Pluto

Once known as the smallest, coldest, and most distant planet from the Sun, Pluto has a dual identity, not to mention being enshrouded in controversy since its discovery in 1930. Pluto is also a member of a group of objects that orbit in a disc-like zone beyond the orbit of Neptune called the Kuiper Belt. This distant region consists of thousands of miniature icy worlds with diameters of at least 1,000 km and is also believed to be the source of some comets.

Pluto has three known moons, Hydra and Nix, besides its companion moon, Charon. At about 1,186 km (737 miles), Charon's diameter is a little more than half of Pluto's.

Pluto and and Charon orbit the Sun in a region where there may be a population of hundreds or thousands of similar bodies that were formed early in solar system history. These objects are referred to interchangeably as trans-Neptunian objects, Edgeworth-Kuiper Disk objects or ice dwarves. Pluto is about two-thirds the diameter of Earth's Moon and may have a rocky core surrounded by a mantle of water ice. Due to its lower density, its mass is about one-sixth that of the Moon. Pluto appears to have a bright layer of frozen methane, nitrogen, and carbon monoxide on its surface. While it is close to the Sun, these ices thaw, rise, and temporarily form a thin atmosphere, with a pressure one one-millionth that of Earth's atmosphere. Pluto's low gravity (about 6 percent of Earth's)causes the atmosphere to be much more extended in altitude than our planet's. Because Pluto's orbit is so elliptical, Pluto grows much colder during the part of each orbit when it is traveling away from the Sun. During this time, the bulk of the planet's atmosphere freezes. In 1978, American astronomers James Christy and Robert Harrington discovered that Pluto has a satellite (moon), which they named Charon. Charon is almost half the size of Pluto and shares the same orbit. Pluto and Charon are thus essentially a double planet. Charon's surface is covered with dirty water ice and doesn't reflect as much light as Pluto's surface. Because Pluto is so small and far away, it is difficult to observe from Earth. In the late 1980s, Pluto and Charon passed in front of each other repeatedly for several years. Observations of these rare events allowed astronomers to make crude maps of each body. From these maps it was learned that Pluto has polar caps, as well as large, dark spots nearer its equator. The duo's gravity has locked them into a mutually synchronous orbit, which keeps each one facing the other with the same side. Many moons - including our own - keep the same hemisphere facing their planet. But this is the only case in which the planet always presents the same hemisphere to its moon. If you stood on one and watched the other, it would appear to hover in place, never moving across the sky.

Charon was discovered in 1978, while two additional moons Hydra and Nix, were discovered in 2005.

In Greek mythology, Charon was the boatman who carried the souls of the dead to the underworld - a kingdom that in Roman mythology was ruled by the god, Pluto. The U.S. Naval Observatory's James Christy suggested the name after he found the moon in 1978.

Seven years later, Charon and Pluto began a five-year period of eclipsing each other from Earth's point of view. That was lucky for us, because it enabled scientists to measure the diameters and masses of both objects as each passed in front of the other.

Charon appears to be covered by water ice, which differs from Pluto's surface of frozen nitrogen, methane, and carbon dioxide. One theory is that the materials that formed Charon were blasted out of Pluto in a collision. That's very similar to the way in which our own moon is thought to have been created.

NASA launched its New Horizons spacecraft to Pluto and Charon in January 2006, and it should arrive in 2015, becoming the first spacecraft to visit them. In preparation, the New Horizons project is organizing a search for additional moons of Pluto, using ground-based telescopes and possibly the Hubble Space Telescope.

Ceres

Ceres has been classified as a dwarf planet that might also be classified as an asteroid. While there are three known dwarf planets, there are only two that have been classified further as plutoids. Ceres remains in the dwarf planet category because of its position in the asteroid belt between Mars and Jupiter. Its name is derived from the Roman goddess Ceres. Discovered on January 1, 1801, by Giuseppe Piazzi, Ceres has a diameter of about 950 kilometers and is by far the largest and most massive known body in the asteroid belt, as it contains approximately a third of the belt's total mass.

The classification of Ceres has changed several times. Even though it was classified as a planet when it was first discovered, because it resembled similar bodies in the asteroid belt it was reclassified as an asteroid for over 150 years, and now retains its name as a dwarf planet.

As the first such body to be discovered, its name was prefixed by the number 1, under the modern system of asteroid numbering. After the discovery of the trans-Neptunian object 2003 UB313 (Eris), a proposition was made by the International Astronomical Union to reinstate Ceres to the status of planet along with Pluto's moon, Charon, and Eris.

Instead, on August 24, 2006, an alternate proposal came into effect labeling Ceres a 'dwarf planet'. It is not yet clear whether dwarf planet status is, like planet status, a sole defining category, or whether dwarf planets also retain their previous minor body classifications such as "asteroid."

(136472) 2005 FY9 or now called MakeMake

The name Makemake has been approved for the plutoid transneptunian dwarf planet (136472) 2005 FY9. Makemake is a creator god of Rapa Nui (Easter Island). Discovered on March 31, 2005 by Michael Brown and his team at the Palomar Observatory, MakeMake is the third largest known dwarf planet in the Solar System and about a third of the diameter of Pluto. Initially known as 2005 FY9 (and later given the number 136472), its discovery was announced on July 29, 2005. On June 11, 2008, the IAU included Makemake in its list of potential candidates to be given "plutoid" status, a term for dwarf planets beyond the orbit of Neptune that would place the object alongside Pluto and Eris. Makemake was formally classified as a plutoid in July 2008.

At first glance, if Earth had a twin, it would be Venus. The two planets are similar in size, mass, composition, and distance from the Sun. 

But there the similarities end. Venus has no ocean. Venus is covered by thick, rapidly spinning clouds that trap surface heat, creating a scorched greenhouse-like world with temperatures hot enough to melt lead and pressure so intense that standing on Venus would feel like the pressure felt 900 meters deep in Earth's oceans. These clouds reflect sunlight in addition to trapping heat. Because Venus reflects so much sunlight, it is usually the brightest planet in the night sky.

Although we cannot normally see through Venus' thick atmosphere, NASA's Magellan spacecraft used radar to image the planet's surface, and the Galileo spacecraft used infrared mapping to view mid-level cloud structure before continuing on its mission to Jupiter.

Like Mercury, Venus can be seen periodically passing across the face of the Sun. These transits of Venus occur in pairs with more than a century separating each pair. Since the telescope was invented, transits were observed in 1631, 1639; 1761, 1769; and 1874, 1882. On June 8, 2004, astronomers worldwide saw the tiny dot of Venus crawl across the Sun; the second in this pair of early 21st-century transits will occur June 6, 2012. 

The atmosphere consists mainly of carbon dioxide, with clouds of sulfuric acid droplets. Only trace amounts of water have been detected in the atmosphere. The thick atmosphere traps the Sun's heat, resulting in surface temperatures over 470 degrees Celsius (880 degrees Fahrenheit). Probes that have landed on Venus have not survived more than a few hours before being destroyed by the incredibly high temperatures.

The Venusian year (orbital period) is about 225 Earth days long, while the planet's rotation period is 243 Earth days, making a Venus day about 117 Earth days long. Venus rotates retrograde (east to west) compared with Earth's prograde (west to east) rotation. Seen from Venus, the Sun would rise in the west and set in the east. As Venus moves forward in its solar orbit while slowly rotating "backwards" on its axis, the cloud-level atmosphere zips around the planet in the opposite direction from the rotation every four Earth days, driven by constant hurricane-force winds. How this atmospheric "super rotation" forms and is maintained continues to be a topic of scientific investigation.

About 90 percent of the surface of Venus appears to be recently solidified basalt lava; it is thought that the planet was completely resurfaced by volcanic activity 300 to 500 million years ago. 

Sulfur compounds, possibly attributable to volcanic activity, are abundant in Venus' clouds. The corrosive chemistry and dense, moving atmosphere cause significant surface weathering and erosion. Radar images of the surface show wind streaks and sand dunes. Craters smaller than 1.5 to 2 kilometers (0.9 to 1.2 miles) across do not exist on Venus, because small meteors burn up in the dense atmosphere before they can reach the surface.

More than 1,000 volcanoes or volcanic centers larger than 20 kilometers (12 miles) in diameter dot the surface of Venus. Volcanic flows have produced long, sinuous channels extending for hundreds of kilometers. Venus has two large highland areas:

Ishtar Terra, about the size of Australia, in the north polar region; and Aphrodite Terra, about the size of South America, straddling the equator and extending for almost 10,000 kilometers (6,000 miles). Maxwell Montes, the highest mountain on Venus and comparable to Mount Everest on Earth, is at the eastern edge of Ishtar Terra.

Venus has an iron core about 3,000 kilometers (1,200 miles) in radius. Venus has no global magnetic field, though its core iron content is similar to that of Earth. Venus rotates too slowly to generate a magnetic field similar to Earth's.

In 1950, Dutch astronomer Jan Oort proposed that certain comets came from a vast spherical shell of icy bodies near the edge of the Solar System. This giant swarm of objects is now named the Oort Cloud, occupying space at a distance between 5,000 and 100,000 astronomical units. (One astronomical unit, or AU, is the mean distance of Earth from the Sun: about 150 million kilometers or 93 million miles.) 

The Oort Cloud contains billions of icy bodies in solar orbit. Occasionally, passing stars disturb the orbit of one of these bodies, causing it to come streaking into the inner solar system as a long-period comet. These comets have very large orbits and are observed in the inner solar system only once. In contrast, short-period comets take less than 200 years to orbit the Sun and they travel along the plane in which most of the planets orbit. They come from a region beyond Neptune called the Kuiper Belt, named for astronomer Gerard Kuiper, who proposed its existence in 1951.

The Kuiper Belt, extending out to about 50 AU around the Sun, is populated with thousands of small icy bodies.  
  
]In 1992, astronomers detected a reddish speck about 42 AU from the Sun-- the first time a Kuiper Belt object (or KBO for short) had been sighted. More than 1,000 KBOs have been identified since 1992. (They are sometimes called Edgeworth Kuiper Belt objects, acknowledging another astronomer who also is credited with the idea, or they are simply called Trans-Neptunian Objects (TNOs.) 

The IAU has been the arbiter of planetary and satellite nomenclature since its inception in 1919. The various IAU Working Groups normally handle this process, and their decisions primarily affect the professional astronomers. But from time to time the IAU takes decisions and makes recommendations on issues concerning astronomical matters affecting other sciences or the public. Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.  
  
The boundary between (major) planet and minor planet has never been defined and the recent discovery of other "Trans-Neptunian Objects" (TNOs), including some larger than Pluto, triggered the IAU to form a Working Group on "Definition of a Planet" from its Division III members.

Quaoar and Orcus

One of the largest KBOs is Quaoar (2002 LM60), named by its discoverers after the mythical creation-force figure of the Tongva tribe of the Los Angeles basin. Quaoar orbits the Sun every 288 years about a billion miles beyond the orbit of Pluto (somewhere around 42 AU). Quaoar was photographed in 1980, but was not recognized as a KBO until 2002, by Astronomer Mike Brown and his colleagues at Caltech in Pasadena, California.

Quaoar is about 1250 km in diameter, roughly the size of Pluto's moon Charon. Nothing larger has been found in our solar system since Pluto was discovered in 1930 (and Pluto's moon Charon in 1978). It's huge. In fact, if you took the 50,000 numbered asteroids and put them together, it would be about the same volume as Quaoar.

An even larger KBO (2004 DW, now officially named Orcus) was found at a distance of about 45 AU from the Sun.

2005 FY9, codenamed "Easterbunny," is a very large Kuiper belt object discovered on March 31, 2005 by the team led by Mike Brown at Caltech. Its discovery was announced on July 29, 2005 on the same day as two other very large trans-Neptunian objects (TNOs), 2003 EL61 and 2003 UB313, now officially known as Eris.

2003 EL61

2003 EL61 is yet another object in the Kuiper Belt, discovered by Mike Brown and his team at Caltech. EL61 is also located in the region of space beyond Neptune that includes Pluto and the large planetoids Quaoar and Orcus, 2005 FY9, and the planet 2003 UB313, among others. 2003 EL61 is currently the third brightest object in this region after Pluto and 2005 FY9. It is so bright that it can readily be seen by high-end amateur telescopes equipped with CCD cameras. Other than being extremely bright, 2003 EL61 appeared at first to be typical of a type of Kuiper belt objects that astronomers call "scattered Kuiper belt objects." They are called "scattered" because it is believed that they once had a close encounter with Neptune, which gravitationally "scattered" these objects onto more eccentric orbits. The mass of 2003 EL61 is about 32% that of Pluto.