The solar system formed about 4.6 billion years ago from the gravitational
collapse of a giant molecular cloud. Most of the collapsing mass gathered at
the center to form the Sun, while the rest flattened into a disk from which
the planets, moons, and asteroids emerged.

Mercury is the smallest planet and the closest to the Sun. Despite that, it is
not the hottest: it has almost no atmosphere to trap heat, so its night side
plunges to minus 180 degrees Celsius.

Venus is the hottest planet. Its thick carbon-dioxide atmosphere creates a
runaway greenhouse effect that keeps the surface near 465 degrees Celsius, hot
enough to melt lead, day and night alike.

Earth is the only planet known to support life. Liquid water covers about 71
percent of its surface, and its magnetic field shields the atmosphere from the
solar wind.

Mars is a cold desert world with the largest volcano in the solar system,
Olympus Mons, which stands about three times the height of Mount Everest.
Evidence suggests liquid water flowed on Mars billions of years ago.

Jupiter is the largest planet, with more than twice the mass of all the other
planets combined. Its Great Red Spot is a storm larger than Earth that has
raged for at least 300 years.

Saturn is famous for its rings, which are made mostly of water ice. The rings
span up to 282,000 kilometers across but are on average only about 10 meters
thick.

Uranus rotates on its side, with an axial tilt of about 98 degrees, probably
the result of a giant collision early in its history. Its seasons each last
about 21 Earth years.

Neptune is the windiest planet, with gusts reaching 2,100 kilometers per hour.
It was the first planet located through mathematical prediction rather than
direct observation.

Pluto, reclassified as a dwarf planet in 2006, orbits in the Kuiper Belt. It
has five known moons, and its largest, Charon, is so big that the two bodies
orbit a point in space between them.