The solar system is often depicted as a closed entity, but how big is it actually? It's a simple question that is difficult to answer.
The Earth is 150 million kilometers from the Sun. The borders of our solar system, however, are so much further away.
So far that we can't see them with telescopes anymore. So, there are objects beyond Pluto, dwarf planets you have probably never heard about.
By Matus Malina

In this article, we are going into space, but not that far away. We are often too focused on what is beyond our own solar system, despite having hardly scraped the surface of what our own solar system has to offer.

So today we’ll talk about our nearest neighbors.

To enlighten us on the topic, we’ve talked to Mads Fredslund Andersen, a telescope and satellite manager at the department of physics and astronomy at Aarhus University.

How Do We Define the Solar System?

The solar system is our home.

We feel comfortable knowing where we are and what is around us. However, most people believe our solar system consists of the Sun, eight planets and a lonely dwarf planet named Pluto, which is an inadequate definition.

It turns out there are fascinating objects that do not get mentioned in the classroom. Objects like asteroids, space dust and more dwarf planets like Pluto.But before getting ahead of ourselves, let us start by getting things straight.

So, what is the definition of the solar system?

There is actually no absolute definition of what the solar system is and where it ends. There are, however, two perspectives on how we can answer these questions.

Either by gravity or solar radiation.

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"That's one of the reasons why it's not very easy to say where the border is, because it depends on the definition."
- Mads Fredslund Andersen

The first perspective is that the solar system is gravitationally bound to the Sun. As a result, it is in orbit around the Sun. But the gravitational force is set to extend so far that it’s midway between our closest neighbor star, Proxima Centauri, which is four light years away.

That is extremely huge.

Under that definition, the radius of our solar system could extend two light years away from the Sun. That is approximately 18 trillion kilometers.

Yes, that’s an 18 with 12 zeros behind it.

The second description, by solar radiation, is, luckily, a bit more moderate.

It is where the quantity of solar radiation equals the amount of cosmic radiation. So, essentially, it is the point at which the energy from our Sun matches the energy levels found in the vacuum of space.

That is typically what is meant by the solar system’s borders.

A point that very little light and energy from our Sun can reach. This distance can be estimated, but it is constantly changing. As a result, it varies with the solar cycle and other factors.

That basically means that all the posters we’ve seen with the planets on them are a little misleading. It represents a closed system that concludes with the eighth planet (plus Pluto). But that is not the case.

However, it kind of represents the solar system from our own perspective. It is our interpretation of reality, which of course changes every time we learn something new. And we still have not discovered everything that lies beyond the poster.

So let us take an outside-in approach. Starting from the furthest edges of our solar system, the unknown zone, slowly moving closer to the Sun into more familiar territory.

What is at The Edge of Our Solar System?

Right at the borders of our solar system is something called the Oort cloud, which is essentially a cosmic junkyard.

It is located roughly at the gravitational border of the solar system. These space particles are the furthest material still bound to the Sun by gravity. As we mentioned previously, that is approximately two light years away.

You may call it “remains of the early solar system” or “things that didn’t get picked up by planets or fall into the Sun.” It is mostly made up of comets and space dust.

So, there’s quite a bit of stuff floating around. In reality, we have no idea how much there is, but it could extend well beyond anything we will ever see, as it may be “unobservable” from a distance.

But why is that?

We can see distant stars because they emit light. This mass, like the dwarf planets, asteroid belts and the Oort cloud, doesn’t shine very brightly. It only reflects solar radiation, which can’t be seen at that distance.

For example, Pluto cannot be seen with the naked eye in the sky, as it is too close to the solar radiation border. This means there is very little solar light for it to reflect at that distance, ultimately, making it harder to observe for scientists on Earth.

So, the radiation border ends a little beyond Pluto, making it difficult to observe anything beyond that point, and the Sun’s gravity reaches all the way to the Oort cloud, attracting objects.

What is then in-between these two borders, orbiting the Sun in almost complete darkness?

The Gap Between the Oort Cloud and Pluto

Scientists began to discover new objects that were similar in size to Pluto but further away in the early 2000s. There aren’t many, but there are at least five or six, and possibly a few more we haven´t seen yet.

This meant that the definition of a planet needed to change.

And it wasn’t clear if it would have to be changed once or several times. There was also no way for scientists to tell how many planets would be found later.

So, a new definition of a planet was created.

A planet must orbit the Sun, be round, not produce light and lastly, it must have cleared its orbit. That means it must have collected all the material in its trajectory; there can be nothing in its way. That is why Pluto and the other dwarf planets are not regarded as planets.

Pluto, for example, is located at the rim of the Kuiper belt, an asteroid belt that roughly marks the solar radiation border. That implies that there is a lot of material and dust in its orbit. As a result, it is constantly being bombarded with asteroids and has not picked up all the mass.

Earth was once likely in an asteroid belt, but over billions of years it absorbed this space debris with gravity, clearing its orbit. All eight planets have done it, meaning we can call them planets.

On the other hand, it means that these dwarf planets are still gathering and collecting mass from asteroid belts, which technically means they are becoming bigger.

Keep that argument in mind if you think that Pluto should be called a planet. Because in millions of years, it just might slowly become one.

It is understandable why we do not have a clear picture of our solar system beyond Pluto. It is dark, far and therefore very hard to observe.

But even the parts closer to us are not completely understood, with a lot of today’s knowledge based only on theoretic understanding.

The “Well-Known” Parts of Our Solar System Are Not That Well Known…

There are multiple theories on how our solar system formed and explanations to why the four rocky planets are closer to the Sun than the gas planets further away.

We have four rocky planets closer to the Sun and gas planets farther away from the Sun, because water in solid form may exist in the outer areas, allowing larger planets with atmospheres to be created. The gas giants.

However, it is difficult to verify.

We can’t send a space probe to one of these gas worlds and then collect a sample to find out. Based on the modern understanding, there is not a surface to land on, at least not in the way we would understand the word “surface.”

So, it is only a hypothesis.

There could, however, be a solid surface beneath the atmospheres of the gas giants, but it’s incredibly compact. So, it’s a different kind of surface than we’re used to seeing on Earth.

The outer regions of their atmospheres are a gas, but the closer to the center it gets, the more compressed the gas becomes. This gas slowly transitions into the liquid state and likely into a solid when extremely compacted. It is more of a slow gradient transition from gas to liquid to solid, instead of the type of surface we know.

In fact, we know very little about objects such as other planets in our solar system. Depending on how scientists quantify their masses, sizes and so on, there are assumptions about what must be within. So, it is an educated guess about what they are made of.

"Of course, early on, we did use binoculars to understand our solar system. Now it's mainly sending missions to do deep investigations of several planets. So, we have had missions to almost all the planets. But not all of them have touched ground, because, for instance, the gas giants, we can't."
- Mads Fredslund Andersen

On only a few planets have there been attempts to measure what’s inside.

More thorough measurements, like seismographs which measure the vibrations below surface and can be used to deduce the composition, have been performed on Mars and Venus. All other planets, we have only been able to study from afar. This includes, for example, space probes that fly by the planet and provide us with data to support or reject various theories.

You may feel like there are no more surprises in the familiar parts of our solar system, but we are still missing out on some fascinating objects in our close neighborhood.

Bear with me, I bet you do not know what is coming next.

What Other Interesting Objects Did Your School Leave Out?

Aside from the five or six dwarf planets that orbit our Sun beyond Pluto. There is one dwarf planet in particular that is very close to Earth. Despite this, it is still very unknown to most people.

You may think it is because it is a new discovery, but we have known about this dwarf planet since 1801.

That is earlier than Pluto, yet it still gets left out despite being equally as intriguing.

Although it is smaller than Pluto, less than half as big in diameter, it is still rotating around the Sun rather than around another planet, such as a moon. Its name is Ceres, and it hangs around in the asteroid belt between Mars and Jupiter.

I bet your school did not tell you about that one!

A mind-blowing discovery from Ceres was that when NASA’s Dawn spacecraft passed this giant asteroid in late 2017, it appeared to be geologically active! Brine, salty water, seems to be seeping onto the surface from the depths of this dwarf planet that is less than a third of the size of our Moon.

Since we find it so fascinating to find water elsewhere than on our own Earth, the moons of some of the larger planets deserve further attention. Jupiter and Saturn have many moons, some of which are coated in ice!

Water is required for life as we know it, so scientists look for signs of it on exoplanets, worlds outside our solar system. However, few people are aware that inside our own solar system, there are moons that are essentially comprised of frozen water.

Mads Fredslund Andersen thinks it would be interesting to land on these moons and see if there’s anything underneath the ice. It seems likely that there may be an ocean underneath the frozen surface, but could there be something more?

There will always be a level of uncertainty and mystery until you can literally “go there and see for yourself.”

Ever since I finished high school, our understanding of the solar system has changed drastically, discovering multiple dwarf planets. Still, experts keep searching, expecting to find more hidden treasures in our own solar system, claiming that there still is so much we don’t know.

It might take many more years before we even get close to a definitive model of the solar system.


If you are still hungry for more knowledge about the solar system, the planets within and whatever lies beyond, tune into our podcast. On Big Ideas Only, we go further down the rabbit hole and discuss many more interesting topics, such as how our solar system came to be and what was before.

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