The
Solar Systemconsists of the
Sunand the
astronomical objects gravitationallybound in
orbitaround it, all of which
formedfrom the collapse of a giant
molecular cloudapproximately 4.6 billion years ago. Of the many objects that
orbitthe Sun, most of the
massis contained within eight relatively solitary
planets. The four smaller inner planets,
Mercury,
Venus,
Earthand
Mars, also called the
terrestrial planets, are primarily composed of rock and metal , the four outer planets, the
gas giants, are substantially more massive than the terrestrials and the two largest,
Jupiterand
Saturn, are composed mainly of hydrogen and helium; the two outermost planets,
Uranusand
Neptune, are composed largely of
ices, such as water, ammonia and methane, and are often referred to separately as "ice giants". In addition to thousands of
small bodies[e]in those two regions, several dozen of which are considered dwarf-planets like
Ceres,
Pluto,
Haumea,
Makemakeand
Eris.The Sun is the Solar System's
star, and by far its chief component. Its large mass (332,900 Earth masses)
produces temperatures and densities in its
coregreat enough to sustain
nuclear fusion,
which releases enormous amounts of
energy, mostly
radiatedinto
spaceas
electromagnetic radiation, peaking in the 400–700 nm band of
visible light.The four inner or terrestrial planets have dense,
rockycompositions, few or no
moons, and no
ring systems. They are composed largely of
refractoryminerals, such as the
silicates, which form their
crustsand
mantles, and metals such as
ironand
nickel, which form their
cores.
