Kuiper Belt

The Kuiper Belt is the region of the Solar System out beyond Neptune where the majority of our short-period comets are found. This is the region where we find the dwarf planets Pluto and Eris. The objects in this region besides being comets and dwarf planets are sometimes referred to as plutinos, KBOs (cubinos or Kuiper Belt Objects), or TNOs (Trans-Neptunian Objects). The belt is generally a torus centered on the Sun populated mostly by icy bodies as the heavier material generally settled in the interior part of the Solar System.

The comets that come from this region are called short-period comets as they have elliptical orbits with large eccentricities and periods of less than 200 years. They are disturbed in their orbits by large passing bodies, like Neptune that can send the object into in the inner solar system.

The Kuiper Belt is named after Gerard Kuiper as he proposed that there must be a disk that has icy remnants from the formation of the Solar System, much like there is a rocky disk in the inner Solar System containing leftovers from the birth of the Sun and planets, the Asteroid Belt. Disks found around other stars indicate that Kuiper Belt like features may be a consequence of planetary formation around stars.

The disk itself ranges from 30 AU to 100 AU in radius and contains at least several thousand objects, many of which are over 100 km in diamter.

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 Moons of Uranus

The Moons of Uranus from closest to farthest starting from the left

Image Credit:

NASA/Voyager

Uranus has 27 known moons which are mostly icy bodies similar to comets. Most of them might be captured comets from the Kuiper Belt or the Oort Cloud. However, there are a few that are considered planetary mass bodies, which means they have enough mass to be differentiated and spherical in shape. Most likely, these five major moons were probably formed at the same time as Uranus.

Titania

Image Credit:

NOAA

Titania has the largest diameter of all of the moons of Uranus, but it is still smaller than our Moon. Its diameter is 1600 km, compared to 3500 km for the Earth. Even though it has the largest diameter, it is not the most massive. That honor goes to Oberon which will be discussed later. Of the five major moons, it is the fourth out from Uranus and has an inclination of 0.34°, meaning it orbits along the equatorial plane of Uranus. William Herschel discovered Titania along with Oberon in January of 1787. Observations of Titania with telescopes and with the Voyager probes tell us a lot of Titania. We know that the moon has two main layers, and icy crust/mantle and a rocky core. The surface is heavily cratered, though not as much as Oberon which also tells us that it was more active in the past than Oberon. Titania was named after the Queen of the Fairies in William Shakespeare's A Midsummer's Night Dream.

Oberon

Image Credit:

NOAA

Oberon has the largest mass, and is second to Titania in diameter. It is also the farthest of the major moons. Discovered at the same time as Titania, Oberon was named after the King of the Fairies in A Midsummer's Night Dream. It is the most heavily cratered moon orbiting Uranus and much like Titania, it has a icy crust/mantle with a rocky core. With an inclination of only 0.058°, it is almost completely aligned with Uranus' equator.

Umbriel

Image Credit:

NOAA

Umbriel is the third largest in mass and diameter of the moons, but was not discovered until 1851 by William Lassell. Like Titania and Oberon, it is an icy and rocky body with an inclination of 0.128°. It is the second most cratered body but went under internal geologic processes in its past to give it the surface it has now. It was named after a character in Alexander Pope's The Rape of the Lock.

Ariel

Image Credit:

NOAA

Ariel is the fourth largest moon and second closest. It was discovered with Umbriel by William Lassell. Again, it is an icy and rocky body that formed in the accretion disk around Uranus based on its inclination of 0.260°. It is also tidally locked in the same way the Moon is tidally locked to Earth. Its name comes from two sources: Pope's The Rape of the Lock and Shakespeare's The Tempest.

Miranda
Image Credit:
NOAA
The last and smallest major moon is Miranda, which is also the closest to Uranus. It is also tidally locked to Uranus and was the last one discovered. Gerard Kuiper (after whom the Kuiper Belt is named - we'll discuss the Kuiper Belt later) found Miranda in 1948. Voyager imaged Miranda as it passed by the Uranus system and discovered that it is the most geologically active because it is the closest and tidal forces keep the interior warm enough. It has the highest inclination at 4.232° and was named after a character in Shakespeare's The Tempest.