Mercury

Mercury is the closest planet to the sun. As such, it circles the sun faster than all the other planets, which is why Romans named it after their swift-footed messenger god.

The Sumerians also knew of Mercury since at least 5,000 years ago. It was often associated with Nabu, the god of writing. Mercury was also given separate names for its appearance as both a morning star and as an evening star. Greek astronomers knew, however, that the two names referred to the same body, and Heraclitus, around 500 B.C., correctly thought that both Mercury and Venus orbited the sun, not Earth.

Venus

Venus, the second planet from the sun, is named for the Roman goddess of love and beauty. The planet — the only planet named after a female — may have been named for the most beautiful deity of her pantheon because it shone the brightest of the five planets known to ancient astronomers.

In ancient times, Venus was often thought to be two different stars, the evening star and the morning star — that is, the ones that first appeared at sunset and sunrise. In Latin, they were respectively known as Vesper and Lucifer. In Christian times, Lucifer, or "light-bringer," became known as the name of Satan before his fall. However, further observations of Venus in the space age show a very hellish environment. This makes Venus a very difficult planet to observe from up close, because spacecraft do not survive long on its surface.

Earth

Earth, our home, is the third planet from the sun. It is the only planet known to have an atmosphere containing free oxygen, oceans of liquid water on its surface, and, of course, life.

Earth is the fifth largest of the planets in the solar systm — smaller than the four gas giants, Jupiter, Saturn, Uranusand Neptune, but larger than the three other rocky planets,

Mars

Mars is the fourth planet from the sun. Befitting the red planet's bloody color, the Romans named it after their god of war. The Romans copied the ancient Greeks, who also named the planet after their god of war, Ares. Other civilizations also typically gave the planet names based on its color — for example, the Egyptians named it "Her Desher," meaning "the red one," while ancient Chinese astronomers dubbed it "the fire star."

Jupiter

Jupiter is the largest planet in the solar systm. Fittingly, it was named after the king of the gods in Roman mythology. In a similar manner, the ancient Greeks named the planet after Zeus, the king of the Greek pantheon.

Jupiter helped revolutionize the way we saw the universe and ourselves in 1610, when Galileo discovered Jupiter's four large moons — Io, Europa, Ganymede and Callisto, now known as the Galilean moons. This was the first time that celestial bodies were seen circling an object other than Earth, major support of the Copernican view that Earth was not the center of the universe.

Saturn

Saturn is the sixth planet from the sun and the second largest planet in the solar systm. Saturn was the Roman name for Cronus, the lord of the Titans in Greek mythology. Saturn is the root of the English word "Saturday."

Saturn is the farthest planet from Earth visible to the naked human eye, but it is through a telescope that the planet's most outstanding features can be seen: Saturn's rings. Although the other gas giants in the solar systm — Jupiter, Uranus and Neptune — also have rings, those of Saturn are without a doubt the most extraordinary.

Uranus

Uranus is the seventh planet from the sun and the first to be discovered by scientists. Although Uranus is visible to the naked eye, it was long mistaken as a star because of the planet's dimness and slow orbit. The planet is also notable for its dramatic tilt, which causes its axis to point nearly directly at the sun.

British astronomer William Herschel discovered Uranus accidentally on March 13, 1781, with his telescope while surveying all stars down to those about 10 times dimmer than can be seen by the naked eye. One "star" seemed different, and within a year Uranus was shown to follow a planetary orbit.

Neptune

Neptune is the eighth planet from the sun. It was the first planet to get its existence predicted by mathematical calculations before it was actually seen through a telescope on Sept. 23, 1846. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest that the gravitational pull from another celestial body might be responsible. German astronomer Johann Galle then relied on subsequent calculations to help spot Neptune via telescope. Previously, astronomer Galileo Galilei sketched the planet, but he mistook it for a star due to its slow motion. In accordance with all the other planets seen in the sky, this new world was given a name from Greek and Roman mythology — Neptune, the Roman god of the sea.

Only one mission has flown by Neptune – Voyager 2 in 1989 – meaning that astronomers have done most studies using ground-based telescopes. Today, there are still many mysteries about the cool, blue planet, such as why its winds are so speedy and why its magnetic field is offset.

Pluto

Pluto, once considered the ninth and most distant planet from the sun, is now the largest known dwarf planet in the solar systm. It is also one of the largest known members of the Kuiper Belt, a shadowy zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets.

In 2006, Pluto was reclassified as a dwarf planet, a change widely thought of as a demotion. The question of Pluto's planet status has attracted controversy and stirred debate in the scientific community, and among the general public, since then. In 2017, a science group (including members of the New Horizon mission) proposed a new definition of planethood based on "round objects in space smaller than stars," which would make the number of planets in our solar systm expand from 8 to roughly 100.