We have all seen the planetary model of our solar system since our school days.
The planets are shown to revolve in a nice circular path, forming concentric rings, with
the Sun sitting smack-dab in the center.
While this vision of planets following a nice circular path around the Sun looks neat and
tidy for a school project, in reality, things aren’t that simple.
The planets in our solar system don’t really revolve in perfectly circular orbits.
Instead, they generally have a STRETCHED version of a circle, which is technically called an
ellipse.
How stretched?
Well, that depends on the orbital eccentricity that the planets possess.
Too much mathematics going over your head already?
Well, let’s get the basics down first!
An ellipse is a symmetrically shaped closed oval.
It has two points, called foci, which act as a combined center for an ellipse.
This ellipse has an important property called eccentricity.
Simply put, eccentricity tells how much the ellipse deviates from the perfect circle.
Mathematically, the value of eccentricity varies between zero and one; with zero representing
a circle and one transforming into another shape, called a parabola.
For planetary objects, when orbital eccentricity crosses 1, the planet no longer remains confined
to the star it is bound to and “escapes” from its gravitational influence.
Now, let’s focus on another question: How much of an elliptical path do planets in our
solar system have?
Or, technically speaking, what is their orbital eccentricity?
Well, as you can see, it is quite evident that the orbits of most planets aren’t very
elliptical.
Mercury is relatively more elliptical than the rest, yet it is still closer to zero than
one, meaning that it is closer to a circle than an (extreme) ellipse.
To those wondering what an “extreme ellipse” could be, there is a great example to look
at—Halley’s comet.
Halley’s comet follows a path with an eccentricity of approximately 0.97.
Just look at how elongated its elliptical path is!
But the question still remains: why aren’t the planetary orbits perfectly circular?
Well, for that answer, we need to go back in history.
For a long time, from Ptolemy to the age of Copernicus, it was widely believed and accepted
that planets revolved in a perfectly circular path.
This was until Kepler came in and laid down important laws pertaining to planetary motion,
with the first one being that planetary orbits are elliptical.
But why elliptical?
Well, in an “ideal” Universe, all orbits would have been “circular”.
In fact, some orbits are perfectly circular, but those instances are very few and far between.
For a perfectly circular orbit, the orbiting planet would need to have mass, velocity,
and distance from the star that precisely matches the gravitational influence of that
star.
A slight change in any of these would derail the planet from maintaining a perfectly circular
path.
For example, if the mass of a star or planet changes, or any other celestial body whizzes
past that planet, it would disturb this delicate balance of mass, velocity, and distance that
keeps a planet in a circular orbit and would change it to an elliptical one.
Even a small tweak in these status quo conditions or any interplanetary interactions would change
the path from a perfect circle.
Despite fretting about orbits being not perfectly circular, it is worth understanding that these
orbits are still closer to being circular than they are to being highly elliptical,
which is perhaps why the planets we see today have survived the test of time.
Planets with highly elliptical orbits are more likely to run into trouble, as compared
to a circular counterpart.
Revolving in highly elliptical orbits makes planets vulnerable to frequent gravitational
interactions and nasty impacts.
Why is that?
Well, think about it like this, the planetary model taught in school, consisting of planets
following circular paths, naturally stacks one planet above the other without crossing
the path of each other.
However, in a case when planets have a highly elliptical path with varying eccentricities,
those orbits are more likely to cross paths with one another in potentially catastrophic
ways.
In short, this makes planetary bodies more susceptible to collision and impacts.
In the aftermath of an impact, a couple of things could happen.
One is that both the colliding objects could break and disperse off into pieces following
the collision.
Or they could simply merge into a single larger entity.
Many astronomers reckon that this sort of activity has been happening in our solar system
for billions of years.
Planets that exist today in our solar system aren’t the only ones that have come into
existence since the birth of our Sun.
However, they are probably the only planets that have endured such cosmic impacts.
The nearly circular orbit with which eight planets in our solar system revolve has certainly
helped them in their quest for survival.