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Our solar system is mostly arranged in a single plane in space, as in this not-to-scale artist’s drawing. . | Credit: NASA/JPL, CC BY
If you have seen Illustrations or models of the solar systemPerhaps you noticed that all the planets orbit the Sun in more or less the same plane, traveling in the same direction.
But what is above and below that plane? And why are the planets’ orbits aligned like this, in a flat pancake, instead of each traveling in a completely different plane?
I am a Planetary scientist Those that work with robotic spacecraft, such as Rovers and circling. When my colleagues and I send them 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 and what’s underneath. Things fall towards the ground, but that direction depends on where you are.
Imagine you are standing somewhere in North America and point down. If you extend a line from your fingertip through the earth, that line points in the direction of someone “up.” On a boat in the southern Indian Ocean.
In the big picture, “down” can be defined as being below the plane of the solar system, as it is called eclipse. By convention, we say that the plane is above where it is The planets appear to rotate in a counterclockwise direction Around the Sun, and from below they appear to rotate clockwise.
More flavors of ‘down’
Is there anything special about the downward direction relative to the eclipse? To answer this, we need to zoom out even further. Our solar system is centered on the Sun, which is only one About 100 billion stars In our galaxy, the Milky Way.
Each of these stars, and their associated planets, are all Orbiting around the center of the Milky WayJust as planets orbit their stars, but on much longer time scales. And just as the planets in our solar system are not in random orbits, the stars in the Milky Way orbit close to the center of the galaxy, which Galactic plane.
This plane is not oriented like the ecliptic of our solar system. In fact, the Angle between two planes It’s about 60 degrees.
Going back a step further, the Milky Way is part of a group of known galaxies local groupAnd — you can see where this is going — these galaxies often lie within another plane, called the supergalactic plane. The supergalactic plane is nearly perpendicular to the galactic plane, with a Angle between the two planes of about 84.5 degrees.
How these bodies end up traveling paths that are close to the same plane is related to how they formed in the first place.
Collapse of the Solar Nebula
The material that eventually composed the Sun and the planets of the Solar System began as a diffuse and very extensive cloud of gas and dust. Solar nebula. Each particle within the solar nebula had a small amount of mass. Because any Mass exerts a gravitational forceThese particles were attracted to each other, although only very weakly.
Particles in the solar nebula began to escape very slowly. But for a long time these particles felt mutual attraction due to gravity The cloud begins to pull inward on itself, contracting.
The solar nebula may also have some very slight overall rotation, perhaps thanks to the gravitational tug of a passing star. As the cloud collapsed, this spiral would increase in speed, just as a spinning figure skater spins faster and faster as she pulls her arms toward her body.
As the cloud contracted, the individual particles grew closer together and had more and more interactions affecting their motion, both due to gravity and collisions between them. these Interactions Individual particles in orbits tilted away from the cloud’s overall direction of rotation to reorient their orbits.
For example, if a particle coming down the orbital plane collides with a particle coming up from that plane, the interaction cancels that vertical motion and Redirect their classes on the plane.
Eventually, what was once an amorphous cloud of particles collapsed into a disk size. After that Particles of similar orbits began to clump togetherEventually forming the Sun and all the planets that orbit it today.
On much larger scales, similar interactions probably account for most of the stars that make up the Milky Way in the galactic plane and confine most galaxies to the Local Group in the supergalactic plane.
The orientations of the ecliptic, galactic, and supergalactic planes return to the direction of the initial random rotation of the clouds from which they formed.
So what lies beneath the earth?
So there’s really nothing special about the direction we define as “down” relative to Earth, other than the fact that the Sun doesn’t orbit much in that direction.
If you go far enough in that direction, you’ll eventually find other stars that have their own planetary systems orbiting in completely different orientations. And if you go farther, you may encounter other galaxies with their rotating planes.
This question highlights one of my favorite aspects of astronomy: it puts everything into perspective. If you ask a hundred people on your street, “Which way is down?” Each of them pointed in the same direction. But imagine you asked that question People all over the worldOr even intelligent life forms in other planetary systems or other galaxies. They were all pointing in different directions.
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