Why Are All the Planets Orbiting the Sun On the Same Plane? (And Why They Might Not)

When you see a model of the Solar System, you will notice that all eight planets of our Solar System orbit on (nearly) the same plane. And you might have even taken this for granted since it’s presented this way for almost every illustration of planetary systems. But have you ever wondered why they are orbiting the Sun on the same plane? Why aren’t their orbits tilting in every direction? Let’s explore this in this article.

To explain this, we first need a quick overview of how the Solar System forms. 4.5 billion years ago, a cloud of gas and dust in the Solar nebula, possibly under the influence of an external event like a supernova explosion, collapsed under its own gravity. As it collapsed, most of the mass is concentrated in the center, which drew more material with its stronger gravitational attraction. At one point, it collected so much mass that nuclear fusion begins at the center of the Solar nebula, and the Sun was born.

But what about the residual materials? The planets are formed from the materials (including heavier elements, for example) leftover from the formation of the Sun. From every possible direction, the particles are attracted by the Sun’s gravity, with its angular momentum in its orbit around the Sun. But from events like collisions, most of the angular momentum essentially cancels out, and you’re left with the net angular momentum of all the materials it started with. Those small random variations in the distribution of mass, causing a small net angular momentum (compared to the mass collapsing into the planetary system), will determine the plane in which the planetary system “takes”, from countless collisions and conservation of angular momentum.

In fact, the plane the planets orbit on is often close to the plane of rotation of the star. This means that most of that “collisional damping”, where the spherical bundle of materials is flattened into a disk, occurs before the star even takes shape and begins to have a clear, dense structure.

But sometimes, orbits of objects might actually tilt everywhere in the planetary system. Take our own Solar System for example. While none of the eight planets are more than 7 degrees from the plane of our Solar System, there are lots of asteroids that orbit in highly inclined trajectories. A particularly large object orbiting in such a path is the asteroid 2 Pallas, which orbits the Sun with about 34 degrees of inclination. Many Kuiper belt objects are also highly inclined, with the dwarf planet Eris having about 44 degrees of inclination. There are many asteroids on these kinds of highly inclined orbits as well, with some even orbiting in retrograde orbits. They orbit in the opposite direction as the rotation of the Sun.

It doesn’t end here: in other planetary systems, there are entire massive exoplanets orbiting in orbits of high inclination. A common type of these inclined orbits seem to be polar orbits, where the inclination is close to 90 degrees.

One reason for that is simply the timescale of planet formation and dispersal of the planetary disk. The planets take up most of the remaining material from the star formation. By the time planets are formed, the particles of the planetary system are sparse enough that collisions don’t occur as frequently. At a stage where the fine particles of the planetary disk are pushed out by radiation pressure and are no longer available for accretion, the collision process almost stops compared to the timescales in the beginning of the formation of the system.

You can think of the amount of particles not being on the same plane as the planets as something like a 1/x curve — as time elapses there are fewer and fewer particles with highly inclined orbits. But at the same time, fewer interactions between them occur, so the rate of reduction of that amount also decreases. Ultimately that amount never reaches zero, which means some massive particles, in the form of asteroids, must have been in a highly inclined orbit.

Another reason, especially applicable to larger objects like planets, could be gravitational disruptions or interactions after formation. You can think of it like gravity assists of space probes — they use the gravity of planets to send them onto different trajectories, like the Ulysses spacecraft, where a flyby of Jupiter sent it from a normal orbit close to the ecliptic to a polar orbit, with an inclination of close to 90 degrees. Likewise, when these objects make close approaches with massive planets — especially near the poles, can significantly change the inclination of the smaller object.

But what about those whose protoplanetary disk itself is tilted compared to the star’s equator? Flybys of nearby stars might play a role in this. Stars often form in clusters, and in the early stages of star formation, two stars might be sent close together. This can have significant effects on the planetary systems of both stars, and in the case where protoplanetary disks are involved in one of the stars, it can excite a whole section of the disk as well, causing the disk to tilt or the objects to orbit with different inclinations.

Other than short-term instabilities, long-term interactions could also cause this phenomenon. The Kozai-Lidov mechanism, where a third massive body perturbs a two-body system from the outside. This can lead to very significant changes in the orbits of the objects, causing dramatic changes to inclination and eccentricity, and even turning a prograde orbit into a retrograde one.

In this article, we’ve talked about how the formation of planetary systems tends to align planets on just about the same plane. Through numerous collisions of particles, the whole system collapses towards one plane with the conservation of angular momentum. But many different types of events and causes, from random variations in formations to gravitational disruptions, can change that picture and cause some of the objects to orbit in highly inclined orbits.

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