There is a power in our solar system. A hidden, unseen power that has shaped our Solar System and I don’t mean the Free Masons or the Illuminati! This force has helped make life on Earth possible, and lies at the very foundation of our stellar neighbourhood. The power I am talking about is not some mysterious force, in fact it is in itslef not really a force at all. In fact it does not even exist in the sense of having a physical from that we can touch or measure with our instruments. Not directly at least. And yet this force, or maybe I should call it a phenomena, has had an almost incalculable effect on the way our solar system looks today, how other planetary systems are arranged, and is probably one of the most important contributing factors to how we came to exist.
At its most basic it is no more than a mathamatical curiosity, so let’s look at what it is and how it has such a huge effect. The force I am talking about is a little thing called “Orbital Resonance”. But what is Orbital resonance, and why is it so important? Well lets’s start by looking at the Solar System we know so well. At its most basic it consists of a bunch of planets and other bodies orbiting the Sun, and a lot of these have moons orbiting them. All these bodies orbit the Sun, or their planet in the case of moons, in regular orbits that take a certain period of time, for us it is about 365 Earth days. But this is a vastly simplified version of the truth, many of these orbits have close relationships, not just to the primary body, but to each other.
Let’s have a closer look at this relationship. If a planet orbits its star in one year, and the next planet out orbits the star in 2 years, then they are said to be in a 2-1 resonance. Big deal. Well actually it is a very big deal. Each time planet 1 has gone round its star twice, planet 2 has gone round once and they line up. What this means is that over time, they pull a little bit on each other more than would be the case if their orbits were not in resonance. The small gravitational tugs they give each other are rythmic and at the same place relative to each other, instead of being random and this makes a big difference over time. I’ve used this little animation before but I will use it again because I think it illustrates what orbital resonance looks like.
It’s worth watching it for a few minutes to really see what is going on, enjoy! These small nudges build up over time and can achieve something remarkable, they can make giant planets like Jupiter and Saturn move through the Solar System, forcing other giant planets out of their orbits and sending a barrage of asteroids crashing into the inner planets. It may seem incredible that small gravitational “nudges” can move a giant planet the size of Jupiter but it seems very likekly that this is exactly what happened. You see there is a problem with our Solar System. Mars is way too small compared with the other planets, and what caused the Late Heavy Bombardment, a huge barrage of asteroids that pummelled the inner Solar System 4.1 to 3.8 million years ago? And why do we have an asteroid belt and not a planet between Mars and Jupiter?
According to the Grand Tack model, Jupiter and Saturn were formed much closer than they are now and that because of the large amounts of gas and dust in their orbits they got slowed down and started moving closer to the Sun, first Jupiter which pulled Saturn with it. As Jupiter gets to roughly where Mars is now, it falls into a 3-2 resonance with Saturn and this pulls it back out again, taking Saturn with it. Jupiter and Saturn’s migration disturbs Uranus and Neptune so that these also get pushed further out and actually swap orbits! This means that they are all interacting gravitationally with the left-overs from the formation of the Solar System, and by left-overs I mean millions of lumps of rock and ice. This results in total chaos, with the orbits of these “lumps” being disturbed so that they get thrown all over the place.
Another, not insignificant, result is that Earth got pelted with water-rich asteroids which might explain where Earth got its water, and not just water, millions of tons of organic compounds, the building blocks of life.
This also means that there is not enough “stuff” in Mars’ orbit for that planet to get any bigger, and that the whole Solar System takes a beating from all the projectiles flying about. It is also thought possible that one or more large, even giant, planets received such a gravitational shove that they got thrown out of the Solar System completely. In fact in recent years we have discovered Brown Dwarfs, objects somewhere between a gas giant planet and a star, floating alone through interstellar space, possible victims of similar events in their original planetary systems, and as our telescopes get better we might one day find planets wandering alone between the stars.
This drawing illustrates the idea very nicely
In other planetary systems we have found gas giants extremely close to their parent stars and this should not be possible. Gas giants can’t form that close to a star because it is simply too hot, so how did they get there? The answer is probably planetary migration where gas giants migrate inwards but haven’t fallen into an orbital resonance with another giant planet that can pull them out again.
Even today it seems that orbital resonance is having a surprising effect on some otherwise insignificant objets in our Solar System. For example, Jupiter’s moon Europa is in orbital resonance with the other Gallillaen moons and this, combined with Jupiters powerful gravity is pulling it out of shape and then letting it fall back again at regular intervals. On Earth it is the Moon’s gravity that pulls at our oceans,
So an apparently insignificant little phenomena actually turns out to have a profound effect on the development of planets and planetary systems and ultimately on the development of life itself. If it wasn’t for resonant orbits Jupiter and Saturn would be very close to the Sun as we see in other systems with Hot Jupiters, and Earth might not exist at all or if it did it might have been a much smaller planet. I have only listed a few of the resonant orbits that exist but I hope I have uncovered a little bit of this unsung hero of our Solar System.