The solar system consists of the sun, the eight planets and several other miscellaneous objects, such as comets, asteroids and dwarf planets. The most abundant elements among these objects are hydrogen and helium, primarily because the sun and the four largest planets are predominantly made up of these two elements.
No. 1: Hydrogen
Hydrogen is the most common element in the universe because it is the simplest element in the universe. A hydrogen atom has one proton, one electron and no neutrons, making it the lightest element. The larger an object is, the stronger its gravitational pull, and thus the more hydrogen it has. The sun is primarily made out of hydrogen, as are the four gas giant planets (Jupiter, Saturn, Uranus and Neptune). The initial formation of the solar system was approximately 75 percent hydrogen.
No. 2: Helium
Helium is the second most common element in the universe, and, like hydrogen, it is relatively simple, as it has two protons and two electrons. Helium made up about 25 percent of the solar system when it was originally forming; however, an isotope of helium is also produced in the sun during nuclear fusion. Nuclear fusion involves four hydrogen atoms coming together to form the helium isotope that has two protons and two neutrons. Helium is also the second-most abundant element in the gas giants.
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Other gases exist in small amounts in the solar system, though none at the level of hydrogen and helium. For example, the Earth's atmosphere is mainly nitrogen, with some oxygen. Neptune, despite being almost entirely hydrogen and helium, has a distinctive blue tint because of methane (a combination of carbon and oxygen) in its atmosphere. Eventually -- around five billion years from now-- when the sun burns out of hydrogen, it will begin to fuse helium in its core and produce more carbon and oxygen in the solar system.
Solids -- a Rarity
Solid elements are exceptionally rare in the overall scheme of the solar system. Despite being predominant on Earth, they constitute less than 1 percent of the total elements in the solar system, primarily because of the size and percentage of gas in the sun and gas giants. Nonetheless, some solid elements exist, the most important of which is iron. Iron is believed to be in the core of every terrestrial planet.