Neptune is the farthest out of the major planets. Much like Uranus in appearance, it too is mostly hydrogen with some helium. High winds periodically produce storms that appear as dark spots. Voyager 2 found 5 new, small satellites. But, the real interest resulting from the Voyager passage is the very large (2400 km) satellite Triton, which appears to be an agglomerate of rock and frozen hydrogen and methane, with subordinate, perhaps trivial, amounts of water. Its southern polar cap has a melange of light and dark deposits of different compositions (and origins?). Pluto (once the 9th planet but now demoted) is farther out and prior to 2006 was considered to be a small planet. It will be visited by the New Horizons spacecraft, now launched, in 2015. In 2003, discovery of Sedna, a planet or planet-like body much farther away brought the total of significant planetary objects to 10. Eris is another Dwarf Planet that was first observed in 2003.

Neptune has eight satellites, all but one irregular in shape and mostly rock and ice. One, Triton, that is farthest out and much larger (2,700 km [1,678 miles] in diameter) than the others, is one of the most intriguing bodies - planets or satellites - in the Solar System. Triton moves in a retrograde circular orbit, i.e., it moves in a clockwise direction (looking down from north) compared to the counterclockwise rotation of Neptune and its other satellites. Triton's axis of rotation tilts 157° relative to its parent's axis. We cite these unusual conditions as evidence, either of its capture or of its assembly after a collision, possibly influenced by interactions with Pluto as that small body periodically orbits inside Neptune's orbit.

The full view (left) of Triton reveals that it is very different from most of the Giant planets' satellites. This satellite has a brownish pink color, has a polar cap (right), and a structured surface. The color may be due to frozen nitrogen and/or the effects of methane.

What has been interpreted as erupting nitrogen gas from structures or fissures made of frozen nitrogen and rock has been called "ice volcanoes" in which liquid nitrogen and/or methane vaporizes in plumes or geyser-like jets at the surface, carrying along particulates that form deposits on the ice cap. Ice "lavas" composed of water ice and some ammonia and methane produce flow features on the surface as well. Here is an area which contains this phenomenon - again the first of its kind in the Solar System. :

Triton's surface contains ridges, fault valleys, and occasional impact craters. This image shows a broad circular feature, with somewhat scalloped edges, that may be some strange volcanic caldera or a distorted impact crater (or one of each).

This computer-generated perspective view helps to visualize this terrain:

Major fracture systems occur over much of Triton, especially in the equatorial regions. Some of the grabens are hundreds of kilometers long.

Terrains on Triton involving fracturing can take on a peculiar "texture" similar to the surface of a cantaloupe (top) or appearing as a series of oval features controlled by ridges (bottom):

While filled grabens (fracture-bounded depressions) are a factor in canteloupe terrains, their specific origin remains unsettled. The oval features are even more mysterious.

19-72: Many of the individual roundish features making up the canteloupe texture look like distorted impact craters. What argues against this possible identity? ANSWER

Nereid, an irregular shaped satellite (maximum dimension = 340 km [220 miles] that is the outermost of the group, had been observed by telescope from Earth (discovered by Gerald Kuiper in 1941). Voyager 2 also got pictures but since Nereid was far away at the time, these are fuzzy:

Largest (418 km; 261 miles) of the six satellites discovered by Voyager is the nearly round Proteus (below). Although bigger than Nereid, it remained undetected until Voyager neared Neptune because it was too close to the planet's surface and was masked by neptunian light.

The remaining smaller moons are Larissa, Galatea, Despina, Thalassa, and Naiad.

The one-time outermost planet in the Solar System, Pluto, is not much different in diameter (about 2,340 km [1,454 mi]) than Triton. It was first discovered (albeit its existence had been predicted) in 1930, by Dr. Clyde Tombaugh, through the large telescope at the Lowell Observatory in Flagstaff, AZ. The best pictures (from HST) today show it is a mix of very dark (organic?) and light zones of unknown nature. Its density suggests mostly rock with some ices (nitrogen, methane, and water), from which an extremely tenuous atmosphere has evaporated.

In a "shocker" of a decision, the International Astronomical Union voted on August 24, 2006 to demote Pluto from the ranks of major planets (see the discussion on page 19-2). For the time being, the writer has opted for keeping Pluto in the ranks of small (dwarf) planets, as considered by the IAU.

Pluto's orbit is so eccentric that at times its path lies inside Neptune's (see diagram below). With this orbit, Pluto at times is within the edge of the Kuiper Asteroid Belt. It also has an inclined orbit at 17.1° relative to the ecliptic. Pluto and Neptune are in a well-known 3:2 resonance which prevents their collision or even any close approaches, regardless of their separation in the z-dimension. The minimum Pluto-Neptune separation is 17 AU, compared to the minimum Pluto-Uranus separation of 11 AU. The libration of Pluto's orbit is due to an exchange of angular momentum with Neptune. When Pluto is "catching up" to Neptune, it gains angular momentum at Neptune's expense. Pluto moves into a higher orbit, but falls behind Neptune, since higher orbits have slower tangential velocities. Thousands of years later, as Neptune seems poised to overtake Pluto, the opposite exchange takes place. Pluto loses angular momentum, falling to a lower - but faster - heliocentric orbit.

Pluto has a relatively large satellite, Charon, whose diameter (1212 +/- 5 km [757 miles]) makes it just a fourth the size of its parent (one opinion considers the two to be a double dwarf planet system), with which it is in synchronous rotation. This diameter was determined from an occultation measurement during a 3 day exposure from the European Southern Observatory telescope, yielding this image:

This HST image shows a better view of Pluto and Charon:

Recently, two more small moons of Pluto were found using the Hubble Space Telescope. These have been named Hydra and Nix.

There is one school of thought - although a minority - that holds Pluto to be a large asteroid that has escaped from the main Kuiper belt beyond. However, others hold that its spherical shape supports the dwarf planet argument, as does the fact that Pluto now has three moons.

For a decade or more, scientists have "agitated" for a spacecraft mission that will go to Pluto and then to the Kuiper Belt asteroids beyond. NASA approved such a mission, the New Horizons spacecraft, and successfully launched it on January 18, 2006. A synopsis of the mission profile (a comprehensive treatment can be found on the Wikipedia web site; the discussion touches upon its 9 year journey to Pluto, a flyby, and then a continuance into the Kuiper Belt of Asteroids. Arrival at Pluto will be in 2015.

The spacecraft and its 7 major instruments are pictured here:

Ralph: Visible and infrared imager/spectrometer; provides color, composition and thermal maps.

Alice: Ultraviolet imaging spectrometer; analyzes composition and structure of Pluto's atmosphere and looks for atmospheres around Charon and Kuiper Belt Objects (KBOs).

REX: (Radio Science EXperiment) Measures atmospheric composition and temperature; passive radiometer.

LORRI: (Long Range Reconnaissance Imager) telescopic camera; obtains encounter data at long distances, maps Pluto's farside and provides high resolution geologic data.

SWAP: (Solar Wind Around Pluto) Solar wind and plasma spectrometer; measures atmospheric "escape rate" and observes Pluto's interaction with solar wind.

PEPSSI: (Pluto Energetic Particle Spectrometer Science Investigation) Energetic particle spectrometer; measures the composition and density of plasma (ions) escaping from Pluto's atmosphere.

SDC: (Student Dust Counter) Built and operated by students; measures the space dust peppering New Horizons during its voyage across the solar system.

New Horizons is slated to arrive at Pluto/Charon on July, 2015. So, "we're off!! But don't expect much of anything on N.H. for nearly a decade.

After 74 years, what was hailed as the first new solar planet-like body since Pluto was publicly announced in March, 2004. Astronomers, led by Michael E. Brown of Caltech and from several universities, using the 48-inch Palomar telescope found (in mid-2003) a large object which they named Sedna. Sedna is located well beyond Pluto's orbit. Being very elliptical, this orbit extends out from about 13 billion km (8 billion miles) at perisol to around 143 billion km (88 billion miles) at apisol. The discovery image is this:

This illustration shows an artist's conception (rather fanciful) of Sedna, which takes into account its redness as seen through the telescope.

These four panels give more information about Sedna, its orbit, and its relation to other named planets and fragmental bodies in the Solar System:

The upper left panel shows the inner Solar System out to beyond the Asteroid Belt (next page); the upper right shows Sedna in relation to the outer Solar System, including the Kuiper Belt of asteroids; the lower left shows the Solar System relative to the Oort Cloud (from which Sedna may have been ejected); the lower right depicts the highly elliptical orbit of Sedna.

This next image is purported to be one showing many of the objects in the Oort Cloud. The writer has greatly stretched the image to bring out the details; the large central circular object is a processing artifact:

Sedna was a motivating factor that raised again questions about what constitutes a planet. Is it size, shape, orbit? One favored criterion is that a planet must be approximately spherical, which means it is big enough to have melted or otherwise reorganized its material into a shape with gravitational equilibrium favoring a globe. Sedna and Pluto meet this criterion but several Kuiper Belt Objects (KBO) approach that defining shape. A good summary of Sedna and similar objects is found at this CalTech site.

The debate has been further complicated by the announcement of still another candidate as a possible 10th planet, for several years designated only as 2003UB313 (for a time, named Xena) but now officially named Eris (the Goddess of Discord, reflecting this body's role in the disputations over what is a planet at the 2006 IAU meeting), that has been found among the KBOs. First examined in late 2003, its description was "leaked" on July 29, 2005. Eris is the circled object shown below:

Eris is about 3 times further than Pluto from the Sun at apihelion (at 97 A.U.) but its highly elliptical orbit (perihelion at 36 A.U.), inclined about 45° from the ecliptic, removes it from the mainstream of solar planets. Here is a diagram indicating its orbit, which reaches distances up to 16 billion km (10 billion miles):

Eris takes about 560 years to fully orbit the Sun. Its size is still only approximated (roughly, around 3000 km [5000 miles]). Some observers think it smaller than Pluto but its discoverers, the same group associated with Caltech that found Sedna, claim it is somewhat larger. It is definitely spherical, seems to have a solid surface, has methane in its atmosphere, and has its own tiny Moon, Dysnomia. Here is a time lapse trio of images that show Eris and Dysnomia:

The discovery of Eris is neatly described on this CalTech website: Eris. Eris' classification was "up in the air" until it was tentatively placed with Pluto and Ceres as a Kuiper Belt Object (KBO). Eris is now considered a possible dwarf planet that transits the Kuiper Belt during part of its orbit.

Before turning to asteroids and comets, we summarize the information on the larger bodies in the Solar System with this diagram (Sedna and the newest "planet" Eris not included):

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Primary Author: Nicholas M. Short, Sr.