Our Earth first came into existence 4,543,000,000 years ago as an inhospitable ball of molten rock that barely survived a crash from a Mars-sized rogue planet whose impact gave rise to the creation of our moon.

In this alternate timeline, Earth was created exactly five billion years ago as the result of a collision between two rogue planets. One was obliterated into pieces while the other was lucky to have some crust and atmosphere left.

One was similar to Venus—a hot ball of molten rock concealed by a thick atmosphere of sulfuric acid, carbon dioxide and methane. The ancient atmosphere was so thick that the pressure would be comparable to a man trying to lift the Queen Elizabeth 2, an ocean liner that weighs roughly 50,000 tons. Beneath the lava-ridden surface was a core of iron, nickel, platinum, titanium, uranium, silver and gold.

To understand the other rogue planet, we must look at one of Jupiter’s moons, Europa. Back home, it has a diameter of 1900 miles, which makes it a little smaller than our moon. But what really does make it stand out is its crust. The first ten to 30 kilometers of it—six to 19 miles—of ice. Beneath that is 160 kilometers—or 100 miles—of liquid water before descending down to a rocky, silicate mantle and finally to a core of iron and sulfur. This second rogue planet is pretty much Europa enlarged to the size of its larger brother, the moon Ganymede, a diameter of 3270 miles.

During the collision, the kinetic energy reduced one rogue planet’s icy surface and the water it might have concealed into a vast nebula of water vapor that enveloped the planet at low orbit. The majority of the gas might have escaped to space, but the pressure of the atmosphere was so high that not all of the water vapor molecules could escape.

After the collision, the two cores merged together to become a brand-new one. The largest pieces of debris clumped together to create the alternate Earth. The smallest of the debris, meanwhile, clumped together to create its moon.

The trapped water vapor cooled down, condensing into rainwater that hit on the still-molten surface, resulting in the release of steam which rose to the atmosphere, gathering more water vapor, which ultimately means more rain. For the first 250 million years after impact, this repeated cycle of evaporation and condensation would create the oceans of Great Lakes Earth.

Is this scenario likely or not?

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    $\begingroup$ Its unusual to post a question of the form: I think x am I right? You also need to give more details. What are the masses and average temperatures of the two rouge planets. Do they stick together after collusion or do parts break off? Are they large enough to hold an atmosphere? Yes some ice will be vaporized weather it will rain back depends on the temperature of the resulting planet and the temperature and mass of the first two planets $\endgroup$ Jan 16, 2016 at 1:27
  • $\begingroup$ I'm going to assume that the ice planet had the majority of our atmosphere encased in ice. It's fine to do that, I think. As long as you ask for the validity of this scenario, and not what people think. You tread a fine line when you do this. $\endgroup$ Jan 16, 2016 at 1:28
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    $\begingroup$ I would say that, given Earth low gravity, such a "vapor nebula" would have been expelled to space by solar wind (in a way similar to what happened to Mars early atmosphere). $\endgroup$
    – SJuan76
    Jan 16, 2016 at 12:43
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    $\begingroup$ @sdrawkcabdear actually we have the "reality-check" tag for just that. Someone asking "I have this scenario, what's wrong with it". $\endgroup$
    – Tim B
    Jan 22, 2016 at 11:12
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    $\begingroup$ You can't have "a thick atmosphere of sulfuric acid, carbon dioxide and methane". Methane and sulphuric acid will react to produce CO2 and SO2. You can have one or the other, but not both. (e.g. Venus has sulphuric acid but not methane.) $\endgroup$
    – Mike Scott
    Jan 22, 2016 at 11:29

3 Answers 3


The problem with your scenario is the time required to do the cooling, the presence of water vapor will make very little difference with that and may in fact slow the process as it works as a greenhouse gas.

If you are prepared to wait for billions of years then the scenario may work, alternatively start with a cooler world and have multiple ice comets/asteroids hit it. Even a rogue planet coming in from interstellar space at an incredibly cold temperature will actually add more heat not cool things down just because of the incredible energy of the impact.


The scenario of the crash is possible. However, with a molten planet I doubt the rain would fall. This is because the huge heat would be made worse by the fact that the water vapour is a greenhouse gas, which will increase the temperature and make it less likely to rain.

You are more likely to end up with a super hot world, rather than a habitable one. Also in liquid forms (not sure about gaseous) mixing water with sulphuric acid can cause heat as they react.

  • $\begingroup$ Do you know any body of ice many times larger than a comet or an asteroid that would bring rain to the planet? $\endgroup$ Feb 12, 2016 at 1:29
  • $\begingroup$ A lot of small comets one after another or a super cooled planet (around -100) might work. $\endgroup$ Feb 12, 2016 at 16:17

The biggest problem I see is that the huge volume of atmospheric gas from the "fire" side of your equation is going to be in that same nebula as the water vapour, you can't really blast the atmosphere off and keep it at the same time so the new world is going to have the same, or at least very similar, atmosphere as the Venusian parent world, the only way I can think of to explain that away is the theory that Earth's current atmosphere isn't the original at all. The theory goes that before the hit from Theia that created the moon the primordial Earth had an atmosphere shielded by a magnetic field much like today but the impact was sufficient to disrupt that field and the atmosphere was destroyed by solar radiation before the new Earth settled enough for the field to reform. Our modern atmosphere is made up of gases brought up by volcanism and down by comets.

To apply this to your scenario imagine the most glancing of blows the fire world is knocked off it's existing axis of rotation and gyrates wildly while it's magnetic field flickers and the sun strips mega-tonnes of atmosphere off, scattering it on the solar winds. Meanwhile the ice worldlet is shattered and scattered in an orbital belt of comets along an elliptical orbit in the same plane as the new, lifeless world. The new world settles and it's magnetic field strengthens. Now two (possibly four depending on the exact parameters) times in every orbit the new world passes through a thick band of cometary debris and picks up water and gases to replenish the atmosphere, and it keeps them this time.


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