Why Are Planetary Orbits Elliptical and Not Circular?

HEY GUYS NOW IT'S TIME FOR SOME SPACE STUFF.

For many children, a popular science project consists of making dioramas of the solar system, with painted styrofoam balls for planets and orbital paths made of wire. To this day, when most adults think of the solar system, they imagine a group of concentric rings, with the furthest planets on the largest circular ring and the Sun smack-dab in the center.

While that makes for a neat and tidy project, it isn’t actually correct. The orbits of the planets in our solar system (and the vast majority of planetary objects in space) are actually elliptical, not circular.

However, since orbits are repetitive patterns based on gravity, inertia, and mass, how can they be anything but a perfect circle?

The 4 Types of Potential Planetary Orbits

The basic science behind orbits is that two objects with mass will have a gravitational attraction towards one another, thus affecting their movement through space. This is a basic principle of astronomical physics. We typically see orbits with one large object and one much smaller one, so that the large one appears relatively still, while the smaller one “orbits”.  To understand orbits, you also need to consider the energy that both objects bring into the system, and the effects that will have on the shape of the orbit.

Let’s take our Sun, for example. When an object approaches the Sun, depending on its energy and trajectory, it will follow one of four possible orbital paths:spiral, hyperbolic, elliptical, or circular.

The spiral option means that the object will be drawn in at a steep angle by the Sun’s gravitational pull, perhaps because it is very low in mass or energy. The object will fall into a tight spiral around the Sun, which can hardly even be called an orbit, dropping lower and lower until it impacts the surface. 

The hyperbolic option occurs with objects possessing a great deal of speed or distance from the Sun’s surface. The object will approach, and its path will be bent in towards the Sun, but its speed and distance allow it to continue past the Sun, and not be pulled into a repetitive orbit. After forming a hyperbolic orbital path that resembles a U, it will fly off into space, and will never return, unlike the last two orbital options.

The circular option is what most children imagine the solar system to be, and while some planets closer to the Sun form nearly perfect circles (the Earth is only off by 3 degrees), a truly circular orbit is very hard to achieve. The conditions have to be absolutely perfect, namely that the energy coming into the system creates an orbit with absolutely no eccentricity, which is possible, but very rare.

via GIPHY

The elliptical orbit option is what all the planets in our solar system follow, and it makes sense why this type is far more common than perfect circles. When an object is too small or slow to escape the gravitational pull of the Sun, it falls into a repetitive elliptical orbit that is largely dependent on its original energy and trajectory when it entered the system. The orbit can also be affected by the gravitational effects of other orbiting planetary objects, as well, making it imperfect, eccentric, and highly dependent on other factors.

Physics Prefers Ellipses…

Imagine it this way: a planetary object soars by the Sun at a high speed; at this point, it only has its own velocity that it gained during the explosion when it was first created. As it passes near the sun, a new force i.e. the gravitational force of the sun acts on the object and starts to pull it in its direction. But as it falls towards the sun, a new component gets added; this is the velocity because of acceleration due to gravity. This component, combined with the initial velocity that a planet has, keep it from falling into the sun and give rise to an elliptical orbit.

In short, a planet’s path and speed continue to be effected due to the gravitational force of the sun, and eventually, the planet will be pulled back; that return journey begins at the end of a parabolic path. This parabolic shape, once completed, forms an elliptical orbit.

Inertia and gravity must combine in impressive fashion for any orbit to occur, and given how many other factors can affect the velocity and path of an orbiting object (e.g., other sources of mass/gravity), a circular orbit is just highly unlikely.

However, if you decide to become an astrophysicist, perhaps that can be one of your career goals… finding as many perfect circular orbits as you can!