Formation and evolution of the Solar System owe its substantially lower mass simply to having formed a few million years after Jupiter, when there was less gas available to consume. T Tauri stars like the young Sun have far stronger stellar winds than more stable, older stars. Uranus and Neptune are thought to have formed after Jupiter and Saturn did, when the strong solar wind had blown away much of the disc material. As a result, those planets accumulated little hydrogen and helium—not more than each. Uranus and Neptune are sometimes referred to as failed cores. The main problem with formation theories for these planets is the
Solar System to as "volatiles". The distance from Earth to the Sun is . For comparison, the radius of the Sun is . Thus, the Sun occupies 0.00001% (10 %) of the volume of a sphere with a radius the size of Earth's orbit, whereas Earth's volume is roughly one millionth (10) that of the Sun. Jupiter, the largest planet, is from the Sun and has a radius of , whereas the most distant planet, Neptune, is from the Sun. With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and