In this artist’s rendering, which is not to scale, our solar system’s components are mostly configured within a single spatial plane. (Image credit: NASA/JPL, CC BY)ShareShare by:
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If you’ve come across images or simulations of the solar system, perhaps you have observed that the planets circle the Sun on roughly the same level, going in the same path.
But what lies above and underneath that plane? Furthermore, why are the planetary orbits arranged this way, in a level configuration, as opposed to each following a completely unique level?
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Which path is ‘down’?
What people consider as up and down is closely tied to Earth’s gravity. Objects descend towards the surface, yet the specific direction relies on the position of the observer.
Visualize yourself standing in North America and pointing downwards. If you prolonged a line from your fingertip straight through the planet, that line would indicate “up” for a person on a vessel in the southern Indian Ocean.
By convention, observing the solar system from ‘above,’ you perceive the planets circling counterclockwise.
In a wider sense, “down” could signify a position below the solar system’s level, referred to as the ecliptic. By convention, we consider the region above this plane as the one where the planets are observed to orbit the Sun counterclockwise, while from below, they seem to orbit clockwise.
Even more variations of ‘down’
Is there something distinct concerning the down direction with respect to the ecliptic? To address this, we should broaden our outlook even more. Our solar system is anchored by the Sun, just a single star from approximately 100 billion in our galaxy, known as the Milky Way.
Every one of these stars, and their associated planets, are all rotating around the heart of the Milky Way, akin to planets orbiting stars, yet on a considerably expanded temporal frame. Additionally, similar to how planets do not have orbits at random in our solar system, the stars inside the Milky Way also circle the galaxy’s center in proximity to a plane, the galactic plane.
This plane does not have an orientation identical to the ecliptic of our solar system. Actually, the disparity between both planes measures around 60 degrees.
A lateral view of galaxy NGC 4217 as captured by the Hubble Space Telescope demonstrating each star with its own planetary systems existing on one level.
Extending this further, the Milky Way makes up part of a grouping of galaxies known as the Local Group, and — it’s easy to see the trend — these galaxies generally sit inside another level, classified as the supergalactic plane. The supergalactic plane stands nearly perpendicular to the galactic plane, displaying an angle of approximately 84.5 degrees between both planes.
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Why these objects journey along comparable routes comes down to the circumstances surrounding their creation.
Collapse of the solar nebula
The material which would eventually form both the Sun and the solar system’s planets initially comprised an extensive cloud of gas and dust, referred to as the solar nebula. Each particle within the solar nebula had a tiny amount of mass. Mass attracts other objects via gravity, so these particles gravitated toward one another, but quite faintly.
The particles inside the solar nebula initially moved at a snail’s pace. But the mutual attraction the particles experienced through gravity eventually forced the cloud to increasingly draw inside on itself, gradually reducing in size.
There would probably have also been a touch of overall rotation affecting the solar nebula, perhaps resulting from the gravitational pull of any stars passing nearby. Because the cloud diminished, this rotation should have picked up speed, similarly to how a whirling figure skater rotates increasingly quicker while drawing their arms close.
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As the cloud persisted in contracting, the various particles moved closer and participated in more interactions affecting their motions, prompted by both gravity and impact. The particles with orbits greatly tilted far from the cloud’s collective rotation realigned their orbits as a result of those interactions.
As an example, if one particle descended through the orbital plane and crashed against a particle ascending through that same plane, this interaction would most likely offset this vertical motion and alter their orbits into the level.
Eventually, the former amorphous particle cloud was crushed into a disc form. Afterward, particles located in similar orbits began clumping together, which led to the eventual genesis of the Sun with all the planets that orbit it nowadays.
The interactions mentioned above very well may have restricted most of the stars comprising the Milky Way into the galactic plane on substantially wider scales, in addition to the majority of the galaxies making up the Local Group into the supergalactic plane.
The directions of the ecliptic, galactic, and supergalactic planes all link back to the starting arbitrary spin path of the clouds from which they were generated.
Traveling in any path from Earth, you’re likely to face galaxies alongside their different up-and-down orientations. Then what is situated beneath the Earth?
Thus, aside from the dearth of things orbiting the Sun that way, there is certainly nothing truly unique in the way we explain “down” as it relates to Earth.
By going far enough that path, you may locate different stars with their own planets spinning in drastically different orientations. Traveling further still, you could potentially encounter galaxies alongside their various rotation planes.
This question brings to light one of my preferred elements concerning astronomy: It renders everything relative. “Where lies down?” would lead all of the people to point in the same general path if asked to a hundred inhabitants of your street. But assume that question was posed to every person across the globe, or even other lifeforms with some level of intelligence in another planetary system or other galaxies entirely. They’d each point somewhere totally different.
This edited article is republished from The Conversation by permission of a Creative Commons license. See the original article.
Jeff MoerschProfessor of Earth, Environmental, and Planetary Sciences, University of Tennessee
Jeff Moersch graduated from Cornell with a Bachelor’s in Physics, and earned Master’s in Geology from Arizona State University, in addition to a Master’s in Astronomy, and later a Ph.D., in Astronomy, from Cornell. After grad school, Moersch spent two and a half years within the Exobiology Branch at NASA Ames Research Center employed as Resident Research Associate through the National Research Council. In June 2000, Moersch shifted to a research faculty role within the Department of Geological Sciences at U.T. (as it was named back then). Moersch got his Assistant Professor position in the Department of Earth and Planetary Sciences at U.T. in August 2003.
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What constitutes the dimmest location in the solar system? What about in the universe?
How perilous are interstellar bodies similar to 3I/ATLAS?
Strange symmetry seems to be cracking between Earth’s Northern and Southern Hemispheres