Sitting 2.73 billion miles away from Earth, the planet Neptune is technically only one year old, thanks to its incredibly long orbit. Though its mass is 21 times larger than that of Earth, Neptune was discovered only 169 years ago, on Sept. 23, 1846. But scientists say they still know very little about the cold, mysterious gas giant. 

As scientific historian Alan Chapman said on the occasion of Neptune's technical first birthday in July, "Let's wish it a happy birthday, but perhaps let's keep as far away from it as we can, as it won't give you a [warm] welcome."

Neptune, the eighth planet, was first discovered after astronomers noticed that some force was pulling Uranus out of its projected orbit during observation as early as the 1700s. At first, scientists believed that Isaac Newton's laws of gravity were flawed, but they soon found out that another, larger planet was disrupting Uranus' orbit.

German scientist Johann Gottfried Galle used Urbain Le Verrier's mathematical calculations to pinpoint the location of the new planet, and first observed it the night of Sept. 23, 1846. The planet was the first in the solar system to be discovered by utilizing mathematical calculations, instead of by observation alone. 

Neptune is the solar system's outermost planet, ever since Pluto was renamed a dwarf planet, or planetoid. Named for the Roman god of the sea, the planet has six rings and 13 moons.

Because of its size and distance from the sun, this young planet has long, harsh seasons and extremely volatile weather. Seasons on Neptune last 40 Earth-years -- half a lifetime on Earth -- and winds during storms can roar in at a devastating 1,200 miles per hour.

Astronauts have had to keep their distance from the planet because they can only approach during spring and summer, and the first and only up-close images came from photographs taken by the NASA space probe Voyager 2 in 1989.