It appears that multiple planets existing in the same orbit is theoretically possible. In that case, how would we go about building a second Earth-sized planet and inserting it into some position in Earth's orbital path? I was thinking planet-construction would be more likely succeed in that region since we know it's a perfect distance from the sun to support life.

Assume we have the technology to supply/harvest, and ship whatever materials required for this endeavor, to whatever location the building takes place. A couple scenarios I've considered:

  • Something similar to the world factory in THGTTG, where we build it offsite such that the new planet doesn't mess with other planets' orbits and whatnot. The final product can be transported into orbit later.
  • Closer to the Death Star process, where construction takes place in the new world's final orbital location. Seems like this would add complications with gravitational effects on and from other planets, although I'm not sure what exactly.

How plausible are these methods, and what other better ones could work?

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    $\begingroup$ If multiple planets co-orbit, are they still planets, by definition? $\endgroup$
    – corsiKa
    Jan 29, 2015 at 23:04
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    $\begingroup$ It would affect the gravity equilibrium within a solar system with very-long-term effects. Which could end up with interesting effects. $\endgroup$
    – Mast
    Jan 30, 2015 at 5:45
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    $\begingroup$ If you insert it opposite of the Earth you have made a fatal mistake. From the abstract you link to "...,while fewer than 7 equal-mass planets are stable only in a configuration where all of the planets remain on the same side of their parent star." $\endgroup$
    – Taemyr
    Jan 30, 2015 at 9:33
  • $\begingroup$ Edited to reflect new understanding $\endgroup$
    – mjr
    Jan 30, 2015 at 16:27
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    $\begingroup$ You can't just say "assume we have the technology" and then ask a question that's entirely technology-dependent. You tell us what the technology actually is, and we might be able to suggest some ways to deploy it. $\endgroup$
    – Mike Scott
    Feb 1, 2015 at 6:51

9 Answers 9


New Earth Recipe:

  • 32 parts iron
  • 30 parts oxygen
  • 15 parts silicon
  • 14 parts magnesium
  • 3 parts sulfur
  • 2 parts nickel
  • 1.1 parts calcium
  • 1.1 parts aluminum>
  • ~1 parts of a varying mixture of metals/metalloids/nonmetals from across the periodic table

Bring all ingredients to a molten state; evenly mix until roughly 7100 miles in diameter; and set to a spin on tilted axis at 23.5 degrees at a rotation speed of 1036 mph, moving at a rate of roughly 18.5 miles/sec in an elliptical formation around a low-grade star emitting 174 PW of energy to the surface.

Heat to 5700 K.

This should be enough to cause the planetary body to undergo an iron catastrophe, which will allow the insides of the cake to separate into layers.

Add moon.

Once magnetic field of sufficient strength is detected (caused by formation of solid iron/nickel inner core, +/- 10% light component combined with gravitational pull developed by spinning), reduce ambient temperature to at least -100 degrees Centigrade (though preferably much lower to speed things up).

Once mantle has sufficiently formed and surface temperatures cooled to 122 degrees Centigrade, sprinkle comets and seed with Xenophilic bacteria. (This may require several hundred thousand bioreactors worth to take.)

Go grab a few hundred million beers.

Once the oceans have formed and volcanic activity has sufficiently subsided, begin tweaking atmospheric concentrations to desired levels. (This may require the sacrifice of several million beers as inoculant. Don't worry, though, this will allow for the brewing of many millions more).

Repeatedly check sodium levels in oceans, and if needed, add salt (this is key!). Once volatile tectonic activity and superstorms have subsided and atmospheric levels have stabilized to appropriate levels, bring in the plants and animals (in that order).

Subcontract to the Magratheans the job of planting/decorating/chemically altering the planet as you desire.

Congratulations, you now have your very own earth. Throw a party, invite your friends.

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    $\begingroup$ "Go grab a few hundred million beers" - LOL'd $\endgroup$
    – user11153
    Jan 30, 2015 at 15:40
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    $\begingroup$ Amazing! :-) This is a genuinely genius answer. $\endgroup$ Jan 30, 2015 at 16:12
  • $\begingroup$ AFAIK, current theories are that Earth's axial tilt of 23.5 degrees was caused by the impact that formed the moon, it didn't start out that way. $\endgroup$
    – Alnitak
    Jan 31, 2015 at 9:13
  • $\begingroup$ <science/astro nerd alert> The calculations listed here are way off, Terra is 24898.3457 Mi (or 40,070 km) in diameter at the equator and spins at 1070 Mph (or 1670 km/h). Unless your assuming the speed of the planet at creation or prior to the Theia collision? Your figures puts Earth at a full rotational day of 6.85 hrs. Making it spin fater than Jupiter. $\endgroup$
    – user6880
    Feb 1, 2015 at 23:50
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    $\begingroup$ @DwightSpencer You've written the circumference of the Earth, not the diameter. No <math nerd alert>? $\endgroup$
    – Samuel
    Feb 2, 2015 at 22:33

Build it in place, certainly. You're going to be collecting mass from all over the solar system to construct such a thing, there is little point in moving it all again after it's collected. Any gravitational effects of moving an entire planet through the solar system would be more tricky than moving all the little bits to the final destination. I wouldn't call it the Death Star process, I'd call it growing a planet.

If you have the ability to move something planet sized around, then I recommend you simply move Mars into a lower orbit. Add a bit of mass, perhaps, then do whatever it was you were going to do to make Earth Mk II habitable. This will save you a lot of energy and time.

I'd rather be a Martian than an "Earth-Mark-Twoian".

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    $\begingroup$ Why not a Mar||ian? $\endgroup$
    – KSmarts
    Jan 29, 2015 at 20:30
  • $\begingroup$ @KSmarts how would you spell that? $\endgroup$
    – PTwr
    Jan 30, 2015 at 7:41
  • $\begingroup$ @PTwr: you mean, "how do you pronounce that". $\endgroup$ Jan 31, 2015 at 21:09
  • $\begingroup$ @DanDascalescu - Mar-two-ian? $\endgroup$ Feb 2, 2015 at 2:59
  • $\begingroup$ Nah, it would be an Earth by then, so the inhabitants would be Second Earthers. $\endgroup$
    – RedSonja
    Feb 2, 2015 at 8:33

Where to put it

The L3 Lagrange point is unstable, so you wouldn't want to put your new planet there. Instead, stick it in the L4 or L5 Lagrange points and you'll have a stable three-body system with that, the Earth, and the sun. As an added benefit, the L4 and L5 points are significantly closer to Earth, so travel time between your planets will be less.

Where to build it

You'll almost definitely want to build it in place. Most of the solar system is dominated by the gravity of large planets, so there isn't much in terms of good real estate further out in the solar system for planet building, and engines that can move planets between stars aren't known for their gas mileage. If you build your planet at the L4, you can just keep dumping materials there until they start to coalesce under the influence of gravity and mush into a sphere. Stability goes both ways, so doing so will neither knock your new planet out of its stable point nor knock Earth out of its orbit. If your new planet gets off of the Lagrange point, it will cause both itself and the Earth to wobble a bit in their orbits, but they'll stay in generally the same place. The heating involved in this process should be enough to get a geodynamo working. It will also probably be enough to liquefy your new planet, so you'll have to wait a few million years for it to cool before starting your terraforming process.

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    $\begingroup$ Good catch with the Lagrange point, I'm definitely not knowledgeable with that stuff. Will the process really create so much heat that millions of years are required to cool it though? Just seems like an astronomically (heh) high amount of time $\endgroup$
    – mjr
    Jan 29, 2015 at 21:15
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    $\begingroup$ Almost definitely. Planets are astronomically huge! This paper estimates that it took the Earth about 100 million years to cool: adsabs.harvard.edu/abs/2001AGUFM.U52A0009M. You can probably improve on that a bit if you're willing to spend a lot of fuel trying to slow all of your materials down before they come to rest, but your planet will compact a bit under its own gravitational pressure no matter what, and when it does so it will need to shed a huge amount of gravitational potential energy in the form of heat. $\endgroup$
    – ckersch
    Jan 29, 2015 at 21:39
  • $\begingroup$ If you have the technology to build a planet, you probably also have some pretty heat sink technology. You might even be able to make the planet cool in just a few hundred thousand years. $\endgroup$
    – KSmarts
    Jan 29, 2015 at 22:35
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    $\begingroup$ Are you sure the L4 and L5 Lagrange points would be stable for an earth-sized object? The Lagrange points calculations seem to assume a small object as the third body. $\endgroup$
    – Ross Ridge
    Jan 30, 2015 at 0:11
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    $\begingroup$ Keep in mind that you want some heat. I think most current models assume a molten core is necessary for a planet to have a protective magnetic field. $\endgroup$ Jan 30, 2015 at 0:47

You will almost have to steal one of the other planets (such as Mars) and move it to that location. The entire asteroid belt mass estimate is 1/1000th of Earth's mass. Most of the rest of the mass in the solar system is in the sun (98.6%) and the other planets (Jupiter is 2/3rd the remaining %1.4) So Pluto would have to go too! Actually it looks like Venus (0.815 Earths) + Mars (0.101 Earths) + Mercury (0.055 Earths) + Asteroid belt, + Pluto would = ~1 earth mass. Could of course steal a few moons from the big planets if you don't want to waste all the planets. Might be able to save Mercury and Mars

If you are moving Venus, then you might as well build it there first, since it is %81 the mass of earth and as you make the collisions you are going to be leaving as much of a mess in the path of the earth (meteors). Once you have it built to size then you can push it into the needed orbit, Though it might be better to be in a different orbit in the goldilocks zone. Someone pointed out that the Earth's L3 Lagrange point is unstable and now you'd have '2' of them each earth causing the point for the other.

  • $\begingroup$ I might be mistaken but I thought they recently determined that Pluto was one of as many 1000 objects in the region. $\endgroup$
    – user458
    Jan 30, 2015 at 22:57
  • $\begingroup$ @fredsbend Pluto is 2/1000 the mass of Earth, so you would need 500 Pluto sized objects. We don't know how many Pluto sized objects are out there, but here's a list of what we do know. Mass is very difficult to estimate, but most are far less than Pluto's size. It would also take a lot of delta-V to move mass from such a distant orbit to a stable Earth orbit. $\endgroup$
    – Schwern
    Jan 31, 2015 at 18:42

The Hitchhicker's Guide to the Galaxy Method

To be entirely clear, this is the process of building planets in something like a shipyard and then "shipping" the planet to its desired location.

This is really infeasible, despite Mag Making the planet and then moving it into place is really energy intensive. Here are some questions which may illuminate why:

  • Where are you going to get materials strong enough to pull a whole planet along?
  • Are you going to make giant rockets on one side and move it into place that way? If so, what are the fuel sources for your rockets? How do you place them so that they don't just drill into the ground or deform your planet?
  • What does the fuel need to combust or otherwise propel? Does this material come from your planet?

This will only work if you have a cheap and effective way of moving the whole planet into position. This problem alone is too large a one for most technologies, even if properly scaled up, to do. You simply need super materials and magic rockets to do so. You're better off trying to find Magrathea with your infinite improbability drive, or ask Slartiblartfast, wherever he is currently.

The "Death Star" Process

This is more likely. You can get a massive asteroid, or several smaller asteroids, and move it/them into position. You can get the correct distance from the sun, the correct speed for you planet, etc. You then send more asteroids or other items to you planet. Eventually, your planet grows to be pluto sized, then moon sized, and so on until you have your planet.

After a while, you may not even need to move that material "gently" into position. You can just send a comet/asteroid of material and have it impact your proto-planet! Talk about reducing costs!

You will have to do some tricky things to maintain angular momentum. This is so that your planet, with increasing mass, does not slow down and potentially crash into the other one. This is easily avoidable with the "death star" process.

The down side of the "death star" process are, of course, any rebels, ewoks, or other random things getting in the way. I suggest marketing it to the galactic empire as a real estate project, even if you never intend to settle people there.

Gravitational Effects of a New Planet

...is actually pretty small. The Solar System is a big, big place. If you do some research into tides, you'll note that the moon is what mostly affects them. All the other planets and even the sun, despite their enormity, don't actually contribute all that much. Unless you build your new planet to be massive, or really close, your other planets shouldn't feel all that much.

  • $\begingroup$ A Hitchhiker's reference to a quest like this and no explicit mention of Magrathea!? :-) $\endgroup$
    – HDE 226868
    Jan 29, 2015 at 22:57
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    $\begingroup$ @HDE226868 What an error on my part! I repent immediately! $\endgroup$
    – PipperChip
    Jan 29, 2015 at 23:38
  • $\begingroup$ The Magrathean movement might be easier if one used Bistromatics, which is presumably what the Magratheans did: hitchhikers.wikia.com/wiki/Bistromatics $\endgroup$
    – glenatron
    Feb 2, 2015 at 15:08

If you want to build a second habitable planet the solution IMHO would be to make the Moon (Luna) larger.

It already has a stable orbit that is very close to an orbit we know to be habitable by humans. The travel time between the new planet and the current one would be minimal, you could conceivably have regular shuttle service. Having a ready core with significant gravity would make the process much easier to start, you'd simply send stuff to the correct orbit and Moon would gobble it up and if the added material had correct orbit to begin with it would not disturb moon orbit. Additionally it would look totally cool; the tax payers would be able to see their money at work. Also AFAIK the double planet system is a stable solution, in any case adding mass to the moon would not make it significantly less stable than it already is. In contrast, making a new planet on Earth orbit or moving Mars or Venus to habitable orbit and enlarging them would cause disturbances in the orbits. I doubt this would be a real problem to a civilization capable of "building planets".

A major problem with something like this would be getting enough mass. But if you start with the Moon and add the material from Mercury, Venus, Mars and the asteroids you'd be close enough. Seriously we probably could terraform Venus, if we really wanted to. It would be too expensive to really make sense, but Venus is close enough to Earth mass (86%) that it shouldn't require super science.

So terraforming Venus would be a decent plan B. Probably cheaper (but not as cool) as terraforming Moon. With Moon you could honestly claim to have built a new planet, Venus already is a planet and quite similar to Earth.

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    $\begingroup$ I would think that the major problem with adding Mercury, Venus, Mars and the asteroids to the moon would be the detrimental influence on Earth's tides. Right? $\endgroup$
    – Samuel
    Jan 29, 2015 at 23:31
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    $\begingroup$ If you'd be doing re-engineering of planets, tides aren't a concern. For an insignificantly tiny fraction of the energy required to transport the material of Earth-II you could install engines that artificially move the ocean in whatever way you want every day - the total mass of all our oceans is ~0.02% of the mass you'd be moving to make a second earth. Making all our oceans into a very large airconditioned indoors swimmingpool is trivial long before you can dream about rearranging planets. $\endgroup$
    – Peteris
    Jan 30, 2015 at 3:01
  • $\begingroup$ @Samuel Thanks, I missed the effect on tides, actually. But peteris actually has a pretty good point about the relative scale of managing tides and building planets. I think most of the increase could be compensated by moving the Moon farther out, a trivial addition to the process. Same with giving a proper day time cycle. The effects on ecology from having fewer but larger tides would be still be considerable. But generally fairly local I think. The disturbances caused by global travel have been much larger and haven't caused an apocalypse. Yet. $\endgroup$ Jan 30, 2015 at 4:37
  • $\begingroup$ @Samuel At closer thought increasing the distance would have no effect on the frequency of the tides. And increasing the Moon mass would move the center of gravity for the Earth-Moon system, so there would be changes to Earth orbit too. Seeing the sheer scale needed to build a planet anyway probably still manageable. $\endgroup$ Jan 30, 2015 at 5:59

If you are actually building a new world, you might want to consider whether to stick to the traditional design of a big spherical of nickel/iron. This leads to "where" options.

If you happen to have cheap mass conversion, you can build a small planet with earth gravity by making it very dense. This will be cheaper in terms of total mass moved. About six lunar masses converted to gold would work nicely, giving earth gravity on a Luna sized planet.

Now, does that mean you can just put it in a Earth orbit?

I'd see the main issue being the tidal effects. To keep to the same as the moon's effect with six times the mass, we need to know that tide varies as the inverse cube of distance. So, put the new planet about 80% further away than the current moon.

You might want to use Luna as a starting point; you can use its gravity to help attract other mass.

Once you'd had practice with the baby planet, you can scale up.


You'll also need a liquid iron core for radiation protection, a moon to stabilize Earth II's spin axis (it's nice to have seasons). While we're at it, why not create some sort of wormhole to resupply the Sun's hydrogen fuel so that it won't ever run out?

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    $\begingroup$ While there are some good ideas in this answer, they could do with a little expansion. How might these things be done? $\endgroup$
    – ArtOfCode
    Jan 30, 2015 at 17:08

The Question has minor flaws. Earth is too near to the sun, and it's only saved by it's moon which provides a large magnetic disruption in the earth.

So, the first thing to ask is: how do you create an earth and moon.

It's a reason why habitable planets can be alot rarer than scientific extrasolar dreamers say it is, the chances of having a moon able to make so much magnetic shield, in a habitable zone is small.

The habitable zone is wide and can be optimized using CO2 and atmospheric greenhouse, you could be 15 million miles and be ok, half way between the earth and mars.

Two planets in the same orbit is naturally unstable and doesn't occur in practice. you would have to place a planet further or closer for more safety. If you had the same orbit and needed to adjust it, it would likely create earthquakes and by counterproductive to the colony.

You can have one planet nearer to the sun, and one further away, by 15million miles.

It should be possible to hurl ice at mars and move it's orbit inwards, if you double it's size, you can provide a certain amount of momentum to send it 15,000,000 kilometers from earth. mars is 30mn away and the moon is 1/4million away.


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