what is above and below the solar system?

It isn’t quite so simple, but pretty close. Why it is approximately contained in a plane, and exceptions to that approximation, help us understand what is above and below the solar system. In terms of above and below, below our solar system is sometimes thought of as the position such that when you look at our solar system it appears to be mostly rotating in a clockwise direction. Above is sometimes defined such that if you look at Earth it is rotating around the sun in a counter-clockwise direction. These are arbitrary, and your question can also be posed as “what is above and below the solar system” with the same answer.

First, let’s make a connection to something here on Earth. You can check out clips on YouTube of ice skaters doing the “scratch spin” in which they launch themselves into a spin with their arms held outward, and then bring their arms inwards resulting in the speed of their spin increasing dramatically. This is due to a law of physics called the “conservation of angular momentum”. Think of the blades of a helicopter, and think of two points on one of those blades. One point we pick very close to the center of rotation, the other point at the very end of the blade. Both points on the blade must make a full circle in the same amount of time (or else the blade will break!) but the outer point has to travel much much further than the inner point to make a full circle. That means it has to go much faster. Things that go faster have more energy. This holds true for the out-stretched arms of a figure skater doing the scratch spin. Think of their fist as one point and the base of their arm as another, and connect that to the helicopter example we just discussed. The fist must go faster than the base of the arms, so in a way, they have more energy. But now what happens to that energy as the skater draws the fist as close to their body as is the base of their arms? It can’t just vanish. Instead, that energy is transferred into speeding up the rest of the body such that the entire body spins faster.

Now let’s talk about the birth of our solar system. Our sun, like most stars, started off in a stellar nursery. This is a large cloud of gas in space that has enough mass to undergo localized gravitational collapse in different spots, and these spots condense gravitationally to eventually form stars. Hubble Space Telescope has taken lots of amazing pictures of such nurseries that you can find online. A team at the University of Texas at Austin recently located a potential sibling star that they believe was born in the same nursery as our sun, so there are probably sibling solar systems out there as well.

Let’s imagine going back in time and zooming in on the blob of gas that would go on to condense into our solar system. Because the stellar nursery likely came from the supernova explosion of an ancient larger star (our sun is quite small compared to the largest possible stars), it is going to have some left over motion from that explosion and from interactions with nearby stars and the interactions of the various embryonic stars in the same nursery. Think of a leaf floating on a pond, and a duck swims by, the leaf is likely going to pick up some motion with a bit of spin. That spin is the key point here.

So let’s imagine we have a roughly spherical blob of gas that is contracting inwards due to its gravitational pull—that is, that it is big enough and dense enough that it reaches a threshold at which it is inevitable that the random interior motion of the blob of gas will be overcome by the gravitational attraction of the interior of the blob itself. This blob is going to have both linear and angular motion. That is, it will be heading in a certain direction in space, but it will also be spinning a bit. At the same time, gravity is pulling the sphere of gas inwards such that it shrinks in radius and becomes more and more compact. That’s when things get super interesting.

Just like with the figure skater, as the gas contracts, its spin gets faster and faster. To make the connection even clearer, think of the standing body of the figure skater as the axis of rotation, and their outstretched arms as the plane of rotation. But there is one more everyday effect we need to think about to make it to becoming a solar system, and that is centripetal force.

Centripetal force is the force that keeps the riders of a Ferris Wheel from flying away as the wheel rotates quickly. You can feel this effect if, for example, you take a grocery bag and put a few apples in it and spin it around you very fast. The apples want to fly away, but the bag and you holding on to the bag keep them from flying away.

As the sphere that will be our solar system collapses, its rotation speeds up. That rotation causes everything in the plane of rotation to resist the gravitational pull inwards. Just like the apples in your bag, things in the plane want to fly away, but the gravity keeps them from doing so. However, that tug of war means that things in the plane of rotation won’t collapse inward as much as things outside the plane. Gravity is pulling everything in the sphere of gas towards the center. Gas which is not in this plane, say below the plane, will be pulled upwards towards the plane and pulled inwards towards the center. Imagine a clump of gas in this sphere which is just exactly below the center and thus doesn’t have much rotation around the center. That clump will be pulled upwards towards the center. Now imagine a clump that is in the plane of rotation. That clump will also be pulled towards the center, but because it is rotating around, just like the apple in the bag you are rotating around you, it will resist that pull inwards a bit and so its net pull inwards towards the center will not be as strong as the clump we considered directly below the center. At some point, those two forces balance and things in the plane don’t fall inward anymore.

Now fast forward hundreds of millions of years, what do you think will happen? For one, the sphere is no longer a sphere, it will look more like a thick disk. Also, it will be much smaller than before but much more dense. It will also be spinning much more faster than before, meaning that things in the disk of rotation will be pushing back against the gravity more strongly than before (though the gravity might be stronger now because much of the gas is now tightly packed in the center). You will also notice that the gas is more clumpy than before. There is a super dense clump in the middle, but smaller clumps spread throughout the disk. This is because areas that were a little more dense than average will have their own localized gravitational pull. Everything that has mass has gravity, of course, but those denser regions are sort of like giant companies that swallow up smaller companies through acquisitions, they dominate due to their size.

The central clump, however, is special. It is far far more dense and more massive than the other clumps in the disk. At some point, for a lucky disk which has enough mass, that central clump will reach a critical mass and ignite, sort of like a thermonuclear explosion. If that clump doesn’t have enough mass, that explosion will scatter it so much that we may never get a star to form. But if, like in our case, the central clump is so massive that its gravity is strong enough to counteract the forces of that explosion, then you get a perpetual (until it runs out of fuel) nuclear explosion being self-contained in a ball of gas by its own gravitational pull. There is lots of fuel in that ball of gas, and if gravity can hold it together, you’ve got yourself a star that will last billions of years.

The other clumps will do similar things, but unless they get super lucky and have enough mass to go critical (in which case you might get a binary star system, though they form in other ways too), they will just condense into spheres and eventually become planets or planetoids or asteroids or comets. Fast forward hundreds of millions of years later, and pretty soon the central star and the proto-planetary clumps have done much to clear out the disk such that instead of a disperse gas, we have a solar system that is fairly empty of dust between the planets and other objects, and that solar system is sort of contained in the plane of rotation, and everything that was above or below this plane has (mostly) been pulled into the plane of rotation.

That means that above and below our solar system, it is pretty empty, but not entirely empty. Internal gravitational interactions can “throw” things into orbits around our sun that can go well above and below our solar system. Pluto’s orbit is pretty inclined relative to the solar plane, for example, as are some comets and astroids. That being said, you really won’t find much above and below the solar system in terms of massive objects.

What you will find, most certainly, is the gravitational and electromagnetic fields of the sun, as well as solar radiation. So even ignoring those rare exception of things that orbit the sun outside of the plane, the space above and below the solar system is just as full of those fields and solar radiation as is the solar system itself. Well, gravitationally at least; the electromagnetic field of the sun has an orientation determined by the rotation of the sun, so the direction and magnitude of those forces will be somewhat different above and below the solar system, but they will still be there and easy to measure. If we sent a probe, like the Voyager probes, but in a direction directly above or below our solar system, and their cameras were facing away from the solar system, we could still very easily tell they were in our solar system based on the gravitational effects and electromagnetic measurements (as well as by measuring interstellar winds).

So the answer to your awesome question is this: in terms of massive objects, there isn’t very much at all below our solar system, but in terms of the various force fields such as gravity and electromagnetism, as well as very very tiny solar particles that emanate from the sun and radiation, there’s plenty. However, you can’t really see that with your eyes, so if you were to get on a space ship and travel below the solar system, it would look super boring and you would only occasionally have to dodge some random asteroid or comet that got kicked up by a planet like Jupiter, or some interstellar asteroid that is passing through our solar system (like Oumuamua, a recently observed asteroid from interstellar space above our solar system).