orm unless the stars are at least 50 AU away (1 AU = distance from the Earth to the sun) and stable orbits can only be achieved where companion stars are at less than 20 million miles apart or farther than one billion miles.3 A planet’s orbit pattern is also of concern. Earth’s orbit is very stable and only has a small degree of ellipticity. A highly elliptical orbit would cause a planet to oscillate in and out of a habitable zone. If a planet could even form in this situation, their orbits would be perturbed by varying gravity of more than one star causing ejection or falling into one of the stars. Another factor in considering the habitable zone is insolation. Insolation is the stellar energy a planet receives. This quantity could only vary by as much as ten percent without affecting its habitability. Much less than ten percent fluctuations on Earth is what causes our climate changes during seasons. Furthermore, the insolation effect would be magnified in a binary star system due to periodic eclipse of one of the stars. Rather than focus on individual star-planet distances in reference to their habitable zones, let us choose the entire Milky Way as a basis. The diameter of our galaxy is about 85000 light-years across. Our sun is located 25000 light-years from the center of the galaxy. Our solar system is located in a region where star density is low, which is indicative of the habitable zone of the Milky Way. Places further toward the center of the galaxy are too densely packed with stars to be in the HZ. The outer zone has a different problem: the wrong type of matter for Earth-like planets exists there, the concentration of heavy elements and rate of new star formation is too low. Even the shape of the Milky Way is important. Our spiral shape is much preferred to elliptical since elliptical galaxies typically contain no heavy elements, little dust, little new stars, and an abundance of asteroids and comets. It has also ...
