I am creating a world for Sci Fi story in which I am trying to have two non-tidally locked earth sized planets in a binary orbit around Alpha Centauri A. The only way I can think of this happening is by a double event in which the original planet, let's say around 3 Earth masses, was struck by a 1 Earth mass body and split to create two roughly Earth sized world's orbiting eachother.

Within a number of millions of years, they would become tidally locked. So I need another event to allow one, or both of them to have their own spin and be habitable. The best suggestion I received to do that is to have an impact event to resurface the planet(s) and add a soon to them. Then potentially they will have a more difficult time to become tidally locked again because of this and the fact that they would probably have different spins at different angles.

The question I have, is how long after this event would the planets become habitable for humans? Intelligent life, or even animal life, does not need to exist/have been formed on the planets. By habitable I mean that they should have a breathable atmosphere with sustainable temperatures. The humans traveling there will have a decent ability to terraform the planet from that point.

Yes, I know this is extremely unlikely to ever actually happen, but the pen is mightier than the astrophysics textbook. If there are any suggestions for another means to achieve my end result then please feel free to share.


  • $\begingroup$ You only need for it to happen once, no matter how extremely unlikely. $\endgroup$ Feb 23, 2020 at 22:36
  • $\begingroup$ 'Within a number of millions of years, they would become tidally locked.' I am not sure why this is absolutely certain. Astronomers are finding exceptions to how things are 'supposed to be' on a regular basis. The only rule seems to be that there are no rules. $\endgroup$ Feb 23, 2020 at 22:40
  • $\begingroup$ It took rough 3 to 3.5 billion years for life on earth to produce the great oxdisation event. I suppose it would take roughly the same amount of time on a new world. The problem is that Oxygen burns with just about anything, so how much stuff is there to oxidise? Only when the oxidisation has been more or less completed will oxygen start to accumulate in the atmosphere in any amount needed to support humans. $\endgroup$
    – Kain0_0
    Feb 24, 2020 at 2:19
  • $\begingroup$ That's a good point, is there a way that oxygen could be artificially added and end with the same result? Part of the story involves a series of robotic ships to arrive 50 to 100 years before the colonists would arrive. At that point, the robotic ships initiate a terraforming procedure. $\endgroup$
    – Markitect
    Feb 25, 2020 at 2:59
  • $\begingroup$ One possible solution would be to make your planets into moons and tidally lock them to a local gas giant--for example, each moon of Jupiter is tidally locked to Jupiter but still has day/night cycles because they're not locked to the sun. $\endgroup$
    – Dragongeek
    Jun 18, 2020 at 12:28

2 Answers 2


We almost had all that here.

Young Earth got hit by a Mars size impactor, resulting in the Earth and Luna.


Known as Theia, the Mars-sized body collided with Earth, throwing vaporized chunks of the young planet's crust into space. Gravity bound the ejected particles together, creating a moon that is the largest in the solar system in relation to its host planet.

You want 2 planets the same size. So in your world the young Moonless earth is bigger, and the impactor creates a moon that is bigger. Pretty much what you said.

Earth has done ok for life.

As regards tidal locking the Moon is locked to earth but not vice versa because of the mass differences. Avoid tidal locking the same way. Your partners are the same size but different mass. You could have /earth sized planets/ but have their composition be different as is the case with the Earth and the moon. Your earth equivalent keeps a metal core. Maybe the impactor is a bare metal core. Your partner planet is made mostly of light crust elements (like Luna is) and so although the same size is less massive.

As regards how long to habitability - the moon was formed 4.5 billion years ago and the Great Oxygenation Event was 2.5 billion years ago. So 2 billion years to a green sea and O2 atmosphere.


If humans have the ability to terraform, they would likely have the ability to speed up the process of breathable atmosphere, especially if all the pieces were already in place for oxygen to occur.

The first place we will terraform will likely be Mars or the Moon, and a large part of that formation would be developing an atmosphere that will eventually lead to an oxygen-rich atmosphere- safe to say that the technology would already exist. Otherwise, what is the point of terraforming as an oxygen-based species from an oxygen-based planet?

The length to habitation would then be reliant on the terraforming technology that has been developed, I see this taking place as a large cryostasis colony ship that would remain in orbit as terraforming bots work their magic technology ability.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .