The Discovery Of Pluto

Believe it or not, the discovery of Pluto was by accident. Astronomers had noted anomalies in the orbit of Neptune, and much like they did with Uranus' orbit, thought there was a planet lying outside Neptune's orbit which would give rise to the anomalies.  So astronomers began to look for it.

It wasn't until Kansas-born Clyde Tombaugh (it's only important that I mention he's from Kansas because so am I), using stereo imaging of photographic plates found what he thought was the new planet. However, the new object he found was Pluto.

Lowell Observatory Archives

Clyde Tombaugh

 

But after more measurements, the new planet was just not large enough to have the effect that they saw on Neptune. After some more calculations, Neptune's orbit was confirmed just using the masses of Neptune and Uranus.

So Pluto was there, but was not what was expected. If the orbit of Neptune had been calculated  correctly initially, it might have been a while longer before Pluto were discovered.

The Great Dark Spots

Neptune's First Great Dark Spot
Image Credit:

Voyager Project, JPL, and NASA

The Great Dark Spot is actually a series of storms that were first discovered by Voyager 2 in 1989. They are anticyclonic (weather systems with high pressure eyes) much like the Great Red Spot on Jupiter. However, unlike the Great Red Spot, these storms are generally cloud-free and only last a few months to few years.

The winds associated with the Great Dark Spots are the fastest known in the solar system, topping out at 2400 kilometers per hour (about 1500 miles per hour). They are thought to be holes in the methane cloud deck, occuring in the troposphere at lower altitudes than the clouds. The first spot to be discovered varied in size and shape as it was viewed from Earth and there had been a plan to photograph the storm with Hubble space telescope in 1994. By the time, Hubble was in position to take an image, the storm had dissipated. However, the spots do reappear on Neptune, and a new storm appeared in the northern hemisphere of Neptune. Unlike the Great Red Spot which is a single storm, the Great Dark Spots is a series of storms with similar appearances and properties to each other.

One of the prevailing theories of what happens to the storms is that as the storms migrate towards the equator of Neptune, the storms break up and disappear. Also, clouds usually appear outside the storms so may indicate that a storm has just dissipated or may appear soon.

The storms themselves are relatively stable because they are vortexes, but again, don't last as long as the Great Red Spot.

Neptune's Rings

Image of Neptune's Rings taken from Voyager 2 in 1989.

Three rings are easily seen in this image: Adams (outermost), Le Verrier (middle) and Galle (inner ring). To the left of the image, in the Adams Ring, Galatea is visible. Between Adams and Le Verrier, two faint rings can be made out: Arago and Lassell.

Image Credit:

NASA/JPL/Voyager

 

Neptune has five main rings: Galle, Le Verrier, Lassell, Arago, and Adams, named after five important astronomers in Neptune's history. Much like the rings of Uranus, the rings of Neptune are made up of dust particles, and kept in place by shepherd moons, which include Galatea (Adams Ring) and Despina (Le Verrier Ring). It is believed that Neptune has other moons which help keep the rings narrow and stable (so to speak), but they have yet to be discovered.

The Adams Ring is unique in that it contains arcs, which are caused by gravity from Galatea. The arcs were discovered when Neptune occulted a star and where the rings should have been, the star shone through the rings. Close ups by Voyager confirmed that the Adams Ring contained arcs.

Much like the rings around Jupiter, Saturn, and Uranus, the material in the rings of Neptune are not permanent. They are continuously replenished by collisions with the moons of Neptune and are kept in orbit by shepherd satellites. The rings themselves are mostly dust with some ice particles and are covered in organic material (carbon compounds) that make them dark.

Image Credit:

Sky and Telescope

Triton and the Other Moons of Neptune

Neptune has two moons that can easily be seen from Earth with the aid of a telescope. One of those moons is one of the few moons in our Solar System with an atmosphere, Triton. The other moon Nereid is smaller, but still visible in a large enough telescope. Proteus is actually larger than Nereid, but not as easily seen as it does not reflect as much light.

Triton with Neptune in the Background

Image Credit:

NASA/Voyager



 

Triton is the largest moon orbiting Neptune, though its size is only 78% that of the Moon. It is one of the few moons (and the largest of all solar system moons) to have a retrograde orbit around its planet. It is believed that some interaction early in the formation of the Neptunian system caused it to reverse its orbit around Neptune. It also may be a Kuiper Belt object, which we will discuss later. Despite having a retrograde orbit, however, it does have a very circular orbit. It is very cold and because it is cold, it has an atmosphere. Our Moon, despite being larger, does not have any atmosphere as it is much closer to the Sun than Neptune. The atmosphere is dominated by molecular nitrogen and methane, but is 10 million times less dense than the Earth's atmosphere. Because it is so cold, there are no clouds in Triton's atmosphere and is transparent, allowing us to see the surface. Triton has an icy surface of nitrogen ice, methane ice, carbon monoxide ice, and carbon dioxide ice. It was found by William Lassell in 1846, just after the discovery of Neptune itself.

Triton has few craters, which tells us that its surface is relatively young, though it does have dark spots on the surface. Those dark regions are from organic molecules, created when methane is exposed to light. And because of the methane on its surface, there are regions near its southern pole where methane geysers can erupt through cracks in the surface. Around these cracks, dark smudges are found which are created in the same way as the dark spots on Triton.

Fuzzy Image of Nereid seen from Voyager 2

Image Credit:

NASA/Voyager

Nereid is unique as it has a very eccentric orbit, which may mean that it is a captured comet, based on its composition.. However, it does orbit prograde, and despite being smaller than Proteus, was actually the second moon discovered around Neptune in 1949 by Gerard Kuiper.

Proteus

Image Credit:

NASA

 

Proteus is the second largest moon but was not discovered until Voyager 2 spotted it in 1989, which is odd considering Nereid was discovered by an Earth-based telescope

The Discovery of Neptune

Neptune was observed before its "official" discovery, starting with Galileo in the 1610s. However, for every observation of Neptune, the astronomer thought he was seeing a star, so would record it as such. After almost two centuries of observation, Neptune's position did not seem to fit what scientists felt should be a planetary orbit.

John Couch Adams

In 1643, Englishman John Couch Adams was a new astronomer who wanted to reconcile the observational data on Neptune with gravitational perturbations in the orbit of Uranus. Using the relatively new invention of calculus (Isaac Newton "discovered" calculus in the 1700s), Adams was able to show that there should be an object at the relative location of Neptune's orbit causing these variations of Uranus' orbit. Showing his calculations to the Astronomer Royal, George Airy, Adams felt that there could be a planet out there. George Airy did not take the calculations seriously, and so did not actively look for Neptune.

Urbain Le Verrier

However, at the same time Adams was performing his calculations, Frenchman Urbain Le Verrier also made the same calculations as Adams. Unlike Adams, his calculations were taken seriously, and Johann Galle at the Berlin Observatory used the predictions given him by Le Verrier to look for Neptune. On September 23, 1846, within half an hour of looking, Galle was the first to see Neptune and realize that it was a planet and not a star. The position of Neptune compared to Le Verrier's calculations was within 1° and 12° of Adams.

Johann Galle

When it was discovered by George Airy that a Frenchman had done the same calculation as Adams and a German had found Neptune, Airy wanted to make sure that an Englishman had made the same calculations. At first, Le Verrier thought that Adams had plagarised his work, but soon it was realized that Adams and Le Verrier had never interacted and that both work independently coming up with the same prediction. As a result, Adams and Le Verrier were considered co-discoverers of Neptune, until the 1990s, when the Royal Observatory decided that since Adams calculations were not used to find Neptune, he would no longer be considered a discoverer.

Neptune was named for the Roman god of the sea because of its blue appearance. It also stayed in line with the naming of the planets after Roman and Greek gods.

Neptune, The Other Blue Planet

Neptune with a Great Dark Spot prominent in the center

Image Credit:

NASA

Earth is not the only blue planet in our Solar System. The planet farthest from the Sun (and Pluto is still not a planet, nor should it be), is also blue. However, Neptune's color does not come from water but from the atmosphere composed mainly of methane. Neptune is also the fourth largest planet in diameter (smaller than Uranus and larger than Earth), but the third largest in mass (between Saturn and Uranus). It orbits at a semi-major axis distance of 30 AU which gives it a orbital period of 165 Earth years. Based on its eccentricity of 0.0087, it is by far the planet with the most circular orbit.

Neptune is also known as the first object to be predicted to exist before it was discovered. In the 1840s, two different astronomers predicted another planet orbited outside the orbit of Uranus and those predictions were confirmed in 1846. Since its discovery, Neptune has only completed one full orbit, way back in 2011.

Neptune also is home to one of the larger moons in the Solar System, Triton. It also has 13 confirmed moons, and may have more awaiting discovery. And just like the other gas planets, Neptune does have a ring system.

As shown in the above image, Neptune has a series of prominent storms on its surface that have persisted since their discovery in 1989 by Voyager 2. Much like Jupiter's Great Red Spot, they are a fascinating set of storms that we are still learning about.