As its name implies, hydrogen was first observed in water. Water consists of one oxygen atom bonded with two atoms of hydrogen. The abundance of water on Earth makes it the most common source of hydrogen on the planet. Pure hydrogen is rare on Earth, however, due to its propensity to react in the presence of oxygen and precipitate out as water vapor.
Atomic and molecular hydrogen are most common beyond the Earth. Hydrogen is the primary constituent of the Sun, as well as most other stars, and it can be observed in vast quantities throughout the universe. Often, hydrogen forms the major component of certain nebulae, the so-called "stellar nurseries," which incubate the next generation of stars.
Hydrogen is also the main component of the atmospheres of gas giant planets. Jupiter, Saturn, Uranus and Neptune all have a proliferation of hydrogen and various compounds of hydrogen in their chemistry. This hydrogen-rich chemistry is an important distinction when describing a planet. Because hydrogen and oxygen cannot be held in equilibrium within the same atmosphere, a planet can be described by which gas dominates its air. An oxygen-rich world, such as Earth, is said to have an oxidizing atmosphere, while a hydrogen-rich world, such as Jupiter, has a reducing one.