It has become accepted knowledge that the planet Earth was birthed via a swirling cloud of dust and gravel clumping into larger and larger pieces of rock. It began 4.54 billion years ago, and 100 million years later, the newly molten Earth was 8,000 miles wide and 25,000 miles around.

But in this alternate universe, Earth was birthed five billion years ago, as the gravitational pull of the newly birthed sun lured in wandering dwarf planets from neighboring star systems. The result is that dozens if not hundreds of dwarf planets (varying in size from that of Ceres, 945 kilometers wide, to that of Ganymede, 5268 kilometers wide) crashed and merged together to form newer, larger bodies before becoming planets. Their surface and core compositions would vary from rock to metal to ice.

This alternate Earth was formed from such collisions before Theia arrived to make an even bigger crash. Back home, it took Earth 100 million years to reach its modern size, and that was from asteroidal clumping. In regards to the speed in which to reach full size, would it make any difference if Earth was instead formed from the colliding and re-merging of wandering dwarf planets captured by the gravitational strength of the sun?


Taken from the comments because it's more of an answer:

Nothing really changes

I think you may misunderstand how the Earth was actually formed. Yes, it did start in a cloud of dust, but that dust didn't suddenly clump together into a planet-sized sphere: it accumulated into larger pieces, which then stuck - basically the "dwarf planet" collisions you're seeking. These pieces were called planetesimals, and the Earth was a protoplanet.

There would be no real change if the sun captured bodies from farther away and combined them - it would be just like planetesimals colliding, and you'd still get an Earth in the end. The Earth's composition may be different, depending on what the "dwarf planets" were made of, but I'm sure even that would even out as the planet settled into layers and life transformed the surface and atmosphere to what we know today. Larger chunks? Gravity can take care of it.

Side note - You may actually want to reconsider having the sun capture the pieces, too - that takes billions of years, at which point the Earth would be much farther along. You'd basically just be adding more (not necessarily stable) moons, and some craters, with material that shouldn't really exist anyways - all the nearby stars were young and just barely forming planets, too.

  • $\begingroup$ The question is about speed. $\endgroup$ – JohnWDailey Jan 29 '17 at 16:07
  • $\begingroup$ @JohnWDailey I addressed that, too - You may actually want to reconsider having the sun capture the pieces, too - that takes billions of years, at which point the Earth would be much farther along $\endgroup$ – Zxyrra Jan 29 '17 at 16:27
  • $\begingroup$ The "speed" is the 100 million years it took for Earth to accrete to its full size. And this is from asteroids. Would a dwarf-planet collision-accretion make a difference? Faster? Slower? $\endgroup$ – JohnWDailey Jan 29 '17 at 16:29
  • $\begingroup$ @JohnWDailey I just answered that in my reply to you. It would take billions of years to pull away, or accrete, dwarf planets from so far. The actual formation of the planet would probably not be so different, because it's just like planitesimals, but to gather the pieces, it would take eons longer. Please reread the question - and my response to your comment - which both already answer it. $\endgroup$ – Zxyrra Jan 29 '17 at 16:31

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