Neptune is more interesting than you think

Neptune on Triton’s horizon (NASA)

Neptune on Triton’s horizon (NASA)

Neptune – which these weeks lies at its brightest and closest of the year – is an amazing place. Give me a chance to prove it.

Neptune is an enormous blue ball 58 times larger than Earth by volume. The only planet that cannot be seen with the naked eye, it lurks among the sprawling hallucinogenic pattern that is Aquarius. It’s highest at midnight, halfway up the southern sky. But we really should start this story at the beginning.

In 1845, the French astronomer Urbain Le Verrier realized that Uranus, discovered 60 years earlier, was not moving in a normal way. Something was mysteriously tugging at it. He calculated where this putative new giant planet should be located and tried to get some astronomer to point a telescope at the spot. No one was interested. Meanwhile, in England, John Couch Adams had the same idea and was meeting with similar indifference.


What finally happened was that Le Verrier convinced someone at the Berlin observatory to point its small nine-inch telescope at his calculated position. Bingo: Within one hour, Neptune was found within a single degree of the predicted spot. That was on September 23, 1846.

Naturally, a huge fight over discovery rights erupted between France and England. To this day, credit is usually shared. In truth, the Frenchman was more accurate. But he had such an annoying personality – Le Verrier wanted the new planet named after himself – that even his fellow Frenchmen backed off and ultimately agreed to the joint-discovery idea. Fortunately, the world quickly agreed to follow the tradition of naming planets for Roman deities, and chose the god of the sea.

A mere 17 days after Neptune’s discovery, an enormous moon fully two-thirds the size of ours was seen orbiting it. For nearly a century, everyone simply (and strangely) called it “Neptune’s moon.” Then, in the mid-1930s people instead referred to it as Triton. In 1949, a second satellite was found – a tiny one, just one-tenth our own Moon’s size – and it was named Nereid, following the rule that everything Neptunian must have a sea-deity nomenclature. Then in 1981 and 1989 came Larissa and then Proteus, first seen by the Voyager 2 spacecraft. The Hubble Telescope eventually found others, and just last year the most recent discovery brought the number to 14.

That’s not so interesting. But this is: These moons are bizarre. Triton is the only major moon that orbits its planet in the wrong direction: backwards. Nor does it circle Neptune’s Equator. These oddities prove that it was a traveling vagabond captured by Neptune’s gravity – except that such captured moons have elliptical orbits, and Triton moves in a perfect circle. Curiouser and curiouser.

By contrast, Nereid has an astonishingly squashed elliptical orbit that brings it seven times closer to Neptune on some occasions than others. Imagine if our own Moon sometimes grew seven times larger in our sky.

Neptune’s newer, most distant moons are also weird, because two of them orbit at the astonishing distance of 30 million miles: by far the most distant satellites from their parent planet in the known universe. That’s farther away from Neptune than Venus is from us. It’s hard to imagine such moons being gravitationally connected to Neptune in a stable way, as they take 26 years to make a single orbit – as compared to four weeks for our own Moon, and a few days apiece for the giant moons of Jupiter.

Neptune has its own peculiarities. Its deep blue color is a mystery. The known presence of natural gas ought to make it greenish-blue like Uranus, so some odd unknown substance must lurk in its atmosphere. Its very round orbit seems to be periodically invaded and crossed by Pluto’s highly elliptical path, when the two orbits are depicted on a chalkboard. But seen in three dimensions, Pluto’s path is so highly tilted that it actually ventures nearer to Uranus than it ever comes to Neptune.

So Neptune is a loner. The farthest planet (after Pluto’s demotion), it’s so distant that it only appears the size of a quarter-dollar coin held up by someone a mile away. Taking 165 years to orbit the Sun and with seasons that each last 41 years, and with winds that blow five times faster than tornadoes, it is an odd place. It is fortunate indeed that the plucky Voyager, operating on a finicky backup radio, spent a day there on August 25, 1989, delivering our only decent look at Neptune and strange Triton, whose surface, the coldest place in the solar system, resembles a cantaloupe.

No further missions to their vicinity are on the drawing boards.

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