Pluto

Pluto is classified as a dwarf planet and 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 realm is populated with thousands of miniature icy worlds, which formed early in the history of our solar system. These icy, rocky bodies are called Kuiper Belt objects or transneptunian objects.

Pluto's 248-year-long elliptical orbit can take it as far as 49.3 astronomical units (AU) from the Sun. (One AU is the mean distance between Earth and the Sun: about 150 million kilometers or 93 million miles.) From 1979 to 1999, Pluto was actually closer to the Sun than Neptune, and in 1989, Pluto came to within 29.7 AU of the Sun, providing rare opportunities to study this small, cold, distant world.

Pluto is about two-thirds the diameter of Earth's Moon and probably has a rocky core surrounded by a mantle of water ice. More exotic ices like methane and nitrogen frost coat its surface. Owing to its size and lower density, Pluto's mass is about one-sixth that of Earth's Moon. Pluto is more massive than Ceres - the dwarf planet that resides in the asteroid belt between Mars and Jupiter - by a factor of 20.

When Pluto is close to the Sun, its surface ices thaw, rise, and temporarily form a thin 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 atmosphere. Because Pluto's orbit is so elliptical, Pluto becomes much colder during the part of each orbit when it is traveling far away from the Sun. During this time, the bulk of the planet's atmosphere is thought to freeze.

Pluto has a very large moon that is almost half its size, discovered in 1978. Named Charon, the moon is so big that the two are sometimes referred to as a double dwarf planet system. The distance between them is 19,640 kilometers (12,200 miles). The Hubble Space Telescope photographed Pluto and Charon in 1994 when Pluto was about 30 AU from Earth. These photos showed that Charon is grayer than Pluto (which is red), indicating that they have different surface compositions and structure.

In 2005, scientists photographing Pluto with the Hubble Space Telescope in preparation for the New Horizons mission found two tiny moons orbiting in the same plane as Charon, but two to three times farther away from Pluto than Charon. Pluto's two small moons, named Nix and Hydra, are thought to be just 50 to 100 kilometers (31 to 62 miles) in diameter.

Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days. Charon neither rises nor sets but "hovers" over the same spot on Pluto's surface, and the same side of Charon always faces Pluto - this is called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus. Pluto's rotation is retrograde: it rotates "backwards," from east to west (Uranus and Venus also have retrograde rotation). It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or no magnetic field.

Because Pluto and Charon are so small and far away, they are extremely 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 rudimentary maps of each body showing areas of relative brightness and darkness.

How Pluto Got its Name
Pluto is named for the Roman god of the underworld (Hades in Greek mythology). The name was originally suggested by 11-year-old Venetia Burney, a girl from Oxford England.

Ceres

Ceres is the largest asteroid in the Solar System, orbiting at an average distance of 413 million km from the Sun. Ceres was discovered on New Years Day in 1801 by Giuseppe Piazzi, a monk and astronomer in Sicily, and the founding director of the Palermo Astronomical Observatory. Ceres was discovered using the Titius-Bode Law that predicts the position of planets based on a mathematical equation of their distance from the Sun. This law predicted the position of Uranus and led to the false belief that Ceres was a planet. Today, Ceres falls into the new classification of dwarf planet.

Ceres is by far the largest and most massive asteroid in the belt between Mars and Jupiter. It's approximately the size of Texas or 975 km by 909 km with a mass of 9.5 x 10²⁰ kg. It actually represents 1/3 of all of the mass of the asteroid belt. It has enough mass to pull itself into a sphere; a major requirement to be considered a dwarf planet. Ceres revolves around the sun every 1679.819 days with a very small axial tilt. The surface is relatively warm. The high temperature is thought to be in the neighborhood of -38°C (235 K). Ceres has a visual brightness magnitude of +6.9 to +9. When it is at the brightest point possible, Ceres is nearly bright enough to be seen with the unaided eye. It can be seen with binoculars, however, whenever it's above the horizon on a completely dark night.

The Dawn space mission is scheduled to visit Ceres in 2015 after stopping by Asteroid Vesta in 2011. There had been some problems with Dawn's ion propulsion system, but it was reignited on June 8, 2009. As of June 15, 2009 (when this article was written), Dawn was only 801 days away from Vesta on its way to Ceres. This space mission hopes to be the first to orbit and observe two planetary bodies, and will make space exploration history if successful.

Although Ceres has not been considered a planet for a very long time, it generates a great deal of interest. The easy visibility of the asteroid and the dark and light spots on its surface make it great fodder for amateur astronomers and their professional counterparts, alike. Hopefully, the upcoming orbital evaluation of Ceres by the Dawn space mission will clear some of the mystery. Whatever the outcome, Giuseppe Piazzi presented astronomers with quite a bit of excitement over the last 200 years.

Eris

Eris, the largest known dwarf planet today, was spotted in January 2005 (first seen in 2003, however important factors were confirmed in 2005) by Michael Brown of the California Institute of Technology and his team. This celestial object was the largest object spotted after Neptune's discovery and its moon in 1846 and outdid Pluto in size. However, beyond the Kuiper Belt (the outer fringe or belt of the solar system), Eris is the most distant object to be discovered in the orbit of the Sun.

Initially this dwarf planet was named 2003 UB313. Later it was nicknamed Xena (a temporary name while awaiting IAU's decision) which became quite popular as well. However, the dwarf planet was officially named Eris on September 16, 2006 by the IAU members. The name Eris comes from Greek mythology, wherein Eris happens to be the Goddess of warfare and strife. According to the mythology, Eris stirred up hatred, jealousy and anger among men and caused them to fight each other. Since this dwarf planet had stirred up a lot of controversies regarding its classification, the IUA members decided that the name Eris suited this object perfectly.

Eris' diameter is 2400 km, which happens to be slightly larger than Pluto at 2320 km. It is approximately 97 AU (Astronomical Units) away from the Sun, and is three times farther than Pluto, taking twice as much time (577 years) to orbit the Sun as compared to Pluto. Eris has an unusually eccentric orbit. At its closest, Eris is only 38 AU from the Sun, which means that at times Eris is closer to us than Pluto. Eris encounters temperatures ranging from -217º Celsius to -243º Celsius.

With the help of several spectral data, scientists today presume that Eris is covered by an outer layer of frozen methane which has actually seeped its way from the interior regions. In Pluto's case, it has been observed that methane on its surface undergoes chemical transformations which causes the methane to redden. The faint solar radiations reaching this planet from the Sun can be the reason behind this. However, since Eris is farther than Pluto, the solar radiations reaching Eris are even fainter, which is why Eris' surface is more yellowish than the reddish yellow surface of Pluto.

Currently Eris has one known moon which orbits it and is called Dysnomia. The name Dysnomia comes from the daughter of the Goddess Eris, who was known to be the cause of lawlessness. Dysnomia takes two weeks to orbit this dwarf planet. Unlike other planets like Mars or Jupiter, Eris cannot be observed by the naked eye or even through binoculars. To view Eris, one needs to have very powerful telescopes with plenty of amateur astronomical experience under his belt.

The discovery of Eris is a landmark in astronomy because it led to the reclassification of celestial objects and formation of a new category known as dwarf planets. From this, we can draw that the universe and our solar system is full of hidden secrets, and what we know today is only like a drop inside an ocean!

Makemake

Makemake is a dwarf planet in our Solar System. Makemake was discovered in March 2005 by a team of astronomers led by Mike Brown. Makemake officially became a dwarf planet in July 2008.

There were three other dwarf planets before Makemake. They are Pluto, Ceres, and Eris. Makemake is a large Kuiper Belt Object (KBO). It is an icy world that orbits far from the Sun on the frozen fringes of our Solar System. Pluto and Eris are KBOs too. Because it is so far away, Makemake takes nearly 310 years to orbit the Sun once! Makemake is usually a bit further from the Sun than Pluto.

Makemake is named after a god of the Rapa Nui people of Easter Island (the place with the cool statues). Makemake is the god of fertility and the creator of humans in Rapa Nui mythology. Makemake was discovered soon after Easter in 2005. The astronomers nicknamed it "Easterbunny". When they chose an official name, they decided they should use a character from the mythology of Easter Island.

Makemake is smaller than both Pluto and Eris. Its diameter is between 1,300 and 1,900 km (808 to 1,181 miles). That makes it about three-fourths the size of Pluto. Astronomers haven't found any moons circling Makemake yet. That is a bit odd, because other large KBOs (including Pluto and Eris) do have one or more moons.

Makemake is very, very cold. The temperature there is about minus 240° C (-406° F)! Makemake may be covered with ice. That ice would probably be made of methane and maybe ethane. The surface of Makemake is very bright, which you would expect of ice. It is also a bit red in color, which you might not expect!

Makemake may have an atmosphere... at least some of the time! When the dwarf planet is closer to the Sun in its orbit, it probably warms up some. Maybe the methane and ethane are gases then, and form an atmosphere. When Makemake is further from the Sun in its orbit, it should be colder. Maybe the gases freeze then, and fall to the ground as snow... and the atmosphere is gone! Scientists don't know if this happens, but it might. The same thing may happen on Pluto, too.

Makemake, along with Pluto and Eris (but not Ceres!), is also considered as a "plutoid". Plutoids are dwarf planets which, like Pluto, orbit the Sun out beyond the planet Neptune.

Haumea

Haumea is a dwarf planet in our Solar System. Haumea is officially the fifth dwarf planet. The four that came before it are Pluto, Eris, Ceres, and Makemake. Haumea was classified a dwarf planet in September 2008.

Haumea is a large Kuiper Belt Object (KBO). It is an icy world that orbits far from the Sun on the frozen fringes of our Solar System. Because it is so far away, Haumea takes 285 years to orbit the Sun once! Haumea is usually a bit further from the Sun than Pluto.

Haumea is smaller than both Pluto and Eris. Haumea has an odd shape. It is not a sphere. It is spinning so quickly that it has been stretched into the shape of an ellipsoid (a 3D ellipse). Haumea is 1,960 km (1,218 miles) across at its longest. It is only about half that far across - 996 km (619 miles) - along its shortest axis.

The "days" are really short on Haumea. This weird object spins around in less than four hours! Haumea is fairly dense. That means it is probably made mostly of rock. Most other KBOs probably have more ice in them than Haumea. This dwarf planet has 2 known moons, both discovered in 2005. Their names are Hi'iaka and Namaka.

Astronomers think Haumea collided with another large object a long time ago. A collision like that might explain Haumea's strange shape and why it spins so fast. The big collision might have knocked away most of Haumea's lightweight ice, leaving behind heavier rock. Haumea's moons may be leftover stuff from the collision.

In the mythology of Hawaii, Haumea is the goddess of fertility and childbirth. Her children include Hi'iaka and Namaka, as well as the Hawaiian volcano and fire goddess Pele.

There are two different groups that might get credit for discovering Haumea. One group is led by Mike Brown of the California Institute of Technology. The other group's leader is José Luis Ortiz Moreno at the Sierra Nevada Observatory in Spain. Haumea was discovered sometime between March 2003 and July 2005.

Haumea was called 2003 EL61 before it got its official, permanent name. It had been nicknamed "Santa" by Brown's group because they discovered it near Christmas in 2004.

NAME OF DWARF PLANET	DISTANCE FROM SUN (in km)	DIAMETER  (in km)	TIME TO SPIN ON AXIS (a day)	TIME TO ORBIT SUN (a year)	AVERAGE TEMP	YEAR OF DISCOVERY	KNOWN MOONS
Ceres	257,031,000	950	9 hours, 5 minutes	4 years, 220 days	-158 °F	1801	None
Pluto	2,761,060,000 to 4,690,700,000	1,413	6 days, 9 hours	248 years	-373 °F	1930	3
Haumea	3,268,000,000 to 4,789,000,000	1,960 x 1,518 x 996	4 hours	285 years	-400 °F	2004	2
Makemake	3,579,000,000 to 4,933,000,000	Between 1300 to 1900	Unknown	309 years	-405 °F	2005	None
Eris	3,518,000,000 to 9,088,000,000	1,489	8 hours	557 years	-386  to -414 °F	2005	1