When it comes to our celestial neighborhood, the two outer planets that everyone will likely be aware of are Neptune and Pluto. Circling the furthest reaches of the outer solar system, these two lonely planets are worlds apart from each other. In this article, we consider why won’t Pluto collide with Neptune.
Okay, technically, only one of these is a planet. Pluto, long since reclassified into a dwarf planet, is so small, that it lost its title as ‘the ninth planet’. It’s so small, that many moons across the solar system are bigger. However, it is no longer classified as a planet because it hasn’t cleared its orbit of other objects.
With such a tiny little celestial body being the next closest object to Neptune, that might raise a few questions in some people.
For example, if Pluto is so small, why hasn’t it been caught in the orbit of Neptune yet? Or, to take this line of questioning another step further, why hasn’t it collided with Neptune? This is especially interesting as at times Pluto is closer to the Sun than Neptune.
There are many reasons for this astronomical phenomenon to remain on the pages of academic textbooks or in science fiction and not an observed celestial event.
And that’s what we’re going to cover in this article! Here, we’ll explain some of the reasons and physics behind why these two (one, sorry Pluto) planets haven’t become a single body yet, in an easy-to-understand format.
There’s a lot of science to cover, so let’s get stuck into this cosmic conundrum!
Two Star-Crossed Planets
So, why haven’t these two planetary objects been on a collision course before? And is there a chance that it could happen in the future?
Well, to tell you the truth, our current understanding of how physics works suggests that these two plants colliding with each other are about as close to zero as numbers ever get when dealing with astrophysics.
And the answer is pretty simple once you consider the vast scales we’re dealing with when talking about these outer planetary bodies.
There’s just too much space!
It’s A Big Solar System Out There
Probably the significant factor that most people will be able to notice is the sheer size of Pluto and Neptune’s orbit around the sun.
Remember that the outer planets of our solar system (Uranus, Neptune, and Pluto) were not always known to us, like the other planets of our solar system.
These planets are so far from the center of the solar system that they are invisible to the naked eye.
So, while we have known about the planets from Mercury to Saturn for literally thousands of years, it was only in the last 250 years that we discovered the last 3 with powerful space-pointed telescopes.
Neptune has an average orbit of around 2.8 billion kilometers from the sun. This almost a billion more than the next closest planet, Uranus.
Pluto, meanwhile, has an orbit even greater than this, being 3.2 billion kilometers away, another 500 million kilometers. You may also be interested to read more about how far Pluto is from the Sun.
This means that their overall orbit times are very different. While Neptune takes around 163 Earth years to complete its orbit around the sun, Pluto takes 248 years.
This gives very little opportunity for these planets to be anywhere close to each other, certainly not enough for their gravitational pulls to affect each other.
Elliptical Orbits
It can be very easy to imagine that every planet’s orbit is a perfect circle. It’s certainly something that looks good on a diagram or a planetarium!
However, the fact is that most, if not all, celestial bodies have an elliptical orbit, meaning that they are more oval-shaped than circular.
This also affects their overall distance from the sun. Sometimes, their orbits will take them noticeably closer to the sun, whilst much further away at other times.
For Neptune and Pluto, they are even less likely to have a point where their orbits come even remotely close!
Different Axes
No, not the woodcutting kind, we mean the angle of their orbits kind!
You’ve also got to consider the axes of these different planets when considering whether they can collide. It might be the most critical factor.
Similar to how we view planetary orbits, thanks to diagrams, we also think of planets as all on the same plane or level.
But, in 3-dimensional space, this is not the case. Many planets have axes that are titled much more than others, meaning that they are traveling in almost entirely different directions in their orbit.
Pluto’s axis, for example, is at a 120° angle, while Neptune’s is at 28°.
A Close Call?
Some folks reading this article may be old enough or interested enough in the subject and might remember that these two celestial bodies did cross orbits at one point.
In February 1979, Pluto crossed into and passed the orbit of Neptune, making it, for the time anyway, the 8th furthest planet from the sun, and Neptune being bumped down to 9th (this was still at a time when Pluto was considered a planet).
This changed back in February 1999, when Pluto moved outside of Neptune’s orbit.
If you’ve been paying attention to what we have covered so far, you’ll likely understand that there was virtually no way for these planets to ever actually meet.
Their axes, coupled with their different elliptic orbits, mean that there was no way that they were remotely close enough to affect each other.
Frequently Asked Question
Why Isn’t Pluto Considered A Planet Anymore?
Pluto was officially declassified as a planet in 2006 because it did not meet the 3 criteria that scientists agreed on:
A planet must orbit a star.
A Planet must have enough mass to pull itself into a spherical shape.
A planet must clear its surrounding space of other objects.
Pluto meets the first two, but thanks to the larger dwarf planets that surround it not being pulled in or moving out of its orbit, Pluto fails on that last account.
Final Notes
So, considering everything that affects these planet’s orbits, from their huge elliptical orbits to their axes, there’s next to no way the two can meet.
Still, you could always cross your fingers and wish for a shooting star. Maybe it’ll knock into Pluto and change its orbital path.
Hi, my name is Jason Anderson, and I am a Physics Professor. Ever since I was a kid, I’ve been fascinated with space, the universe, the moon, you name it. I spent hours and hours at the planetarium close to my hometown, wondering what else could be out in the universe.
Since then, I’ve been an avid stargazer and astronomer, and love nothing more than spending my time charting stars, observing planets, and finding constellations.
This is why I decided to start Telescope Guru. I only wish to share this fun pastime with the world. With this site, I hope to answer all of your questions relating to astronomy, telescopes, and stargazing.