zone extends much further from the star. Studies in 2013 indicated an estimated frequency of 22±8% of Sun-like stars having an Earth-sized planet in the habitable zone. Venus zone The Venus zone is the region around a star where a terrestrial planet would have runaway greenhouse conditions like Venus, but not so near the star that the atmosphere completely evaporates. As with the habitable zone, the location of the Venus zone depends on several factors, including the type of star and properties of the planets such as mass, rotation rate, and atmospheric clouds. Studies of the Kepler spacecraft data indicate that 32% of red dwarfs have potentially Venus-like planets based on planet size and distance from star, increasing to 45% for K-type and G-type stars. Several candidates have been identified, but spectroscopic follow-up studies of their atmospheres are required to determine whether they are like Venus. Galactic distribution of planets The Milky Way is 100,000 light-years across, but 90% of planets with known distances are within about 2000 light years of Earth, as of July 2014. One method that can detect planets much further away is microlensing. The upcoming Nancy Grace Roman Space Telescope could use microlensing to measure the relative frequency of planets in the galactic bulge versus the galactic disk. So far, the indications are that planets are more common in the disk than the bulge. Estimates of the distance of microlensing events is difficult: the first planet considered with high probability of being in the bulge is MOA-2011-BLG-293Lb at a distance of 7.7 kiloparsecs (about 25,000 light years). Population I, or metal-rich stars, are those young stars whose metallicity is highest. The high metallicity of population I stars makes them more likely to possess planetary systems than older populations, because planets form by the accretion of metals. The Sun is an example of a metal-rich star. These are common in the spiral arms of the Milky Way. Generally, the youngest stars, the extreme population I, are found farther in and intermediate population I stars are farther out, etc. The Sun is considered an intermediate population I star. Population I stars have regular elliptical orbits around the Galactic Center, with a low relative velocity. Population II, or metal-poor stars, are those with relatively low metallicity which can have hundreds (e.g. BD +17° 3248) or thousands (e.g. Sneden's Star) times less metallicity than the Sun. These objects formed during