Astronomers estimate that the Milky Way is home to at least as many planets as stars – 100 billion at a minimum. However, not all parts of the galaxy are suited to producing planets, let alone ones with the right conditions for life. Significant quantities of elements heavier than hydrogen and helium are needed in order for rocky terrestrial planets to arise as a byproduct of star formation, and so planets are naturally more likely to occur among the Population I stars of the galactic disc. However, some research suggests that an excess of these heavy elements will produce planetary systems with large numbers of gas giant planets, whose gravitational interactions may tend to disrupt the orbits of smaller worlds.