Kortenkamp said that in combination with other studies, the new model hints at another idea: Depending on how close two stars are to each other in a binary system, planets might form around either star by the runaway growth method.

Researchers have suspected that if a pair of stars were separated by great distances, say 1,000 times the distance from Earth to the Sun (1,000 AU, or astronomical units) then planet formation around either of the stars might proceed based on currently understood rules.

But could there be terrestrial planets in such a system? And is there a lower limit? What if the two stars are just 10 AU apart?

Kortenkamp said that while the model doesn't prove that terrestrial planets could form around stars that are so close together, "you could have terrestrial planets forming when the stars are closer together than traditionally thought."

So if Earth-like planets formed around these stars, they would have had to form after the larger planet migrated in. While the new model says such a formation is possible, it's uncertain whether there would be enough material left to build a planet out of.

Even the largest extrasolar planets can't be seen. They are found by noting a gravitational wobble in the host star. The technique only allows the detection of very large planets that are very close to a star.

But as techniques improve, researchers fully expect to find giant planets orbiting at greater distances from other stars.

In such systems, Kortenkamp says terrestrial planets will have had a chance to form, regardless of how the giant planets formed. These Earth-like planets might travel in a so-called "Goldilocks" orbit around a star, a region of space that is not too hot and not too cold -- just right for the development of life.