The Edge of the Solar System

The Edge of the Solar System

In the past century, we have learned incredible amounts about our solar system. But what lies at the very edge? Darkness? A portal to another dimension? Or perhaps the point at which our solar system merges into the other parts of the universe, unaffected by the Sun at the center?

Now, after decades of sending probes into space, we are finally beginning to get some answers.

First, let's talk about the structure of our solar system. The inner part of our solar system is fairly well understood, because well, our planet lies within it. There's the Sun at the center, a planet that orbits it, billions of asteroids and comets that are held in its gravity, and masses of gas and dust that swirl around close to the Sun.

Our four rocky planets, Mercury, Venus, Earth, and Mars, lie within the inner solar system. Beyond Mars, there is actually room for another planet, and a material present that could have formed into one if it wasn't for the massive gas giant Jupiter that comes next.

Following Jupiter, of course, comes Saturn, another gas planet. Then come the smaller, colder gas planets Uranus and Neptune.

Beyond Neptune, things are more disorganized, with a lower gravitational pull from the Sun. Billions of clumps of rock and ice are in orbits, but never themselves formed into planets. The belt of comets further out than Neptune is called the Kuiper Belt. And the enormous bubble of materials around the solar system is known as the Oort Cloud.

At the center of the solar system, around which everything else orbits, the Sun exerts its influence to the furthest reaches, with both its gravity and the particles that it emits. These charged particles far out into space from the Sun in all directions, creating a sphere around it called the heliosphere.

This reaches out about eleven billion miles, or the distance equivalent to 100 times the distance from the Sun to the Earth. The further these particles go, the less energy they have. And at the edge of the solar system, the energy of uncharged hydrogen atoms in interstellar space push back against those emitted by the Sun. This boundary is known as the heliopause.

It's believed that here there is a buildup of hydrogen atoms from interstellar space, and this invisible wall scatters incoming ultraviolet lights. And with the vast distances involved, there have only been two spacecrafts that have passed through the heliopause: Voyager 1 in 2012, and Voyager 2 in 2018.

It was from these spacecrafts that we first got proof that the edge of the solar system existed, where it was, and even if it expands and contracts. But more recent probes have taught us a lot more.

The New Horizons spacecraft, which recently conducted a flyby past Pluto and the Kuiper Belt, used its Alice UV spectrometer to take readings of the heliosphere between 2007 and 2017. It detected an ultraviolet glow on the boundary where the solar particles hit the hydrogen atoms beyond, and this is called the Lyman-alpha line.

This glow can actually be seen across the entire solar system, but it's more intense at the edge of the solar system than anywhere else. However, the theory and understanding behind this phenomenon is not completely understood yet. So New Horizons will continue to collect data twice a year as it journeys deeper into space. Eventually, it won't itself cross the boundary, possibly within 15 years. And with the new technology aboard, it can help us to uncover more information about this mysterious region.

As it is so far out, it seems like a missed opportunity to send a probe to explore the outer edge without first visiting other unexplored areas within the solar system. The Voyager probes only reached an edge because they continue flying after the mission completed, and New Horizons is only just expected to remain operational by the time it reaches it.

Since the discovery of the border, there are now calls for a specialized robotic probe to make the trip. If the proposal is accepted, it should be able to reach the edge six times faster than the Voyagers did, and could reach its target in less than ten years. Traveling to a distance of 90 billion miles from the Sun, it will be able to automatically explore the region and then send back the data to answer some of the biggest questions that remain unanswered about the solar system.

Now, every time a new probe is sent out to explore the solar system, something else is discovered that piques the interest of researchers even further. The mysterious boundary between our Sun's influence and interstellar space really is still unknown. But the New Horizons and future probes will really get us a lot closer to a better understanding.

Thanks for reading!