How many solar systems are there?

How far can two solar systems be without disrupting eachother?

• I am making a sci-fi story, and I was wondering how far apart two solar systems could be. Each wuld have four planets that are the size of earth.

• Answer:

  Scientifically, your question is asking about the "Roche Limit." In a work of fiction, you can refer to that and then describe in more sci-fi ways what is happening in your story. The Roche Limit is the point at which the gravitational pull between masses, and the tidal effects caused by momentum are balanced. That means that the pull of gravity facing the masses is trying to pull them together and momentum is carrying them apart. Depending on what the bodies are made of, this can have either no effect, a shape changing effect - where the body or bodies elongate toward the other, depending on which is more massive and how solid they are, or a disintegration effect, where the smaller is torn apart and part becomes a ring of material in orbit around the other. This limit is determined by the masses involved. It can be seen in planetary rings such as those around Saturn, and in binary stars where material from the secondary is falling across trillions of miles into the more massive star. In your story, the planets farthest from their star would be affected first as their orbits were elongated toward the approaching star. This could be how astronomers in your story would discover the disaster that was approaching. In reality, an approaching star would be detected far too soon to make the story exciting. Astronomers would say to each other, "Oh, oh. It looks like that star is going to destroy us in about a million years or so." So to make it exciting, perhaps a dark star, a neutron star, or a black hole, that can not be seen visually is first discovered because of its disruption of the orbits of the outer planets. Then they could say, "Oh my God! There is something out there headed this way! Alert the President immediately!" :-D You can adjust the distance at which occurs simply by changing the mass of the approaching star to suit your needs. Higher mass = farther away, smaller mass = closer. You could have sections in your book related to 1. Discovery 2. Determining the magnitude of the danger (disaster) 3. Plans for survival (Interstellar colonization/building underground cities) 4. The event. Good luck!