Is there anything “under” the Earth in space?

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If you saw illustrations or models of the solar systemyou may have noticed that all the planets orbit the sun more or less in the same plane, traveling in the same direction.

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

i am one planetary scientist working with robotic spacecraft such as Rover and orbiters. When my colleagues and I send them out to explore our solar system, it is important for us to understand the 3D map of our space neighborhood.

Which way is “down”?

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

Imagine you’re standing somewhere in North America and pointing down. If you extended a line from your fingertip all the way through the Earth, that line would point “up” to someone. on a boat in the southern Indian Ocean.

In the big picture, “down” could be defined as below the plane of the solar system, which is known as ecliptic. By convention, we say that above the plane is the place the planets are seen to orbit counterclockwise around the Sun, and from below they are seen to orbit clockwise.

And more “down” flavors

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

Each of these stars and associated planets are all orbiting the center of the Milky Wayjust as 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 in the Milky Way orbit the center of the galaxy close to a plane, which is called galactic plane.

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

Taking a step back, the Milky Way is part of a known group of galaxies Local Groupand—you can see where this is going—these galaxies mostly fall into another plane, called the supergalactic plane. The supergalactic plane is almost perpendicular to the galactic plane, with an angle between the two planes of approximately 84.5 degrees.

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

The collapse of the solar nebula

The material that would eventually make up the Sun and the planets of the solar system began as a diffuse and very extensive cloud of gas and dust called solar nebula. Each particle in the solar nebula had a small amount of mass. Because whatever the mass exerts gravitational forcethese particles were attracted to each other, although only very weakly.

The particles in the solar nebula began to move very slowly. But over time, the mutual attraction that these particles felt due to gravity caused cloud to begin to draw in on itself, shrinking.

There would also have been a very slight overall rotation of the solar nebula, perhaps due to the gravitational pull of a passing star. As the cloud collapsed, this rotation would have increased in speed, just as a figure skater spins faster and faster while pulling her arms toward her body.

As the cloud continued to shrink, the individual particles drew closer together and had more and more interactions that affect their motion, both due to gravity and collisions between them. These interactions determined individual particles in orbits that were tilted away from the general direction of rotation of the cloud to reorient their orbits.

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

Finally, what was once an amorphous cloud of particles collapsed into a the shape of the disk. Then particles in similar orbits began to clump togethereventually forming the Sun and all the planets that orbit it today.

On a much larger scale, similar types of interactions are likely what have come to confine most of the stars that make up the Milky Way in the galactic plane and most of the galaxies that make up the Local Group in the supergalactic plane.

The orientations of the ecliptic, galactic, and supergalactic planes all return to the original direction of random rotation 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” relative to the Earth, other than not much orbiting the Sun in that direction.

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

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 you asked that question people all over the Earthor of intelligent life forms in other planetary systems or even other galaxies. They would all point in different directions.

This edited article is republished from conversation under a Creative Commons license. Read on original article.