, in turn, determines whether a planet can retain an atmosphere and sustain geologic activity. The abundance of metals is measured by metallicy-the ratio of the number of metal atoms to the number of hydrogen atoms. Particularly, the lower the metallicy of a planet, the lower is the abundance of metals. In fact, without enough metals, large planets can not form at all. On the other hand, too high of a metallicity can also be a problem. High metallicity increases the density of the protoplanetary disk and therefore induces the giant planets to shift position. The result of such shift is that such planet will throw any smaller, Earth-like bodies out of the system all together or push them into the sun. b. Metallicy is not enough to determine hospitality of a planet. To contain life, a planet must also be kept reasonable safe from outside threats, such as the impact of asteroids and comets. Blasts of radiation also present a problem. Until a certain point, a planet’s magnetic field can fend off most particle radiation and its ozone layer can screen out dangerous electromagnetic radiation. However, sufficiently energetic radiation can ionize the atmosphere and generate nitrogen oxides in amounts capable of wiping out the ozone layer. Energetic radiation hitting the atmosphere can also let loose a deadly rain of secondary particles. Effects on EarthIn many ways, the Milky Way is unusually hospitable: a disk galaxy with orderly orbits, comparatively little dangerous activity (comets and asteroids) and plenty of metals. Earth satisfies the requirement for optimal metallicity. The mix of land and sea on Earth is important for atmospheric temperature control and other processes. In our solar system, the frequency of asteroid impact depends on the details of Jupiter’s orbit and formation; the rest of the galaxy has no direct effect. As far as radiation is concerned, the nucleus of the Milky Way is currently relativ...
