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Without giving spoilers, this idea comes from the movie Titan A.E. How would one create an Earth-like planet in a reasonable time frame (100-250 years) that has both the gravity of Earth and a magnetic field?

For the purposes of this question, assume that the people have sufficiently advanced interstellar space travel to make this viable and they want to create an exact replica of Earth as it was before the Andromeda-Milky Way collision after our Sun expands and the Solar System is thrown out of the Galaxy.

EDIT:

It took the earth 200 Million years to cool after its initial formation 4.5 billion years ago. To clarify, I am more interested in how one would drastically speed up the natural formation of an Earth-sized planet and its cooling afterward to the 250 years time frame listed above. After that it should be trivial for a civilization of this technology level to add a breathable atmosphere and some ocean water.

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    $\begingroup$ The biggest challenge I would see would be to set it on a proper Orbit with a viable rotation etc..., not the creation of the mass(planet) itself $\endgroup$
    – Spacemonkey
    Commented Apr 9, 2015 at 15:03
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    $\begingroup$ You have got to stop linking the mobile Wikipedia pages. A phone browser will automatically get the mobile version while desktop browsers end up getting the weird mobile version. Just remove the "m." at the start. $\endgroup$
    – Samuel
    Commented Apr 9, 2015 at 15:04
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    $\begingroup$ @Samuel sorry, I assumed most people today use phone browsers ;) $\endgroup$
    – Jax
    Commented Apr 9, 2015 at 15:05
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    $\begingroup$ Ship all the required material to your building site. build from the core outside. Then ship your finished product (gift-wrapped, if required) to your target destination. And don't forget to award Slartibartfast with a prize for the fjords. Or in other words: I guess your question deserves a little more background information, as it appears too broad to me. $\endgroup$
    – Burki
    Commented Apr 9, 2015 at 15:06
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    $\begingroup$ @Burki It took 200 million years for the Earth to cool to a point where the crust could form. I was more hoping for an answer on how to drastically speed up the process of planetary formation. $\endgroup$
    – Jax
    Commented Apr 9, 2015 at 15:15

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Paradoxically, you want to assemble your planet slowly.

If you smash rocks into each other at high speed (as happens in natural planet formation), the energy of the collision is released as heat, and you have a very hot ball of molten rock.

Conversely, if you push the components together slowly, the outer layers remain relatively cool. The interior would still heat up somewhat, from radioactive decay and compression due to gravity.

A world assembled very slowly might be too cold to have much geological activity, which is not necessarily what you want. You could seek a middle ground by colliding chunks of rock at carefully controlled speeds (although I'm not sure this is compatible with a 250-year time frame).

As per Anixx's comment, it would also take a very long time for the crust to oxidize to the point where it would support an oxygen-rich atmosphere. (We need free oxygen in the air, instead of having it immediately bind to the rocks.) This is somewhat at odds with your goal of rapid cooling, since oxidization will occur more quickly with a hot crust.

A possible solution is heating only the top layer of the crust, with the deeper layers being kept cool by slow assembly. But I'm not a geologist and I don't know if it would be feasible to get the balance right in your time frame.

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    $\begingroup$ Once you start to accrue sufficient mass you don't have any choice -- the rocks will smash together at high speed.. For example, Ceres has an escape velocity of over 1000 miles per hour and it is far too small to really be considered an earth-like planet. Ceres mass is < 1% of earth mass so 99%+ of the mass you add will slam together at high speed unless you have controller descent. $\endgroup$
    – Gary Walker
    Commented Apr 9, 2015 at 20:23
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    $\begingroup$ "unless you have controller descent" -- yes, that is exactly what I'm saying they need. $\endgroup$
    – Royal Canadian Bandit
    Commented Apr 10, 2015 at 7:04
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    $\begingroup$ Controlled descent did not sound like, pushing the rocks together slowly to me, as controlled descent requires a much greater investment, but OK. $\endgroup$
    – Gary Walker
    Commented Apr 10, 2015 at 12:38
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It is not possible to do this in 250 years without truly magical technology.

Earth is easily massive enough that your cannot simply put the right materials in the right place even using teleportation tech and get it to work. Materials that we think of as incompressible are not when subjected to sufficient pressure (much less high temperature and pressure).

The earth is still rebounding from the relatively small pressures of the last ice age. Water, Granite. Iron even diamond are compressible given the pressures at the core.

The only way you could do this is by assembling layers upon layer using materials that are already at the desired composition, temperature and pressure and motion as they are supposed to be when completely assembled. There is no time for materials to naturally transform to the required properties during the construction phase unless they start out very close to the desired end result.

You have to pressurize the planet as you are constructing it, i.e., when you have only 1 km of iron core assembled, it needs to be pressured to 380 GPa via force field or suchlike to keep it from massive deformation during the construction process.

As you add more and more layers the required force field containment pressure has to drop correspondingly to match the natural pressure of that layer for the final product.

Even the oceans and the atmosphere will need similar treatment as transient conditions by doing this carelessly will undo your hard work. To get an ideal match you also need to seed life as needed during construction phase too -- it is really hard to seed life on sea floor or deeply underground without having access to that layer.

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