Neptune: The Other Blue Planet

Neptune is the most distant planet in our Solar System—2.8 billion miles from Earth!

How Neptune Was Discovered

In 1845, the French astronomer Le Verrier realized that planet Uranus—discovered 60 years earlier—was not moving in a normal way.  Something was mysteriously tugging at it. He calculated where this unseen planet should be located and tried to get someone to aim a telescope at the spot.  No one was interested.

Meanwhile an English astronomer had the same idea and was meeting with similar indifference. Finally, Le Verrier convinced someone at the Berlin observatory to point their small 9 inch telescope at his calculated position.  Bingo.  Within one hour, Neptune was found within a single degree of the predicted spot. That was September, 1846.

Credit: NASA Voyager 2. Taken August, 1989.

Just 17 days later, a giant moon was seen orbiting it. It strangely had no name for 75 years, but in the 1930s people started calling it Triton after the 3-pronged staff held by the god of the sea.  A second satellite was found in 1949, and the Hubble telescope eventually discovered a bunch more.

Neptune’s Moons

Planet Neptune’s moons are bizarre.  Triton is the only major satellite that orbits its planet in the wrong direction (to which Neptune orbits).  Backward!  And its orbit is a perfect circle.

Credit: NASA. Composite illustration of Neptune as seen from its moon, Triton, using images from Voyager spacecraft.

Neptune’s tiny moon Nereid is the opposite, with an weirdly squashed elliptical path that brings it SEVEN times closer to Neptune on some occasions than others. Imagine if our own moon sometimes got seven times larger than before?

Other newly found moons are also weird, because two of them orbit at the astonishing distance of 30 million miles—the most faraway satellites from their planet in the known universe. That’s farther than Venus is from us.  It’s hard to imagine such moons even being gravitationally connected with mysterious Neptune.

So it’s a strange place, that huge blue world, so enormous that 58 Earths could fit inside. Even if we can’t see it floating halfway up the southern sky, among the dim stars of Aquarius.