By Drew Lichtenstein, updated March 24, 2022
The solar system is composed of the Sun, eight planets, and a variety of smaller bodies such as comets, asteroids, and dwarf planets. The most abundant elements across these bodies are hydrogen and helium, owing to the Sun and the four gas‑giant planets being largely made of them.
Hydrogen is the lightest and simplest element—one proton, one electron, and no neutrons. It dominates the solar system, accounting for roughly 75 % of its mass. The Sun’s core is almost entirely hydrogen, and the gas giants—Jupiter, Saturn, Uranus, and Neptune—share the same composition.
Helium, the second most common element, comprises about 25 % of the solar system’s original material. In the Sun, nuclear fusion turns four hydrogen atoms into one helium nucleus (two protons, two neutrons). Helium also dominates the atmospheres of the gas giants.
While hydrogen and helium reign supreme, trace gases give each planet a distinct character. Earth’s atmosphere is largely nitrogen with oxygen, whereas Neptune’s blue hue comes from methane (carbon and hydrogen). Over the next five billion years, as the Sun exhausts hydrogen, it will fuse helium into heavier elements like carbon and oxygen, enriching the solar system further.
Solid elements are a minority in the solar system, making up less than 1 % of its total elements because of the Sun’s massive gaseous envelope. Nonetheless, iron is a key solid, believed to compose the cores of all terrestrial planets.
