eventually coalesce into planets.
Terrestrial planets and gas giants: the frost line
Close to the host star, due to the high temperatures only heavier elements can condense into solid particles. This means there are fewer grains around to form clusters, so the inner planets form slowly. By the time they have, most of the light gas in the disk (hydrogen and helium) has escaped, which is why the inner planets lack large hydrogen/helium envelopes. On the other hand, further out in the disk the temperature is low enough that ices (mostly water, ammonia and methane) can form on the grains; the distance from the star at which this happens is known as the frost line. This makes the grains much more massive, allowing planets to form relatively quickly.12 As a result, the outer planets are large enough early enough to gravitationally attract and hold a large amount of gas, which is why they have an icy core and a gas envelope.13
Why is the initial cloud of gas and dust spinning?
This is the first question that occurred to me when I learnt about this process. Somewhat frustratingly, the answer more or less amounts to “it just is”. If you sum up all the motions of its constituent particles, the cloud is always going to have some small overall momentum. This becomes obvious when you consider the opposite: for the cloud as a whole to have zero net motion, the random motions of all the particles would have to cancel each other out, which is an absurdly unlikely and extremely unstable configuration.14 Thus, the cloud can be thought to have some small overall motion, which gets greatly amplified as the particles in it collapse towards the centre.
How about moons, asteroids and comets?
Asteroids and comets are essentially “leftovers”: large bits of debris that never made it to being a full-size planet. Dwarf planets like Pluto are another example of this15 Moons have two possible origins: Either they formed out of the same collapsing region of disk as their parent planet (in which case they’re expected to show the same sort of coplanar, ordered orbit), or they’re captured leftovers, sometimes additionally fragmented by collisions (in which case they’re expected to show more distant, inclined or eccentric16 orbits like our own moon.)
How do we explain observations of giant planets orbiting close to their parent stars?
These are generally Jupiter-sized objects observed much, much closer to their host starts than would be expected from the simple model described above, and are therefore termed “hot Jupiters”. They end up migrating via gravitational interactions with nearby dust and planetesimals (they pull on the matter around them, the matter pulls back17).
Where do we go from here?
As the course progresses, we’ll be starting in the centre of the solar system and working our way outwards. Next time, we’ll taking a closer look at the Sun (and stars in general), followed by the inner planets and so on. Stay tuned! (If you want to be notified of new posts, as I don’t quite have a schedule yet, feel free to follow the Facebook or Twitter accounts and I’ll post there when the next installment is ready.)