Dwarf Planets: Interesting Facts about the Five Dwarf Planets Interactive Solar System Model Dwarf Planet Facts There a currently five officially classified dwarf planets in our solar system . They are Ceres , Pluto , Haumea , Makemake and Eris . Ceres is located inside the asteroid belt between the orbits of Mars and Jupiter , while the other dwarf planets are located in the outer solar system in, or near to, the Kuiper belt . Another six objects are almost certainly dwarf planets, but are waiting for official classification, and there may be as many as 10,000 dwarf planets in the solar system. What is a dwarf planet? In 2006, a new and distinct class of celestial objects was named by the International Astronomical Union (IAU).

They were called “dwarf planets.” According to the IAU, a celestial body is classified as a dwarf planet if it exhibits the following characteristics: It is in orbit around the Sun It has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape It has not cleared the neighborhood around its orbit; and It is not a satellite (a moon) This definition of a dwarf planet differentiates it from a planet on two criteria: unlike the dwarf planet, a planet has cleared the neighborhood around its orbit, and the definition of a planet does not include the satellite distinction. Size of the dwarf planets Side by side comparison of the dwarf planet sizes The largest dwarf planet is either Pluto or Eris, followed by Makemake, Haumea and Ceres being the smallest dwarf planet. Currently it is believed that Pluto is the largest but we will know more once the New Horizons mission by NASA reaches Pluto and beyond sometime in 2015.

The order of the dwarf planets from closest to the Sun outwards is Ceres, Pluto, Haumea, Makemake and Eris is the furthest from the Sun at 96.4 astronomical units (AU) – almost 14 billion km (9 billion miles) away. Dwarf planet facts Eris Pluto reclassified as a dwarf planet The enormity of the impact of this distinction between a planet and a dwarf planet had an immediate victim. With the new definition, Pluto , which from its discovery in 1930 up until that year was considered the ninth planet in the Solar System , would now be stripped of that position and become a dwarf planet. The main reason that relegated Pluto to the dwarf planet status is that its orbit is riddled with other celestial objects that crisscross its path around the Sun. These objects include other celestial objects in the region outside the planet Neptune known as the Kuiper Belt . Pluto and Eris , another dwarf planet, are actually part of the Kuiper Belt. In fact, the orbital paths of Eris and Pluto cross each other. In comparison, the orbits of the remaining eight planets do not contain other celestial bodies, and they never cross each other’s path.

History of the dwarf planet Pluto’s status as the ninth planet in the solar system began to unravel in August 1992 when astronomers at the University of Hawaii discovered more than a thousand objects orbiting the Solar System beyond Neptune. These objects were given the name trans-Neptunian Objects (TNOs). They were of varying sizes and those that had been detected were smaller than Pluto. However, on October 2003, astronomers at the Palomar Observatory in California identified a new trans-Neptunian object that was more massive than Pluto. This object also had a satellite and was initially named 2003 UB313. The discovery of 2003 UB313 led astronomers to believe that more of these massive objects existed. It also made them question whether 2003 UB313 could be considered a new planet or not. The important question of what makes a celestial object a planet led the IAU to form a committee that was tasked to gather the opinions of professionals from a wide range of interests including astronomers, educators, writers and planetary scientists. Based on their opinions, the committee created a draft resolution on the definition of a planet.

This draft proposal was then presented during IAU’s 2006 General Assembly in Prague. After much debate from its members, a revised version was then put to vote during the Closing Ceremony. By the end of the General Assembly, the members had approved Resolution B5: Definition of a Planet in the Solar System. The Resolution also included the definition of a dwarf planet and thus created a new class of celestial objects that was distinct from planets. The Resolution automatically made Pluto a dwarf planet. After the Assembly, 2003 UB313 was given the name Eris. Pluto, Eris and Ceres (an asteroid in the asteroid belt between the orbits of Mars and Jupiter ) were the first members of the dwarf planet class. There are currently five recognized dwarf planets, with Makemake and Haumea joining the first three members. Dwarf planet controversy The resolution that led to the classification of Pluto as a dwarf planet was not without its share of controversy. Some planetary scientists objected to the third criterion of the Resolution because they believed a planet, and therefore a dwarf planet, should be judged based on its intrinsic qualities and not what is or isn’t in its neighborhood (orbital path).

This criterion also creates a different standard for an object to be considered a planet, one that is based on size and distance: If an object is deemed too small and is too far away from the Sun, then it must not be a planet. Scientists opposed to the Resolution believe that dwarf planets should be categorized as a “true planet” in the same way that gas giants (e.g. Jupiter) and terrestrial planets (e.g. Earth) are considered “planets”. Most Popular Dwarf Planets: Classification, Discovery, Definition, Kuiper Belt - Windows to the Universe The Poles of the Dwarf Planets Dwarf Planets In 2006 the International Astronomical Union (IAU) approved a new classification scheme for planets and smaller objects in our Solar System . Their scheme includes three classes of objects: "small solar system bodies" (including most asteroids and comets ), the much larger planets (including Earth , Jupiter , and so on), and the new category of in-between sized "dwarf planets". There are currently five official dwarf planets. Pluto , formerly the smallest of the nine "traditional" planets, was demoted to dwarf planet status.

Ceres , the largest asteroid in the main asteroid belt between Mars and Jupiter, was also declared a dwarf planet. The three other (for now!) dwarf planets are Eris , Makemake , and Haumea . Pluto, Makemake, and Haumea orbit the Sun on the frozen fringes of our Solar System in the Kuiper Belt . Eris, also a Trans-Neptunian Object , is even further from the Sun. What's the difference between regular planets and dwarf planets? As you might guess, it's partly an issue of size, with dwarf planets being smaller. But just how big does a planet need to be to become a full-fledged planet instead of a dwarf? You might think the minimum size requirement is arbitrary, but the size cutoff is actually based on other properties of the object and its history in the Solar System. Both planets and dwarf planets orbit the Sun , not other planets (in which case we call them moons). Both must be large enough that their own gravity pulls them into the shapes of spheres; this rules out numerous smaller bodies like most asteroids, many of which have irregular shapes . Planets clear smaller objects out of their orbits by sucking the small bodies into themselves or flinging them out of orbit.

Dwarf planets, with their weaker gravities, are unable to clear out their orbits. Though there are just five dwarf planets now, their number is expected to grow. Scientists estimate there may be 70 dwarf planets amongst outer solar system objects that have been discovered already. Since we don't know the actual sizes or shapes of many of the objects we've found (because they are so far away), we can't yet determine whether they are actually dwarf planets or not. More observations and better telescopes will help us determine which other objects are dwarf planets. Astronomers speculate that there may be 200 or so dwarf planets out through the distance of the Kuiper Belt , an icy band of frozen planetoids on the edge of our Solar System. Last modified June 9, 2009 by Randy Russell . Shop Windows to the Universe Science Store! The Winter 2010 issue of The Earth Scientist , which includes articles on meteor cratering, classroom glaciers, podcasts in the classroom, and pyro-cumulonimbus clouds, is available in our online store .

Windows to the Universe Community News Meet the Solar System's Dwarf Planets Meet the Solar System's Dwarf Planets By Mike Wall, Space.com Senior Writer | October 26, 2011 02:00pm ET MORE Credit: David Aguilar/Center for Astrophysics Introduction For three-quarters of a century, schoolkids learned that our solar system has nine planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. But things changed five years ago. On Aug. 24, 2006, the International Astronomical Union (IAU) struck Pluto from the list, demoting it to the newly created category of "dwarf planet." The move was spurred by the discovery of multiple large bodies orbiting even farther from the sun than distant Pluto — particularly an object called Eris, which appeared to be bigger than Pluto.

As a result, the IAU came up with a new definition of "planet": A body that circles the sun without being some other object's satellite, is large enough to be rounded by its own gravity (but not so big that it begins to undergo nuclear fusion, like a star) and has "cleared its neighborhood" of most other orbiting bodies. Since Pluto shares orbital space with lots of other objects out in the Kuiper Belt — the ring of icy bodies beyond Neptune — it didn't make the cut. So Pluto was newly classified as a dwarf planet, which tend to be smaller than "true" planets and fall short on the "clearing your neighborhood" criterion. Although hundreds, or perhaps thousands, more solar system bodies may eventually join the list, the IAU officially recognizes just five dwarf planets at the moment. Here's a brief tour of all five: Pluto, Eris, Haumea, Makemake and Ceres. 2 of 7 Credit: NASA, ESA, and M. Buie (Southwest Research Institute) Pluto: The demoted former planet Pluto was discovered by American Clyde Tombaugh in 1930.

It's about 1,455 miles (2,352 kilometers) across — less than 20 percent as big as our planet. And Pluto is just 0.2 percent as massive as Earth. Pluto has an extremely elliptical orbit that's not in the same plane as the eight official planets' orbits. On average, the dwarf planet cruises around the sun at a distance of 3.65 billion miles (5.87 billion km), taking 248 years to complete one circuit. Because it's so far from the sun, Pluto is one of the coldest places in the solar system, with surface temperatures hovering around minus 375 degrees Fahrenheit (minus 225 degrees Celsius). Pluto has four known moons: Charon, Nix, Hydra and a newly discovered tiny satellite currently called P4. While Nix, Hydra and P4 are relatively small, Charon is about half as big as Pluto. Because of Charon's size, some astronomers regard Pluto and Charon as a double dwarf planet, or binary system.

3 of 7 Credit: NASA/JPL/Caltech Eris: The troublemaker Caltech astronomer Mike Brown led the team that discovered Eris in 2005. The find spurred the IAU to strip Pluto of its planethood and create the "dwarf planet" category a year later. That decision remains controversial to this day, making Eris' name quite fitting: Eris is the Greek goddess of discord and strife, who stirred up jealousy and envy among the goddesses, leading to the Trojan War. Eris has one known moon, Dysnomia. Eris is virtually the same size as Pluto, but it's about 25 percent more massive, suggesting that Eris contains considerably more rock (and less ice) than its Kuiper Belt neighbor. Like Pluto, Eris has a highly elliptical orbit. But Eris is even more far-flung, orbiting the sun at an average distance of about 6.3 billion miles (10.1 billion km). It takes Eris 557 years to complete one lap around the sun.

4 of 7 Credit: SINC/José Antonio Peñas Haumea: The oddball Haumea, a Kuiper Belt denizen orbiting slightly beyond Pluto, was discovered by Brown and his team in late 2004. It's one of the weirdest objects in the solar system. Haumea measures about 1,200 miles (1,931 km) across, making it nearly as wide as Pluto. But Haumea is just one-third as massive as Pluto, partly because it's not spherical. Rather, Haumea is shaped like a giant American football. The dwarf planet also completes one full rotation in less than four hours, making it one of the fastest-spinning bodies in the solar system. This super-charged spin is responsible for Haumea's oblong shape, pushing the dwarf planet outward substantially at the equator. Haumea makes one complete lap around the sun every 283 years. Haumea has two known moons, Hi'iaka and Namaka. 5 of 7 Credit: IAU/M. Kornmesser Mysterious Makemake Brown's team also discovered Makemake, spotting the dwarf planet in 2005.

Astronomers aren't sure of Makemake's exact size, but the dwarf planet is thought to be about three-quarters as big as Pluto. It's therefore likely the third-largest dwarf planet, after Eris and Pluto. Makemake orbits the sun from slightly farther away than Pluto, at an average distance of 4.26 billion miles (6.85 billion km), and completes an orbit every 310 years or so. Makemake is the second-brightest Kuiper Belt object (after Pluto) and can be seen with a high-end amateur telescope, according to the IAU. 6 of 7 Credit: NASA, ESA, J. Parker (Southwest Research Institute), L. McFadden (University of Maryland) Ceres: Queen of the asteroid belt Ceres is the only dwarf planet not found in the freezing cold, faraway Kuiper Belt. Rather, it orbits in the main asteroid belt between Mars and Jupiter, completing one lap around the sun every 4.6 years. Ceres is by far the largest object in the asteroid belt, containing about one-third of the belt's mass.

However, at 590 miles (950 km) across, it is the smallest known dwarf planet. Because it's so much closer to Earth than the other dwarf planets, Ceres was discovered far earlier. Italian astronomer Giuseppe Piazzi spotted it first, on Jan. 1, 1801. For the next half-century, many astronomers regarded Ceres as a true planet. That changed when it became apparent that Ceres was just one of many bodies hurtling through space in the asteroid belt. These days, most astronomers regard Ceres as a protoplanet, saying it likely would have continued growing into a full-fledged rocky planet like Earth or Mars if Jupiter hadn't shaken up the asteroid belt long ago. 7 of 7 Dwarf Planets Dwarf Planets Illustration by David A. Aguilar Click the full-screen arrows in the upper right to see the whole image! Poor Pluto. As if this faraway frozen ball’s long journey around the sun —a squashed trajectory that takes 248 years—wasn’t strange enough, Pluto’s story took an even stranger turn in 2006, when astronomers stripped it of its status as our ninth planet.

But Pluto’s loss was actually a gain for wannabe astronauts. Suddenly, the solar system offered new “planets” to explore. Astronomers coined the term “dwarf planet” to reclassify Pluto, which would become a representative of this new club of heavenly bodies. And membership is growing fast. While standard planets must meet three criteria, dwarf planets only need to check off two: They must orbit the sun and be of sufficient size for their gravity to squish them into a spherical shape. Along with Pluto, astronomers have identified four other dwarf planets. Most of them join Pluto in the so-called Kuiper Belt of comets -in-waiting beyond the orbit of Neptune . There’s Eris, which rivals Pluto in size, and egg-shaped Haumea, one of the fastest spinning objects in the solar system (its “day” lasts just four hours). Mighty Ceres, the largest of the asteroids at nearly 600 miles (966 kilometers) in diameter, joins the dwarf-planet club from the Asteroids Belt. Astronomers suspect that dozens—possibly hundreds—of dwarf planets await discovery. Perhaps someday you’ll find one of these itty-bitty worlds.

MORE FACTS In July 2015, Pluto became the first dwarf planet to be explored by spacecraft. NASA's New Horizons reached Pluto after traveling more than 3 billion miles over nine years. The five confirmed dwarf planets—including Pluto—are all smaller than Earth ’s moon , but some of them have moons themselves. Astronomers suspect that the asteroid Ceres might have geysers that spew water from its rocky crust. Asteroid Belt Facts - Interesting Facts about the Asteroid Belt Asteroid Belt Facts Facts Asteroids What is the asteroid belt? The vast majority of asteroids in the solar system are found in a region of the solar system out beyond Mars. They form the Asteroid Belt. Others orbit in near-Earth space and a few migrate or are thrown out to the outer solar system by gravitational interactions. The four largest asteroids in the belt are Ceres, Vesta, Pallas, and Hygiea. They contain half the mass of the entire belt. The rest of the mass is contained in countless smaller bodies. There was a theory once that if you combined all the asteroids they would make up the missing “Fifth” rocky planet.

Planetary scientists estimate that if you could put all that material together that exists there today, it would make a tiny world smaller than Earth’s moon. Where is the asteroid belt located? The Asteroid Belt is located in an area of space between the orbits of Mars and Jupiter. That places it between 2.2 and 3.2 astronomical units (AU) from the Sun. The belt is about 1 AU thick. The average distance between objects in the Asteroid Belt is quite large. If you could stand on an asteroid and look around, the next one would be too far away to see very well. Various Asteroids – Asteroid Mining The solar system contains many different types of asteroids, grouped by the minerals they contain. The abundances of precious metals such as nickel, iron, and titanium (to name a few), and water make asteroids an attractive target for mining operations when humans decide to expand their presence through interplanetary space. For example, water from asteroids could serve colonies in space, while the minerals and metals would be used to build habitats and grow food for future space colony inhabitants. Beginning 2013, companies interested in asteroid mining began announcing their plans for future operations on distant planetoids.

In addition, NASA is looking into similar missions. The biggest obstacles to asteroid mining are the need to develop affordable spaceflight technology that would allow humans to get to the asteroids of interest. Facts about the Asteroid Belt What other fascinating things do we know about the Asteroid Belt? Asteroid Belt objects are made of rock and stone. Some are solid objects, while others are orbiting “rubble piles”. The Asteroid Belt contains billions and billions of asteroids. Some asteroids in the Belt are quite large, but most range in size down to pebbles. The asteroid 1/Ceres is also designated as a dwarf planet, the largest one in the inner solar system. We know of at least 7,000 asteroids. The Asteroid Belt may contain many objects, but they are spread out over a huge area of space. This has allowed spacecraft to move through this region without hitting anything. Asteroids get their names from suggestions by their discoverers and are also given a number. The formation of Jupiter disrupted the formation of any worlds in the Asteroid Belt region by scattering asteroids away. This caused them to collide and break into smaller pieces. Gravitational influences can move asteroids out of the Belt.

The Asteroid Belt is often referred to as the “Main Belt” to distinguish it from other groups of asteroids such as the Lagrangians and Centaurs. Dwarf Planets, Pluto, Kuiper Belt & Oort Cloud A diagram showing solar system orbits. The highly tilted orbit of Eris is in red. Dwarf Planets: 10 Need-to-Know Things Smaller Than Our Moon If the sun were as tall as a typical front door, Earth would be about the size of a nickel and dwarf planet Pluto, one of the larger known dwarf planets, would be the size of the head of a pin. Long Trip Dwarf planets orbit our sun, a star. Most are located in the Kuiper Belt, a region of icy and rocky debris beyond Neptune. Ceres, the closest dwarf planet, is the largest world in the main asteroid belt between Mars and Jupiter. What's in a Day? Days on dwarf planets vary. One day on dwarf planet Ceres, for example, takes about nine Earth hours (the time it takes for Ceres to rotate once). It takes Ceres about 4.5 Earth years to go around the sun. Hard Worlds Dwarf planets are solid rocky and/or icy bodies.

It all depends on their location in the solar system. Moon Check Dwarf planets cannot support life as we know it. World Class Pluto was considered a planet until 2006. But the discovery of similar worlds prompted scientists to change the definition of a planet to better describe the different kinds of worlds of our solar system. Dwarf Planets: Overview While similar to planets in many ways, dwarf planets share their orbits around the sun with other objects such as asteroids or comets. The first five recognized dwarf planets are: Ceres | Pluto | Eris | Haumea | Makemake Dwarf Planets: In Depth Dwarf planets are a lot like regular planets: They both have enough mass and gravity to be nearly round - unlike odd-shaped asteroids. They both travel through space in a path around the sun. The big difference? A dwarf planet's path around the sun is full of other objects like asteroids and comets. A regular planet has a clear path around the sun. Most of the major impacts with other objects in its orbit happened billions of years ago. There is not much left over to get in the way. There may be dozens of dwarf planets in our solar system.

So far, we've classified just a handful: Closest to home, Ceres is the largest and most unique resident of the main asteroid belt between Jupiter and Mars. It also is the first dwarf planet to be visited by a spacecraft - NASA's Dawn mission. Pluto is the most famous dwarf planet. Discovered in 1930, it was long classified as our solar system's ninth planet. Pluto and its busy system of moons orbits the sun in the Kuiper belt, a region of icy debris beyond Neptune. Astronomers discovered Eris, a Pluto-sized world, in 2003. It takes icy Eris 557 Earth years to complete a single orbit around our sun. Two more confirmed orbit the sun in the icy zone beyond Neptune - Haumea and Makemake. Dwarf Planets: Trivia Little Rocks Dwarf planets are very different from their larger planetary cousins. It would take about 30,000 of them to create a world the size of Earth. Closest Dwarf Planet Ceres is the only dwarf planet found outside of the Kuiper Belt. Fast Football Dwarf planet Haumea is one of the fastest rotating objects in our solar system.

Haumea spins so fast that it has pulled itself into the shape of a squashed American football. Bring a Flashlight High noon on Pluto would look a lot like a moonlit night here on Earth. The sunlight that reaches Pluto is about 1,000 times dimmer than what we see here on Earth and provides little warmth. Distance Runner Even though it traveled at speeds 30 times greater than the fastest fighter jet, NASA's New Horizons spacecraft still took more than nine years to get to Pluto. Camera-Shy Planet Even our most powerful telescopes show little more than fuzzy blobs when pointed across billions of kilometers at Pluto and Charon. It took the New Horizons spacecraft nine and a half years of space travel to get the only close up images. Mission Milestone New Horizons is the only spacecraft to explore the Kuiper Belt up close. Long, Strange Trip Pluto is so far away it takes 248 years to orbit the sun -- so long it still hasn't finished one orbit since it was discovered in 1930. Pluto's orbit is so elliptical -- picture a squashed circle -- its distance from the sun varies more than 2.7 billion km (1.9 billion miles).

New Class The discovery of Eris -- about the same size as Pluto -- far beyond Neptune sparked intense debate about the definition of a planet. In the end, Eris and Pluto were classified as dwarf planets. Deep Freeze Pluto is so cold even the air can freeze and fall to the ground like snow. The planet's average temperature is about -387 degrees Fahrenheit (-233 degrees Celsius). For comparison, the coldest spot on Earth gets down to -128 degrees Fahrenheit (-89 degrees Celsius). Little Worlds, Big Numbers Scientists now consider that dwarf planets may be the most common type of planet. There are likely hundreds more dwarf planets out there waiting to be discovered. Faint Discovery There are millions of stars brighter than dwarf planet Pluto in the night sky, but observant astronomer Clyde Tombaugh caught a faint image -- light equal to a candle seen at a distance of 480 km (300 miles) -- on a photographic plate. Tombaugh had discovered the mysterious Pluto.

Pluto Pluto nearly fills the frame in this image from the Long Range Reconnaissance Imager (LORRI) aboard NASA's New Horizons spacecraft, taken on July 13, 2015, when the spacecraft was 476,000 miles (768,000 kilometers) from the surface. Pluto: 10 Need-to-Know Things If the sun were as tall as a typical front door, Earth would be the size of a nickel and dwarf planet Pluto would be about the size of the head of a pin. Pluto orbits our sun, a star, at an average distance of 3.7 billion miles (5.9 billion kilometers) or 39.5 AU. One day on Pluto takes about 153 hours. That's the time it takes for Pluto to rotate or spin once. Pluto makes a complete orbit around the sun (a year in Plutonian time) in about 248 Earth years. It is thought that Pluto has a rocky core surrounded by a mantle of water ice with other ices coating its surface. Pluto has five known moons.

Pluto is sometimes called a double-planet system due to the fact that its moon Charon is quite large and orbits close to its parent planet. NASA's New Horizons spacecraft is the first mission sent to encounter the Pluto-system and other members of the Kuiper Belt. There are no known rings around Pluto. Pluto has a thin, tenuous atmosphere that expands when it comes closer to the sun and collapses as it moves farther away -- similar to a comet. Scientists do not think Pluto can support life as we know it. Although, some scientists believe it is possible Pluto could possess a hidden ocean under its surface. Pluto was considered a planet from 1930, when it was first discovered, until 2006. The discovery of similar-sized worlds deeper in the distant Kuiper Belt sparked a debate which resulted in a new official definition of a planet. The new definition did not include Pluto. Pluto: Overview Discovered in 1930, Pluto was long considered our solar system's ninth planet. But after the discovery of similar intriguing worlds deeper in the distant Kuiper Belt, icy Pluto was reclassified as a dwarf planet.

Findings by NASA's New Horizons in 2015 revealed for the first time how that Pluto is a complex and mysterious world. Information about Pluto and its largest moon, Charon, provide insight into the collision believed to have formed the dwarf planet and moons we see today. Dwarf planets may provide the best evidence about the origins of our solar system. Pluto: In Depth 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 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.

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 km 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. Since its orbit is so elliptical, 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. 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 named Charon, which was discovered in 1978.

This moon is so big that Pluto and Charon are sometimes referred to as a double dwarf planet system. The distance between them is 19,640 km (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 appears to have a slightly red hue), indicating that they have different surface compositions and structures. 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 rotations). 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. 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. These two moons, named Nix and Hydra, are two to three times farther away from Pluto than Charon. In 2011 and 2012, scientists used Hubble to spot two more moons (originally designated P4 and P5). In 2013, the two moons were named Kerberos (P4) and Styx (P5). Late in 2014 and early in 2015, image animations displayed the mutual orbital waltz of Pluto and Charon around their center of mass. Beginning in the spring of 2015, New Horizons started its detailed studies of Pluto including searches for additional moons and for rings.

Various studies continued through its close approach on July 14 at a distance of 8507 miles (13,691 kilometers) and after. The return of New Horizons' best data began shortly after its close approach and will continue for more than a year due to the large transmission distance. Because of the speed of the flyby and Pluto's slow rotation rate only one hemisphere of this dwarf planet has been photographed and measured at high resolution, limiting generalizations about all of Pluto's surface. Still, Pluto's diameter could be measured and had to be revised upward to 1475 miles (2,374 kilometers) based on New Horizons' imagery. Results from New Horizons, besides forcing the revision of textbooks, left planetary scientists struggling to explain this cold, distant world and its system of moons. Pluto's surface exhibits craters as large as 162 miles (260 kilometers) in diameter on the dayside, near encounter hemisphere. Craters are widely distributed there and show degradation or infill. This was a surprise to find on a dwarf planet that was anticipated to be heavily cratered and lacking activity that might affect craters.

Dense cratering is seen in some areas, but so are tectonic features including scarps and troughs as long as about 370 miles (600 kilometers). Mountains are also seen rising 6500 to 9800 feet (2 to 3 kilometers) above their surroundings. The likely materials that can hold up the mountains and maintain their shapes over millions of years in Pluto's cold is limited. The science team concludes that the mountains are made of "water ice-based 'bedrock'." Frozen gases on Pluto's surface (the temperature there is about -391 F = -235 C = 38 K) include nitrogen (N2), carbon monoxide (CO),and methane (CH4). These were detected by ground-based telescopes and are now thought to be thin layers on top of the 'bedrock' water ice. Dark surface coloring appears to be due carbon residues called tholins. These are created by solar ultraviolet rays or charged particles falling on mixtures of nitrogen and methane.

Detailed views of a plain on this dwarf planet show no confirmed craters but it has large (tens of kilometers) polygonal or egg-shaped features defined and separated by troughs between neighbors. Features similar in look to glaciers on Earth are seen in this region. This is consistent with the strength and flow properties of the frozen gases mentioned above. Radio transmission measurements from New Horizons measured Pluto's atmosphere having a pressure of 10 microbars (millionths of a bar). For comparison, Earth's atmospheric pressure at sea level is about 1 bar, 100,000 times greater than Pluto's surface pressure. Those measurements also showed that Pluto has a shallow tropospheric boundary layer (Earth has a troposphere too). Imagery from the cameras shows a haze layer and even some structure in it. It isn't known whether Pluto has a magnetic field, but its small size and slow rotation suggest little or none. Data from two of New Horizons instruments may give an indirect answer to this question. Charon was also studied in detail. Its diameter came out slightly larger than expected, at 753 miles (1212 kilometers).

Surface variations of 9800 feet (3 kilometers) seen on this moon imply that it, like Pluto, has water ice that runs deep in its structure. Also similar to Pluto, Charon exhibits cratered and smooth plains, fault scarps, and an extensive system of faults and graben. (A graben [Grah-ben] is a block of surface material that has dropped lower than its surroundings, creating a wide valley whose walls are the fault planes along which the block dropped.) Rolling plains are moderately cratered. They even show several rille-like structures (first observed on Earth's moon). Craters with rays (both light, as on Earth's moon, and dark, as sometimes seen on Mars) indicating freshness are found on Charon. Other craters show evidence of aging. The two largest fractures (visible during the flyby) on Charon extend at least 650 miles (1050 kilometers) across the surface and at least one other, with a depth of 3 miles (5 kilometers) is seen going over the horizon to Charon's night side. One of the dayside fractures is seen as a double-walled graben-like structure. There is no evidence that Charon has an atmosphere.

Nix is not spherical. It has three different diameters (making it a tri-axial ellipsoid): 67 x 51 x 45 miles (108 x 82 x 72 kilometers). There are variations in composition over its surface but it reflects more light than Charon. This suggests that its ice is cleaner than Charon's. Hydra is also not spherical with rough diameters of 53 x 41 miles (86 x 66 kilometers) with the third dimension not well determined. It also is more reflective than Charon, leaving scientists to puzzle over how the ice on these small moons could stay so bright over billions of years given the processes known to darken material over time. More information on Kerberos and Styx awaits the return of more data from New Horizons. Data used for searches for additional moons and rings has not detected any. How Pluto Got its Name Pluto is the only world named by an 11-year-old girl. In 1930, Venetia Burney of Oxford, England, suggested to her grandfather that the new discovery be named for the Roman god the underworld. He forwarded the name to the Lowell Observatory and it was selected.

Pluto's moons are named for other mythological figures associated with the underworld. Charon is named for the river Styx boatman who ferries souls in the underworld; Nix is named for the mother of Charon, who is also the goddess of darkness and night; Hydra is named for the nine-headed serpent that guards the underworld; Kerberos is named after the three-headed dog of Greek mythology; and Styx is named for the mythological river that separates the world of the living from the realm of the dead. Pluto's place in mythology can get a little muddled, so we asked Dr. Elizabeth Vandiver, chair of the Department of Classics in Whitman College in Walla Walla, Washington, to clarify the origins of the name: "Pluto is the name of the Roman god of the Underworld, equivalent to the Greek Hades. However, the Greek name "Plouton" (from which the Romans derived their name "Pluto") was also occasionally used as an alternative name for Hades. But Pluto is definitely the Roman spelling." The symbol for dwarf planet Pluto is a monogram made up of P and L in Pluto (and also the initials of Percival Lowell, who predicted its discovery).

Pluto: Significant Dates 1930: Clyde Tombaugh discovers Pluto. 1977-1999: Pluto's lopsided orbit brings it slightly closer to the sun than Neptune. It will be at least 230 years before Pluto moves inward of Neptune's orbit for 20 years. 1978: American astronomers James Christy and Robert Harrington discover Pluto's unusually large moon, Charon. 1988: Astronomers discover that Pluto has an atmosphere. 2005: Scientists using the Hubble Space Telescope announce the discovery of two additional moons of Pluto. Named Nix and Hydra, the little moons may have formed at the same time as Charon did, perhaps all three splitting off from Pluto in a giant impact event. 2006: NASA's New Horizons mission launches on a path to explore Pluto and the Kuiper Belt region. The spacecraft is scheduled to reach Pluto in 2015. 2006: The International Astronomical Union classifies Pluto as a dwarf planet and recognizes similar worlds beyond the orbit of Neptune as plutoids.

2011: Astronomers using the Hubble Space Telescope discovered a fourth moon, named Kerberos, orbiting the icy dwarf planet. 2012: Astronomers using the Hubble Space Telescope to search for potential hazards to the New Horizon mission discovered a fifth moon, named Styx, orbiting the icy dwarf planet. Pluto: Trivia Pluto's estimated surface temperature falls between --378 to --396 degrees F (-228 to -238 C). Planet Hunter Pluto was discovered in 1930 by American astronomer Clyde Tombaugh. Big Companion Charon is half of Pluto's diameter - making it the largest satellite relative to the planet it orbits. Long Year Pluto orbits the Sun once every 248 years. That's longer than the entire history of the United States of America. Slow Communication A radio signal moving at the speed of light takes about 4 hours to reach Pluto from Earth. Shifting Scene Pluto's brightness changes as it rotates, revealing large light and dark regions. Dark Mythology Pluto is named after the Roman god of the underworld. How Pluto Got Its Name Student Venetia Burney suggested the name Pluto in 1930.

Scary Boat Charon is named after the mythological boatman who ferried souls across the river Styx to Pluto for judgment. Tribute The symbol for Pluto ("PL") is tribute to Percival Lowell, who started the search for the ninth planet in the early 1900s. Small World At 1,470 miles (about 2,370 kilometers) across, Pluto could fit between Washington, D.C. and Denver, Colorado. Sideways Like Uranus, Pluto rotates on its side. Far From the Sun Pluto's last time at aphelion - the farthest point in its orbit - was 1865. It will next reach aphelion in 2112. Lightweight A person on Pluto would weigh 1/15 what they weigh on Earth . The astronauts on the Moon had 1/6 of their Earth weight. Close Companion Charon is 20 times closer to Pluto than our moon is to Earth. Moon Surprise American astronomers James Christy and Robert Harrington discovered Charon in 1978. Kuiper Belt Artist's concept of Eris and its moon. The sun is in the distance. Image credit: Robert Hurt (IPAC).

Kuiper Belt: 10 Need-To-Know Things The Kuiper Belt and the Oort Cloud are regions of space. The known icy worlds and comets in both regions are much smaller than Earth's moon. The Kuiper Belt and the Oort Cloud surround our sun, a star. The Kuiper Belt is a doughnut-shaped ring, extending just beyond the orbit of Neptune from about 30 to 55 AU. The Oort Cloud is a spherical shell, occupying space at a distance between five thousand and 100 thousand AU. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud. Short-period comets (which take less than 200 years to orbit the Sun) originate in the Kuiper Belt. There may be are hundreds of thousands of icy bodies larger than 100 km (62 miles) and an estimated trillion or more comets within the Kuiper Belt. The Oort Cloud may contain more than a trillion icy bodies. Some dwarf planets within the Kuiper Belt have thin atmospheres that collapse when their orbit carries them farthest from the sun.

The Kuiper Belt and the Oort Cloud are regions of space and therefore do not have moons. However, several dwarf planets within the Kuiper Belt have moons. The Kuiper Belt and the Oort Cloud are regions of space and therefore do not have rings. No objects within these two regions are known to have rings. The first mission to the Kuiper Belt is New Horizons. New Horizons will reach Pluto in 2015. These two regions of space are not capable of supporting life as we know it. Fact: Both the Kuiper Belt and the Oort Cloud are named for the astronomers who predicted their existence during the 1950s: Gerard Kuiper and Jan Oort. Kuiper Belt: In Depth The Kuiper Belt is a disc-shaped region beyond Neptune that extends from about 30 to 55 astronomical units (compared to Earth which is one astronomical unit, or AU, from the sun). This distant region is probably populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets. Dwarf planet Pluto may be the best known of the larger objects in the Kuiper Belt.

Comets from the Kuiper Belt take less than 200 years to orbit the sun and travel approximately in the plane in which most of the planets orbit the sun. .Objects in the Kuiper Belt are presumed to be remnants from the formation of the solar system about 4.6 billion years ago. The first of these strange bodies, which astronomers call Kuiper Belt Objects (KBOs), came to light in 1992, discovered by Dave Jewitt and Jane Luu -- a pair of scientists who didn't believe the outer solar system was empty. Beginning in 1987 they had doggedly scanned the heavens in search of dim objects beyond Neptune. It took five years, looking off-and-on through the University of Hawaii's 2.2 m telescope, but they finally found what they were after: a reddish-colored speck 44 AU from the Sun -- even more distant than Pluto!

Jewitt (University of Hawaii) and Luu (UC Berkeley) wanted to name their find "Smiley," but it has since been cataloged as "1992 QB1." That discovery marked our first glimpse of the long-sought Kuiper Belt, named after Gerard Kuiper who, in 1951, proposed that a belt of icy bodies might lay beyond Neptune. It was the only way, he figured, to solve a baffling mystery about comets: Some comets loop through the solar system on periodic orbits of a half-dozen years or so. They encounter the Sun so often that they quickly evaporate -- vanishing in only a few hundred thousand years. Astronomers call them "short-period comets," although "short-lived" is more to the point. Short-period comets evaporate so quickly compared to the age of the solar system that we shouldn't see any, yet astronomers routinely track dozens of them. It was a real puzzle. Kuiper's solution was a population of dark comets circling the Sun in the realm of Pluto -- leftovers from the dawn of our solar system when planetesimals were coalescing to make planets.

The ones beyond Neptune, Kuiper speculated, never stuck together, remaining instead primitive and individual. Nowadays they occasionally fall toward the Sun and become short-period comets. Because KBOs are so distant, their sizes are difficult to measure. The calculated diameter of a KBO depends on assumptions about how reflective the object's surface is. With infrared observations by the Spitzer Space Telescope, most of the largest KBOs have known sizes. One of the most unusual KBOs is Haumea, which is a part of a collisional family orbiting the sun. The parent body, Haumea, apparently collided with another object that was roughly half its size. The impact blasted large icy chunks away and sent Haumea reeling, causing it to spin end-over-end every four hours. It spins so fast that it has pulled itself into the shape of a squashed American football. Haumea and two small moons -- Hi'iaka and Namaka -- make up the family. In March 2004, a team of astronomers announced the discovery of a planet-like transneptunian object orbiting the sun at an extreme distance, in one of the coldest known regions of our solar system.

The object (2003VB12), since named Sedna for an Inuit goddess who lives at the bottom of the frigid Arctic ocean, approaches the sun only briefly during its 10,500-year solar orbit. It never enters the Kuiper Belt, whose outer boundary region lies at about 55 AU -- instead, Sedna travels in a long, elliptical orbit between 76 and nearly 1,000 AU from the sun. Since Sedna's orbit takes it to such an extreme distance, its discoverers have suggested that it is the first observed body belonging to the inner Oort Cloud. In July 2005, a team of scientists announced the discovery of a KBO that was initially thought to be about 10 percent larger than Pluto. The object, temporarily designated 2003UB313 and later named Eris, orbits the sun about once every 560 years, its distance varying from about 38 to 98 AU. (For comparison, Pluto travels from 29 to 49 AU in its solar orbit.) Eris has a small moon named Dysnomia. More recent measurements show it to be slightly smaller than Pluto.

The discovery of Eris -- orbiting the sun and similar in size to Pluto (which was then designated the ninth planet) -- forced astronomers to consider whether Eris should be classified as the tenth planet. Instead, in 2006, the International Astronomical Union created a new class of objects called dwarf planets, and placed Pluto, Eris and the asteroid Ceres in this category. In 2015, NASA's New Horizons spacecraft flew past Pluto, making the first up-close exploration of a Kuiper Belt Object. The spacecraft is continuing deeper into this region of icy debris and may be able to explore at least one more object. How the Kuiper Belt Got Its Names The region is named for the astronomer who predicted its existence -- Gerard Kuiper. It is sometimes called the Edgeworth-Kuiper Belt, recognizing the independent and earlier discussion by Kenneth Edgeworth. Objects discovered in the Kuiper Belt get their names from diverse mythologies. Eris is named for the Greek goddess of discord and strife. Haumea is named for a Hawaiian goddess of fertility and childbirth. Comets from both regions are generally named for the person who discovered them.

Significant Dates 1943: Astronomer Kenneth Edgeworth suggests that a reservoir of comets and larger bodies resides beyond the planets. 1950: Astronomer Jan Oort theorizes that a vast population of comets may exist in a huge cloud on the distant edges of our solar system. 1951: Astronomer Gerard Kuiper predicts the existence of a belt of icy objects just beyond the orbit of Neptune. 1992: After five years of searching, astronomers David Jewitt and Jane Luu discover the first KBO, 1992QB1. 2002: Scientists using the 48-inch Oschin telescope at Palomar Observatory find Quaoar, the first large KBO hundreds of kilometers in diameter. This object was photographed in 1980, but was not noticed in those images. 2004: Astronomers using the 48-inch Oschin telescope announce the discovery of Sedna (2003VB12). 2005: Astronomers announce the discovery of 2003UB313. This object, later named Eris, is slightly larger than Pluto.

2008: The Kuiper Belt object provisionally known as 2005FY9 ("Easterbunny") is recognized in July as a dwarf planet and named Makemake (pronounced MAHkeh-MAHkeh) after the Polynesian (Rapa Nui) creation god. In September, 2003EL61 ("Santa") was designated a dwarf planet and given the name Haumea after the Hawaiian goddess of fertility and childbirth. This 2013 graphic puts huge solar system distances in perspective. Oort Cloud: 10 Need-To-Know Things The Oort Cloud is a region of space. The known comets in this region are much smaller than the Earth's moon. The Oort Cloud surrounds our sun, a star. The Oort Cloud is a spherical shell, occupying space at a distance between five and 100 thousand AU. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud. The Oort Cloud may contain more than a trillion icy bodies. Comets that originate from the Oort Cloud gain atmospheres (the coma) when they near the sun.

This atmosphere collapses when the comet's orbit carries it farthest from the sun. There are no known moons of Oort Cloud objects. The are no known rings around objects in this region of space. There have been no missions sent to the Oort Cloud. The Oort Cloud is not capable of supporting life as we know it. The Oort Cloud is named for the astronomer who predicted its existence during the 1950s: Jan Oort. Oort Cloud: In Depth The Oort Cloud is believed to be a thick bubble of icy debris that surrounds our solar system. This distant cloud may extend a third of the way from our sun to the next start -- between 5,000 and 100,000 astronomical units. Earth is about one astronomical unit from the sun (roughly 93 million miles or 150 million kilometers). Consider this: At its current speed of about a million miles a day, NASA's Voyager 1 spacecraft won't reach the Oort Cloud for about 300 years. And it will take about 30,000 years to reach the other side.

Dutch astronomer Jan Oort first proposed the idea of this region of space to explain the origins of comets with that take thousands of years to orbit the sun. These are called long-period comets and most have been seen only once in recorded history. More frequent visitors to the inner solar system are called short-period comets. There may be hundreds of billions, even trillions, of icy bodies in the Oort Cloud. Every now and then, something disturbs one of these icy worlds and it begins a long fall toward our sun. Two recent examples are comets C/2012 S1 (ISON) and C/2013 A1 Siding Spring. ISON was destroyed when it passed too close the the sun. Siding Spring, which made a very close pass by Mars, will not return to the inner solar system for about 740,000 years. How the Oort Cloud Got its Name This region of space is named for Dutch astronomer Jan Oort who predicted its existence in 1950. Discovery The Oort Cloud still hasn't been discovered. It is just a theory, although we have studied several comets believed to have come from this distant region of our galaxy.

Eight planets Mike Brown On August 24th 2006 the word "planet" was given its first-ever scientific definition by a vote of the International Astronomical Union. With the raising of a few yellow cards in Prague, Pluto was demoted from full-fledged planet to "dwarf planet." Eris, originally called 2003 UB313 , sometimes called Xena, sometimes called the "10th planet," which in many ways precipitated this final debate, becomes the largest known dwarf planet. Unless astronomers revisit this issue at some point in the future, it is unlikely that there will ever be more than eight planets. While some still continue to mourn the loss of Pluto from the pantheon of planets, it is much more interesting to consider this new class of dwarf planets, instead. What is a dwarf planet? When the final vote on the definition of "planet" was made, and the eight dominant bodies in the solar system were declared (quite rationally) a class separate from the others, a new class of objects was defined.

The "dwarf planets" are all of those objects which are not one of the eight dominant bodies (Mercury through Neptune) yet still, at least in one way, resemble a planet. The best definition heard so far is that a dwarf planet is something that looks like a planet, but is not a planet. The dwarf planets are bodies in the solar system which are large enough to become round due to their own gravitational attraction. Why do astronomers care about round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object without and with interesting geological and planetary processes occuring (there are many many other transitions that are equally important, however, a fact that tends to be ignored in these discussions). How many dwarf planets are there? There are two ways to look at this question. First is to ask: acording to the official list kept by the IAU, how many dwarf planets are there?

The current answer (as of 1 April 2007) is only three. The asteroid Ceres is the only asteroid known to be round. Pluto is also considered a dwarf planet. And Eris, a little larger than Pluto, is a dwarf planet also. Three is all the IAU will currently deal with. Reality, however, does not pay much attention to official lists kept by the IAU or by anyone else. A more interesting question to ask is: how many round objects are there in the solar system that are not planets? These are, by the definition, dwarf planets, whether or not they ever make it to any offiicially sanctioned list. If the category of dwarf planet is important, then it is the reality that is important, not the official list. So how many dwarf planets are there? Ceres is still the only asteroid that is known to be round. After that it gets complicated. All of the rest of the new dwarf planets are in the distant region of the Kuiper belt, where we can't actually see them well enough to know for sure if they are round or not.

While we can't see most of the objects in the Kuiper belt well enough to determine whether they are round or not, we can estimate how big an object has to be before it becomes round and therefore how many objects in the Kuiper belt are likely round. In the asteroid belt Ceres, with a diameter of 900 km, is the only object large enough to be round, so somewhere around 900 km is a good cutoff for rocky bodies like asteroids. Kuiper belt objects have a lot of ice in their interiors, though. Ice is not as hard as rock, so it less easily withstands the force of gravity, and it takes less force to make an ice ball round. The best estimate for how big an icy body needs to be to become round comes from looking at icy satellites of the giant planets. The smallest body that is generally round is Saturn's satellite Mimas, which has a diameter of about 400 km. Several satellites which have diameters around 200 km are not round. So somewhere between 200 and 400 km an icy body becomes round. Objects with more ice will become round at smaller sizes while those with less rock might be bigger.

We will take 400 km as a reasonable lower limit and assume that anything larger than 400 km in the Kuiper belt is round, and thus a dwarf planet. How many objects larger than 400 km are there in the Kuiper belt? We can't answer this question precisely, because we don't know the sizes of more than a handful of Kuiper belt objects (for an explanation why, see the discussion on the size of Eris) , but, again, we can make a reasonable guess. If we assume that the typical small Kuiper belt object reflects 10% of the sunlight that hits its surface we know how bright a 400 km object would be in the Kuiper belt. As of late August 2006, 44 objects this size or larger in the Kuiper belt (including, of course, Eris and Pluto), and one (Sedna) in the region beyond the Kuiper belt. In addition our large ongoing Palomar survey has detected approximately 30 more objects of this size which are currently undergoing detailed study. We have not yet completed our survey of the Kuiper belt.

Our best estimate is that a complete survey of the Kuiper belt would double this number. For now, the number of known objects in the solar system which are likely to be round is 53, with the number jumping to 80 when the objects from our survey are announced, and to ~200 when the Kuiper belt is fully surveyed. Beyond the Kuiper belt there may be even more dwarf planets than in the Kuiper belt. Our best guess is that the region where Sedna resides could contain another ~2000 dwarf planets. The new dwarf planets in the solar system are very different from the previous 8 planets. Most are so small that they are smaller across than the distance from Los Angeles to San Francisco. They are so small that about 30,000 of them could fit inside the earth. What does the new solar system look like? Here is the before (black circles with nine planets) and after (add red ellipses to show the dwarf planets) views of the solar system: (images may be freely used; credit: Mike Brown, Caltech) In order from closest to furthest, the dwarf planets are shown below (with the planets, shown for comparison, in blue).

A few of them have been studied well enough that we know interesting details. Click the links to learn more....... name 1800 Who has discovered dwarf planets? Many people have found dwarf planets, starting with the discovery of Ceres by Piazzo in 1801, continuing with Pluto by Tombaugh in 1930, and the moving to the discovery of the many many objects in the Kuiper belt begining in 1992. For the likely dwarf planets listed above, here is the list of discoverers (some planets were discovered by teams which change a bit, in general the team name or the team leader is listed. Sometimes no discoverer is recorded): Brown et al.: 15 dwarf planets Deep Ecliptic Survey: 8 dwarf planets Near Earth Asteroid Tracking Survey: 4 dwarf planets Spacewatch: 4 dwarf planets StarChild: The Asteroid Belt Listen to an audio version of this page. Sorry, your browser does not support the audio element, please consider updating. The Asteroid Belt The dwarf planet called Ceres orbits the Sun in the asteroid belt. An asteroid is a bit of rock.

It can be thought of as what was "left over" after the Sun and all the planets were formed. Most of the asteroids in our solar system can be found orbiting the Sun between the orbits of Mars and Jupiter. This area is sometimes called the "asteroid belt". Think about it this way: the asteroid belt is a big highway in a circle around the Sun. Think about the asteroids as cars on the highway. Sometimes, the asteroid cars run into one another. When this happens, the asteroids may break up into smaller asteroids. Scientists think that most asteroids are the result of collisions between larger rocky space bodies. Asteroids can be a few feet to several hundred miles wide. The belt probably contains at least 40,000 asteroids that are more than 0.5 miles across. If an asteroid is captured by the gravitational pull of a planet, the asteroid can be pulled out of the belt and go into orbit as a moon around the planet that pulled on it. A Question

Question: Which of the five dwarf planets is in the asteroid belt?
Answer: Ceres