Solar System moons. The centaurs and many short-period comets also orbit in this region. Due to their greater distance from the Sun, the solid objects in the outer Solar System contain a higher proportion of volatiles, such as water, ammonia, and methane than those of the inner Solar System because the lower temperatures allow these compounds to remain solid. The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. Jupiter and Saturn are together more than 400 times the mass of Earth and consist overwhelmingly of hydrogen and helium.
Exploration of Jupiter is also searching for clues about how Jupiter formed, including whether the planet has a rocky core, the amount of water present within the deep atmosphere, and how the mass is distributed within the planet. "Juno" also studies Jupiter's deep winds, which can reach speeds of 600 km/h. ESA's Jupiter Icy Moon Explorer (JUICE) has been selected as part of ESA's Cosmic Vision science program. It is expected to launch in 2022 and, after a series of flybys in the inner Solar System, arrive in 2030. In 2012, the European Space Agency's selected the "JUpiter ICy moon Explorer" (JUICE) as