We all know what a planet is. If not, move away from computer and take a walk outside. Feel the amazing habitat we are living in. There are different type of planets, rocky planets like our Earth, gas giant planets like the Jupiter. Interestingly, the other type of planets is missing in our solar system, i.e., gas dwarf planets whose radii lie in between about 2 to 4 times of Earth radii. Formation of different type of planets depends on how much metals are available in a protoplanetary disk, where planets form. The metals astronomers refer to are elements with atomic numbers higher than 2, i.e., every element except for Hydrogen and Helium.
The correlation between the gas giant planet occurrence and the metallicity of their host stars has long been established. The correlation can be explained by the following theory. Gas giant planets have only a few million years to form, which is short compared to the life time of a star, i.e., typically a few giga years. During the few million years, gas giant planets start from dust, to pebbles, to planetesimals, then to planet embryos that are massive enough to accrete their surrounding materials in a runaway fashion. Without sufficient metals in the system, which are the fundamental elements in dust, pebbles, planetesimals, and embryos, gas giant planets are very difficult to form. On the other hand, the more metals are available in the system, the more efficient gas giant planet form. This theory provides a physical explanation of the observed planet-metallicity correlation for gas giant planets.
Despite tremendous success in confirming the planet-metallicity correlation time and time again for gas giant planets, the one for smaller planets, such as rocky planets and gas dwarf planets, has not been established. The correlation has significant implications to future missions in the search for Earth-like planets. If rocky planets are more common around metal-rich stars, then metal-rich stars should be prioritized in the target selection and observation. Using the Kepler data, I find that the planet-metallicity correlation is universal. Metal-rich stars tend to have more planets. Although the correlation becomes weaker for smaller planets, the correlation holds for rocky planet, gas dwarf planets and gas giant planets.