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I'm not sure if this post should be in the physics subject forum, but this seems to fit here too. I have been reading a book about chemistry and how the universe came to be with the theory of stars creating most of the elements we know of. I began wondering how the Earth came to be in orbit around the Sun. I understand that we are in a constant free fall towards the Sun, but our motion causes us to "miss" the Sun stopping us from burning up. However, I'm confused how we were put into motion in the first place. My best guess is that we were captured by the Sun's gravity, and were put into orbit, which would mean the Earth was once a meteor, right? If someone could please explain this, it'd be very helpful because I can't seem to find the answer anywhere.

Thank you

P.S. I'm just a 9th grader, so I don't have much knowledge in astrophysics.

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  • $\begingroup$ Hi bob. It seemed to me this would indeed fit better on another site, but I decided that site was Astronomy. $\endgroup$ – kim holder Jun 6 '16 at 1:06
  • $\begingroup$ One of my favorite minute-physics videos, and it touches on your question. youtube.com/watch?v=tmNXKqeUtJM $\endgroup$ – userLTK Jun 6 '16 at 3:35
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The picture was so much cleaner 20 to 25 years ago. I'll present that nice clean picture first. Stars form from the gravitational collapse of huge clouds of interstellar gas. Those gas clouds inevitable have some net non-zero angular momentum. This forces the gas cloud to change shape from being more or less spherical to being disk-shaped. (Why? That's a different question. Ask it.)

While this protoplanetary disk continued to feed mass to the growing protostar, it also set the stage for the formation of planets. The gas cloud was mostly primordial hydrogen and helium, but it also contained heavier elements thanks to stellar fusion and supernovas in the billions of years that preceded the formation of our solar system.

Those heavier elements behave quite differently than do hydrogen and helium. They have chemistry. The planets started as microscopic clumps of mass of these heavier elements, bound together chemically. These microscopic clumps occasionally collided, eventually forming larger clumps of mass. These larger clumps in turn collided with one another, forming even larger clumps of mass. Eventually the clumps became large enough that they interacted gravitationally, making them grow even larger. This process continued, eventually forming protoplanets, and then planetary embryos, and finally planets.

Temperature in the protoplanetary disk were high near the forming protostar but dropped precipitously with increasing distance from the protostar. At some point, volatiles such as water, ammonia, methane, and carbon dioxide become as solid as rock. This is the ice line, aka the snow line or frost line. Asteroids inside of Ceres' orbit tend to be rocky. Asteroids outside of Ceres' orbit tend to be icy.

Planets that form outside the ice line can grow very quickly and then they can grow very, very large. The stuff that comprises the protoplanetary disk orbits the growing protostar at something other than the rate suggested by Kepler's laws thanks to the pressure of all that stuff in the disk. Thanks to the square-cube law, larger objects aren't as subject to that pressure. Those larger objects orbit at a Keplerian rate. Planets that form outside the ice line grow very quickly and then sweep up gas and ice because they are orbiting at a different speed than the immediate surroundings. The result is gas giants such as Jupiter and Saturn and further out, ice giants such as Uranus and Neptune. Planetary growth is a much more difficult process and much slower process inside the ice line. That's why Mercury, Venus, the Earth, and Mars are rocky and so much smaller than Jupiter, Saturn, Uranus, and Neptune.


That's the pretty picture. The not so pretty picture:

  • Why are Mercury and Mars so much smaller than Venus and the Earth?
    Simulations suggest that the rocky planets should all be more or less the same size. That is not the case in our own solar system, let alone elsewhere.

  • How could Uranus and Neptune have formed?
    Simulations cannot recreate Uranus and Neptune at their current distances from the Sun. The material in the protoplanetary disk should have been too sparse at those distances to form large planets.

  • Much, much worse, what's the deal with all the weird exoplanets scientists have found?
    Scientists have found Jupiter-sized objects orbiting very close to their sun, Neptune-sized objects orbiting where the simple model would have only rocky planets forming, and planets in highly inclined (and sometimes retrograde) orbits that don't make sense.

These simulations (which have become to be very good) and the plethora of exoplanets have pushed the theory of how planets form back into the “that’s funny” stage. ("The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” but “That’s funny...”", a quote widely attributed to Isaac Asimov.)

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  • $\begingroup$ David Hammerman's answer is good, and I just want to augment it. This should be a comment, but I don't have the rep on this stack exchange yet, but I'm trying to save you from asking another question as he suggests. If you want to know why some things in space are flat and others are round, I suggest checking out these videos: youtube.com/watch?v=Aj6Kc1mvsdo and youtube.com/watch?v=tmNXKqeUtJM $\endgroup$ – user13097 Jun 8 '16 at 17:24
  • $\begingroup$ @ user13097 - My name has been misspelled and mispronounced from day one (my birth certificate has a corrected spelling error). I don't quite get it. What's so hard about spelling Neilsen, which is supposed to be my last name? My last name was misspelled 55 years before I was born. Your misspelling, however, is a new one. $\endgroup$ – David Hammen Aug 8 '18 at 21:30
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Earth used to be many, many asteroids and meteorites. The larger asteroids would pull meteorites and smaller asteroids, crash with them, and bundle into a bigger and bigger clump, that eventually became so heavy, that rock began behaving like liquid under the pressures, forming the neat sphere Earth is now.

That all rubble was already orbiting the Sun when that happened; some of it arrived from outside the Solar System, but most was just a nebula of gases and other matter that coalesced into the current solar system.

As for "falling and missing"... that's a semi-accurate description of orbital mechanics. Taking the distances into account, "missing" the central body is actually quite easy; look at the sky at night - some of the brighter stars are actually other planets in our system. That's it - these tiny dots. The Sun is bright, but it's also tiny in the sky. There's a lot of room to "miss", and instead of crashing, to fly in an elliptic trajectory around the central body - passing it by, faster when you're close, then getting ejected "around the other side" to slow down, and fly away, only to return down the same (missing) path.

Now... how comes Earth's, and most planets' orbits are not elongated ellipses, as is most common with most randomly moving objects in gravitational field - but pretty near circles - that's a subject for a different question and a longer debate.

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Planetary systems are formed from clouds of gas and dust.

The gravity of the cloud's mass holds it together; the densest part at the center of the cloud collapses until it's dense enough to begin nuclear fusion, turning into a star.

Things further out randomly collide and stick until a few of them become big enough to have significant gravity of their own. When those "planetesimals" collide with each other, on average their orbits become more circular. After billions of years of this, you wind up with relatively few bodies in nearly circular orbits: the planets of a solar system.

Capture of bodies arriving from outside the solar system does happen, but formation from a proto-stellar gas cloud is the more common way planets are formed.

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The Nice theory of planetary formation:

Created in Nice, France

Also answers the not so pretty picture (except for Planet 9). In the beginning, the sun, as a protostar, sucked up mass from a nebula and became a rapidly spinning star. The matter flattened out into a disk (Why? See Why are some galaxies flat.) The Nice theory explains the Late Heavy Bombardment, after most matter had already coalesced.

The Grand Tack

Around 5 billion years ago, the Late Heavy Bombardment happened as Jupiter and Saturn locked into a strong resonace, which brought them closer to the Sun. As this happened, icy debris that was still present in the outer solar system was dragged inward to the inner solar system. This prompted a bona fide "Grand Attack" on the inner solar system.

The Grand Attack

The Grand Attack happened after Jupiter got out of the resonance, flinging it (and Saturn) out back into the outer solar system. The debris it dragged into the solar system locked into swarms of debris that would have siphoned enough energy off of any super-Earths to drag them into the sun. The remaining debris coalesced into the four planets of the inner solar system.

The rest of the theory is consistent with what everyone else already states

  • Why are Mercury and Mars so much smaller than Venus and the Earth?
  • What's the deal with all the weird exoplanets scientists have found?

    If I have made any mistakes in conventions, please tell me. Us sixth-graders are very inexperienced on StackExchange.

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