What is below Earth, since space is present in every direction?

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This article was originally published on The conversation. The publication contributed the article to Space.com Expert voices: opinion pieces and perspectives.

If you saw illustrations or models of the solar systemmaybe you noticed that all the planets revolve around the sun in more or less the same plane, traveling in the same direction.

But what is above and below this plane? And why are the planets’ orbits aligned like this, in a flat pancake, rather than each traveling in a completely different plane?

I am a planetologist who works with robotic spacecraft, such as rovers And orbiters. When my colleagues and I send them to explore our solar system, it is important for us to understand the 3D map of our spatial neighborhood.

Which direction is “down”?

Earth’s gravity has a lot to do with what people think is up and down. Objects fall towards the ground, but this direction depends on where you are.

Imagine you are somewhere in North America and you are pointing down. If you extend a line from the tip of your finger to the end the Earththis line would point “up” to someone on a boat in the southern Indian Ocean.

From a broader perspective, “down” could be defined as being below the plane of the solar system, known as ecliptic. By convention, we say that above the plane is the place where we see the planets rotating counterclockwise around the sun, and from below, we see them rotating clockwise.

Even more “down” flavors

Is there anything special about the direction down relative to the ecliptic? To answer this question, we need to zoom out even further. Our solar system is centered on the sun, which is only one of about 100 billion stars in our galaxy, the Milky Way.

Each of these stars and their associated planets are all orbiting the center of the Milky Wayjust like planets orbit their stars, but on a much longer time scale. And just as the planets in our solar system are not in random orbits, the stars of the Milky Way orbit the center of the galaxy near a plane called the galactic plane.

This plane is not oriented in the same way as the ecliptic of our solar system. In fact, the angle between the two planes is about 60 degrees.

Going back a little further, the Milky Way is part of a cluster of galaxies known as the the local groupand – you can see where this is going – these galaxies are mostly in another plane, called the supergalactic plane. The supergalactic plane is almost perpendicular to the galactic plane, with a angle between the two planes of approximately 84.5 degrees.

How these bodies end up traveling on paths close to the same plane has to do with how they formed in the first place.

Collapse of the solar nebula

The material that would ultimately make up the Sun and the planets of the Solar System was originally a widespread, diffuse cloud of gas and dust called the solar nebula. Each particle in the solar nebula had a tiny amount of mass. Because any mass exerts a gravitational forcethese particles were attracted to each other, although very weakly.

The particles of the solar nebula began to move very slowly. But for a long time, the mutual attraction felt by these particles thanks to gravity caused the the cloud begins to fold in on itself, shrinking.

There would also have been a very slight overall rotation toward the solar nebula, perhaps thanks to the gravitational pull of a passing star. As the cloud collapsed, this rotation would have increased in speed, much like a figure skater spinning faster and faster by bringing his arms toward his body.

As the cloud continued to shrink, individual particles got closer to each other and had more and more interactions affecting their motion, both due to gravity and collisions between them. These interaction caused individual particles in orbits tilted away from the cloud’s overall rotation direction to reorient their orbits.

For example, if a particle descending through the orbital plane collided with a particle rising through that plane, the interaction would tend to cancel this vertical motion and reorient their orbits on the plane.

Eventually, what was once an amorphous cloud of particles collapsed into a disc shape. SO particles in similar orbits began to clump togetherultimately forming the sun and all the planets that orbit it today.

On much larger scales, it is likely similar types of interactions have ended up confining most of the stars that make up the Milky Way to the galactic plane, and most of the galaxies that make up the Local Group to the supergalactic plane.

The orientations of the ecliptic, galactic, and supergalactic planes all trace back to the initial random rotation direction of the clouds from which they formed.

So, what’s under the Earth?

So there’s nothing special about the direction we define as “down” from Earth, other than the fact that there isn’t much else orbiting the sun in that direction.

If you go far enough in this direction, you’ll eventually find other stars with their own planetary systems orbiting in completely different orientations. And if you go even further, you might encounter other galaxies with their own rotation planes.

This question highlights one of my favorite aspects of astronomy: it puts everything into perspective. If you asked a hundred people on your street, “Which way is down?” » each of them would point in the same direction. But imagine asking this question to people all over the Earthor intelligent life forms in other planetary systems or even other galaxies. They were all pointing in different directions.