The gases that surround a star or planet — called the object’s atmosphere — vary from one celestial object to the next. Many planets have complex atmospheres, but stars usually have simple ones. As with the Earth’s sun, the atmospheres of most stars are composed of hydrogen and helium. However, a few neutron stars with oxygen-rich atmospheres have been documented.
Mercury is thinly covered in hydrogen, oxygen, water vapor and potassium, but most of its atmosphere has been blasted away by the Sun. By contrast, Venus has a very thick atmosphere composed primarily of carbon dioxide. The Earth’s atmosphere is primarily composed of nitrogen and oxygen, although carbon dioxide, argon, neon, helium, krypton, hydrogen, ozone, neon, carbon monoxide, ammonia, nitrous oxide and iodine are present in small amounts. Water vapor is also an important component of the Earth’s lower atmosphere. Mars has very little atmosphere, and what is left is composed of carbon dioxide, nitrogen, oxygen, argon, methane and water vapor.
Determining where a planet ends and its atmosphere begins is difficult for gaseous planets, such as Jupiter and Saturn. However, scientists normally place the lower boundary of the atmosphere as the place at which the pressure is equal to 1 bar. Jupiter’s atmosphere is primarily composed of hydrogen and helium, but methane, water, ammonia and hydrogen sulfide are present in trace amounts. Saturn’s atmosphere is nearly identical to Jupiter’s except that some of the chemicals in Saturn’s atmosphere combine to form smog. Uranus and Neptune are similar in composition to Jupiter and Saturn, being primarily composed of hydrogen and helium.