Pluto Facts for Kids | Facts About Dwarf Planet Pluto Interesting facts about space, the planets and our solar system Pluto Facts and Information Pluto is a dwarf planet and the second closest to the Sun of its kind. Before it was re-classified to a dwarf planet Pluto was thought to be the ninth planet in our solar system and the one furthest away from the Sun. It has recently been explored by New Horizons, the first spacecraft to ever reach Pluto and we are learning fascinating things about this far away planet. Pluto was discovered in 1930. It was seen using a telescope as it is too far away from Earth to be seen without the use of equipment. Pluto was discovered by Clyde Tombaugh and was initially thought to be a ninth planet. However even then people were unsure as to whether or not it could be classed as a planet like the other eight in the solar system. Pluto does not orbit the Sun like other planets in the solar system. Instead of taking a clear path around the sun Pluto goes in more of an oval and has a tilted orbit when compared with other planets.

This means not only does Pluto get closer and further away from the Sun, it is also higher in the solar system on one side of the Sun than the other. Pluto actually spends 22 years of its orbit closer to the Sun than Neptune . This last happened between 1979 and 1999 and will happen again in 2227. It takes 248 Earth years for Pluto to orbit the Sun. This means that Pluto hasn’t made a full trip around the Sun since it was found in 1930. Scientists have predicted its orbit path. Pluto takes 153.3 hours to rotate on its axis. This means a day on Pluto is 6.4 Earth days. Pluto is in the Kuiper Belt. This is an area in space containing thousands of objects orbiting the sun. Most of these are very small and made of ice. It is sometimes called the “Third Zone” and is the space further away from the sun than Neptune. Pluto is 1,473 miles (2,370 kilometers) in diameter. This is quite small compared to the other eight planets in the Solar System apart from Mercury which is even smaller.

Pluto gets its name from the Roman god of the underworld. The name was actually chosen by a young girl in England who was only 11 at the time. Her name was Venetia Burney. In 2006 it was decided that Pluto should no longer be classed as a planet. It is now called a dwarf planet. The decision was made after another dwarf planet named Eris was discovered in 2003 and astronomers began to talk about what actually makes a planet a planet. This was mostly their size, location and ability to clear their own orbit path. Clearing an orbit path is where a planet has enough gravity to push other items out of its way or bring them into their own atmosphere. There are five dwarf planets in total. The others are called Makemake , Ceres and Haumea . Of the dwarf planets, Pluto is second closest to the Sun. Ceres is the closest. Pluto is the second largest of the dwarf planets with Eris being the biggest by roughly 24 miles (40 kilometres) difference in their diameters. Pluto has 5 known moons. These are named Charon, Styx, Nix, Kerberos and Hydra.

Charon is the largest of the five and is similar to the Earth’s moon – this makes it about half the size of Pluto itself. Both are tidally locked to their planets meaning the same side of the moon faces the planet at all times. The first spacecraft to visit Pluto was a NASA mission called New Horizons and flew by the dwarf planet in July 2015. The probe was the fastest spacecraft ever launched and left Earth on the 19th of January 2006. The mission is not over and is planned to continue until 2026 after spending a few years looking at objects in the Kuiper Belt. The spacecraft is carrying a small container holding the ashes of Clyde Tombaugh who first discovered the dwarf planet. Pluto is grey in colour with a slight red tinge to it. Scientists were expecting it to be a dark, bluish icy planet and were surprised to discover its rusty tint – not too different to the colours of Mars . The photos revealed that Pluto has a heart shape on it! This heart shape is thought to be made of ice and is changing over time.

This region of the planet is now called the Tombaugh Reggio and does not have the craters that are found on the rest of the planet. This makes it likely that this region is much younger than Pluto itself and is probably no older than 100 million years. That is pretty young as far as planets go! Pluto’s atmosphere is made up mostly of nitrogen. There are also low levels of carbon monoxide and methane. This atmosphere actually grows bigger as the planet gets closer to the sun in its unusual orbit and some of the ice on the surface evaporates. Below all of the gas is a rocky core covered in ice. New information has been gathered by New Horizons on Pluto’s atmosphere that shows it to be much thinner than had been predicted. There is still a lot to learn about this, hopefully New Horizons collected enough data to answer many questions. Pluto has ice caps. People have thought for a long time that Pluto has ice caps and the pictures from New Horizons has proved them right. The other planets with ice caps are Earth and Mars. Pluto has mountains as tall as 3,500 metres (11,000 feet). Scientists think these are formed on top of layers of water and ice.

Mountains on planets are usually surrounded by craters but not on Pluto, not many were discovered from the images sent back from the spacecraft. This could meant that there has been recent geological activity (still millions of years ago!) on the dwarf planet which would have smoothed the craters out. We do not know if Pluto has a magnetic field or not. Scientists think it probably does not have one because of how small it is and the fact that it rotates on its axis so slowly. Pluto is surrounded by a thick and very visible haze. This was only discovered once the New Horizons spacecraft had already passed Pluto and took a photo looking back towards the dwarf planet. Little is known about the haze at this point but it is very different to what scientists were expecting to see. New Horizons got closest to Pluto on the 14th of July. This allowed the spacecraft to take photos of the planet’s surface with detail that had never been seen before. Information will be downloaded until 2017 due to the distance of the spacecraft from Earth. There is still lots more information to receive and exciting things to learn about Pluto!

Picture of Pluto Pluto and the Solar System | IAU Pluto and the Solar System Pluto and the Developing Landscape of Our Solar System The discovery of Pluto Nearly eighty years ago an astronomer working at the Lowell Observatory in the United States made a discovery that would ultimately initiate a dramatic change in the way we look at our Solar System. The young astronomer was Clyde Tombaugh, an observing assistant working at the observatory made famous by the great astronomer Percival Lowell. Tombaugh was continuing the search for an elusive planet – planet X – that Lowell had believed (incorrectly) to be responsible for perturbing the orbits of Uranus and Neptune. Within a year, after spending numerous nights at the telescope exposing photographic plates and months tediously scanning them for signs of a planet, Tombaugh saw what he was looking for. At around 4pm on the afternoon of 18 February 1930 Tombaugh began comparing two plates taken in January that year showing a region in the constellation of Gemini. As he flicked from one plate to the other, trying to see if something moved slightly between the two (the tell-tale sign of the planet he was hunting), he spotted something.

In one part of the frame a small object flitted a few millimetres as he switched between the two plates. Tombaugh had found his new planet! (Stern & Mitton, 2005) The changing landscape of the Solar System The object Tombaugh had discovered was named Pluto, a name officially adopted by the American Astronomical Society, the Royal Astronomical Society in the UK and the IAU. It is a frigid world, billions of kilometres from Earth, and 30 times less massive than the then-smallest known planet, Mercury. But Pluto was not alone. It was found to have five satellites. The largest, Charon, was discovered in 1978. The smaller four were discovered using the Hubble Space Telescope in 2005, 2011 and 2012 and officially named Nix, Hydra, in early 2006 ( read more ) , Kerberos and Styx in 2013 ( read more ) by the IAU.

The view of our Solar System's landscape began to change on August 30, 1992 with the discovery by David Jewitt and Jane Luu from the University of Hawaii of the first of more than 1000 now known objects orbiting beyond Neptune in what is often referred to as the transneptunian region. More generally these bodies are often simply labelled as Trans-Neptunian Objects (TNOs). With so many Trans-Neptunian Objects being found, it seemed inevitable that one or more might be found to rival Pluto in size. On the night of the 21 October 2003, Mike Brown from Caltech, Chad Trujillo from the Gemini Observatory and David Rabinowitz from Yale University were using a telescope and camera at the Palomar Observatory in the US to search the edge of the Solar System. That night they imaged a region of sky showing an object moving relative to the background stars. Later analysis showed that they had discovered another cold world, around 2500 km across, orbiting the Sun.

Subsequent observations showed that the new object, initially named 2003 UB313 according to the International Astronomical Union's protocol on the initial designation of such objects, was more massive than Pluto and that it too had a satellite ( read more ). With an object larger and more massive than Pluto now beyond Neptune and ever more of these Trans-Neptunian Objects being discovered, astronomers were beginning to ask: "Just what constitutes a planet?" A new class of objects and how to define a planet The IAU has been responsible for the naming and nomenclature of planetary bodies and their satellites since the early 1900s. As Professor Ron Ekers, past president of the IAU, explains: 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 .( read the full article on page 4 of the IAU GA Newspaper ) The IAU decided to create a committee to gather opinions from a broad range of scientific interests, with input from professional astronomers, planetary scientists, historians, science publishers, writers and educators. Thus the Planet Definition Committee of the IAU Executive Committee was formed and quickly went about preparing a draft resolution to put to the members of the IAU. After the final meeting in Paris the draft resolution was completed. One crucial aspect of the resolution is described by Professor Owen Gingerich, Chair of the IAU Planet Definition Committee: "On the scientific side, we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighbouring objects". ( read more in the IAU GA Newspaper , starting on page 4) The final resolution The first draft proposal for the definition of a planet was debated vigorously by astronomers at the 2006 IAU General Assembly in Prague and a new version slowly took shape.

This new version was more acceptable to the majority and was put to the members of the IAU for a vote at the Closing Ceremony on the 24 August 2006. By the end of the Prague General Assembly, its members voted that the resolution B5 on the definition of a planet in the Solar System would be as follows: A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. ( read more ) Dwarf Planets, plutoids and the Solar System today The IAU Resolution means that the Solar System officially consists of eight planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called dwarf planets was also decided on. It was agreed that planets and dwarf planets are two distinct classes of objects. The first members of the dwarf planet category are Ceres, Pluto and Eris, formerly known as 2003 UB313.

Eris was named after the IAU General Assembly in 2006 ( read more ) Eris is the Greek god of discord and strife, a name which the discoverer Mike Brown found fitting in the light of the academic commotion that followed its discovery. The dwarf planet Pluto is recognised as an important prototype of a new class of Trans-Neptunian Objects. The IAU has put given a new denomination for these objects: plutoids. Today the resolution remains in place and is a testament to the fluid nature of science and how our view of the Universe continues to evolve with changes made by observations, measurements and theory. The latest observations On 14 July 2015, NASA's New Horizons spacecraft flew past Pluto, providing numerous imaging, spectroscopy, and in situ datasets that have dramatically altered our knowledge about Pluto and its system of five moons. The images established that Pluto is larger than Eris and is the largest body in the Kuiper Belt. The images also revealed a remarkable landscape containing a variety of landforms, including broad plains, mountain ranges several kilometres high, and evidence for volcanoes. Pluto's surface is unusual for its diversity of surface compositions and colours.

Some regions are as bright as snow and others are as dark as charcoal. Colour imaging and composition spectroscopy revealed a highly complex distribution of surface ices, including nitrogen, carbon monoxide, water, and methane, as well as their chemical byproducts produced by radiolysis. It has also been determined that some surfaces on Pluto are completely free of visible craters, indicating that they have been modified or created in the recent past. Other surfaces are heavily cratered and appear to be extremely old. Pluto is shrouded by a cold, nitrogen­‐dominated atmosphere that contains a thin, highly extended haze layer about 150 km thick. Pluto’s large moon Charon displays impressive tectonics and evidence for a heterogeneous crustal composition, but no evidence for an atmosphere; its pole displays puzzling dark terrain. No new satellites were detected, nor were rings. Small satellites Hydra and Nix have brighter surfaces than expected. These results raise fundamental questions about how a small, cold planet can remain active over the age of the Solar System. They demonstrate that dwarf planets can be every bit as scientifically interesting as planets.

Equally important is that all three major Kuiper belt bodies visited by spacecraft so far — Pluto, Charon, and Triton — are more different than similar, bearing witness to the potential diversity awaiting the exploration of their realm. References: Stern, A., & Mitton, J., 2005, Pluto and Charon: Ice Worlds on the Ragged Edge of the Solar System, Wiley-VCH 1997 Planets, Dwarf Planets and Small Solar System Bodies Questions and Answers Q: What is the origin of the word planet? A: The word planet comes from the Greek word for "wanderer", meaning that planets were originally defined as objects that moved in the night sky with respect to the background of fixed stars. Q: Why is there a need for a new definition for the word planet? A: Modern science provides much more information than the simple fact that objects orbiting the Sun appear to move with respect to the background of fixed stars. For example, recent new discoveries have been made of objects in the outer regions of our Solar System that have sizes comparable with and larger than Pluto. Historically Pluto has been recognised as the ninth planet.

Thus these discoveries have rightfully called into question whether or not the newly found Trans-Neptunian Objects should also be considered as new planets. Q: How did astronomers reach a consensus for a new definition of planet? A: The world's astronomers, under the auspices of the International Astronomical Union, deliberated on a new definition for the word planet for nearly two years. The results of these deliberations were channelled to a Planet Definition Committee and ultimately proposed to the IAU General Assembly. Continued evolution of the definition through debate and further discussion allowed a final consensus and vote. Q: What new terms are used in the official IAU definition? A: There are three new terms adopted as official definitions by the IAU. The terms are: planet, dwarf planet and small Solar System body. Q: In plain language, what is the new definition of planet? A: A planet is an object in orbit around the Sun that is large enough (massive enough) to have its self-gravity pull itself into a round (or near-spherical) shape. In addition a planet orbits in a clear path around the Sun.

If any object ventures near the orbit of a planet, it will either collide with the planet, and thereby be accreted, or be ejected into another orbit. Q: What is the exact wording of the official IAU proposed definition of planet? A: A planet is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit. Q: Does a body have to be perfectly spherical to be called a planet? A: No. For example, the rotation of a body can slightly distort the shape so that it is not perfectly spherical. Earth, for example, has a slightly greater diameter measured at the equator than measured at the poles. Q: Based on this new definition, how many planets are there in our Solar System? A: There are eight planets in our Solar System; Mercury, Venus, Earth,Mars, Jupiter, Saturn, Uranus, Neptune. Mnemonic: *My Very Educated Mother Just Served Us Nachos (Phyllis Lugger, * ). Q: Is that all, only eight planets?

A: No. In addition to the eight planets, there are also five known dwarf planets. Many more dwarf planets are likely to be discovered soon. Q: What is a dwarf planet? A: A dwarf planet is an object in orbit around the Sun that is large enough (massive enough) to have its own gravity pull itself into a round (or nearly round) shape. Generally, a dwarf planet is smaller than Mercury. A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects. Q: How many dwarf planets are there? A: Currently there are five objects accepted as dwarf planets. Ceres, Pluto, Eris, Makemake and Haumea. Q: What is Ceres? A: Ceres is (or now we can say it was) the largest asteroid, about 1000 km across, orbiting in the asteroid belt between Mars and Jupiter. Ceres now qualifies as a dwarf planet because it is now known to be large enough (massive enough) to have self-gravity pulling itself into a nearly round shape.

(Thomas, 2005) Ceres orbits within the asteroid belt and is an example of the case of an object that does not orbit in a clear path. There are many other asteroids that can come close to the orbital path of Ceres. Q: Didn't Ceres used to be called an asteroid or minor planet? A: Historically, Ceres was called a planet when it was first discovered in 1801, orbiting in what is known as the asteroid belt between Mars and Jupiter. In the 19th century astronomers could not resolve the size and shape of Ceres, and because numerous other bodies were discovered in the same region, Ceres lost its planetary status. For more than a century, Ceres has been referred to as an asteroid or minor planet. Q: Why is Pluto now called a dwarf planet? A: Pluto now falls into the dwarf planet category on account of its size and the fact that it resides within a zone of other similarly-sized objects known as the transneptunian region. Q: Is Pluto's satellite Charon a dwarf planet? A: For now, Charon is considered just to be Pluto's satellite.

The idea that Charon might qualify to be called a dwarf planet in its own right may be considered later. Charon may receive consideration because Pluto and Charon are comparable in size and orbit each other, rather than just being a satellite orbiting a planet. Most important for Charon's case as a dwarf planet is that the centre of gravity about which Charon orbits is not inside of the system primary, Pluto. Instead this centre of gravity, called the barycentre, resides in free space between Pluto and Charon. Q: Jupiter and Saturn, for example, have large spherical satellites in orbit around them. Are these large spherical satellites now to be called dwarf planets? A: No. All of the large satellites of Jupiter (for example, Europa) and Saturn (for example, Titan) orbit around a common centre of gravity (called the "barycentre") that is deep inside of their massive planet. Regardless of the large size and shapes of these orbiting bodies, the location of the barycentre inside the massive planet is what defines large orbiting bodies such as Europa, Titan, etc. to be satellites rather than planets.

[Actually, there has been no official recognition that the location of the barycenter is involved with the definition of a satellite.] Q: What was 2003 UB313? A: 2003 UB313 was a provisional name given to a large object discovered in 2003 that resides in an orbit around the Sun beyond Neptune. It is now called Eris and is recognised as a dwarf planet. Q: Why is Eris a dwarf planet? A: Hubble Space Telescope images have resolved the size of Eris showing it to be as large as, or larger than Pluto, Brown (2006).More important, Eris was found to have a satellite, which was later named Dysnomia, after the Greek demon of lawlessness who was Eris' daughter. In 2007, the mass of Eris was determined to be (1.66 ± 0.02)×1022 kg, 27% greater than Pluto, based on observations of the orbit of Dysnomia. Eris also orbits within the transneptunian region - a region that has not been cleared out. Therefore Eris is a dwarf planet.

Q: What is an object called that is too small to be either a planet or dwarf planet? A: All objects that orbit the Sun that are too small (not massive enough) for their own gravity to pull them into a nearly spherical shape are now defined as being small Solar System bodies. This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars and Jupiter Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs) and comets. Q: What is a small Solar System body? A: The term "small Solar System body" is a new IAU definition to encompass all objects orbiting the Sun that are too small (not sufficiently massive) to satisfy the definition of planet or dwarf planet. Q: Is the term minor planet still to be used? A: The term "minor planet" may still be used. But generally the term small Solar System body will be preferred. Q: How will an official decision be reached on whether or not to call a newly discovered object a planet, dwarf planet, or a Solar System body? A: The decision on how to classify newly discovered objects will be made by a review committee within the IAU.

The review process will be an evaluation, based on the best available data, of whether or not the physical properties of the object satisfy the definitions. It is likely that for many objects, several years may be required to gather sufficient data. Q: Are there additional planet candidates currently being considered? A: No. None appear likely in our Solar System. But there are planet discoveries galore around other stars. Q: Are there additional dwarf planet candidates currently being considered? A: Yes. Some of the largest asteroids may be candidates for dwarf planet status and some additional dwarf planet candidates beyond Neptune will soon be considered. Q: When will additional new dwarf planets likely be announced? A: Probably within the next few years. Q: How many more new dwarf planets are there likely to be? A: There may be dozens or perhaps even more than a hundred waiting to be discovered. Q: What are plutoids? A: Plutoids are celestial bodies in orbit around the Sun at a semimajor axis greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium (near-spherical) shape, and that have not cleared the neighbourhood around their orbit.

Satellites of plutoids are not plutoids themselves, even if they are massive enough that their shape is dictated by self-gravity. The two known and named plutoids are Pluto and Eris. It is expected that more plutoids will be named as science progresses and new discoveries are made. ( Read more ) Q: Can a satellite orbiting a plutoid be a plutoid too? A: No, according to the IAU Resolution B5 a dwarf planet can not be a satellite, even if they are massive enough that their shape is dictated by self-gravity. Why Pluto is No Longer a Planet - Universe Today Universe Today Why Pluto is No Longer a Planet Article Updated: 16 Oct , 2016 This article was originally written in 2008, but we created a cool video to go along with it yesterday Let’s find out why Pluto is no longer considered a planet. Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X.

Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next. After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld). The Solar System now had 9 planets. Astronomers weren’t sure about Pluto’s mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across.

Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune. Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun). Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto , that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object. Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a little smaller than Pluto .

And there are several other Kuiper Belt objects in that same classification. Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt. And in 2005, Mike Brown and his team dropped the bombshell. They had discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Since its discovery, astronomers have determined that Eris’ size is approximately 2,600 km (1,600 miles) across. It also has approximately 25% more mass than Pluto. With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter?

Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic. Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale, and a third would drop the number of planets down to 8, and Pluto would be out of the planet club. But, then… what is Pluto? In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarf planet”. Is Pluto a planet? Does it qualify?

For an object to be a planet, it needs to meet these three requirements defined by the IAU: It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet. It needs to have enough gravity to pull itself into a spherical shape – Pluto…check It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet. What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit. Any object that doesn’t meet this 3rd criteria is considered a dwarf planet . And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.

It’s not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that could qualify for planethood status. Then our Solar System would have 9 planets again. Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has sent their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf) planet’s surface. Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We’ll just be able to appreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted. If you’d like more information about Pluto , we did two podcasts on this topic at Astronomy Cast. The first discusses the IAU’s decision , and the second is about Pluto and the Icy Outer Solar System . Check them out. Here is much more info about Pluto , including pictures of Pluto.

References: Dwarf Planets of Our Solar System (Infographic) Dwarf Planets of Our Solar System (Infographic) By Karl Tate, SPACE.com Infographics Artist | November 21, 2012 01:00pm ET MORE Pluto was demoted to dwarf planet status in 2006, joining Eris, Haumea, Makemake and Ceres. Credit: Karl Tate, SPACE.com contributor In 2006 the organization responsible for classifying celestial bodies, the International Astronomical Union, decided that a new class of objects was needed. The solar system's erratic ninth planet, Pluto, was assigned to the new “ dwarf planet ” category along with four other bodies, all tinier than Earth’s moon. Some astronomers expect there may be as many as 50 dwarf planets in the solar system. Eris , the largest dwarf planet, is only slightly bigger than Pluto, at 1,445 miles in diameter (2,326 km).

Discovered in 2003, Eris orbits at an average distance of 68 AU (that is, 68 times the Earth’s distance from the sun) and takes 561.4 Earth years to circle the sun. Eris has the orbit that is most highly inclined of all the dwarf planets, tilted nearly 47 degrees from the plane of the planets’ orbits. A day on Eris takes 25.9 hours. Eris has one moon, Dysnomia. Pluto , discovered in 1930, orbits the sun at an average of 39.5 times the Earth’s distance. Its diameter is 1,430 miles (2,302 km). Pluto takes 247.9 Earth years to orbit the sun, and its day is 6.39 times as long as Earth’s. Pluto has five known moons: Charon, Nix, Hydra and two that were recently discovered and have not yet been named. Haumea was discovered in 2003. This dwarf planet has an extremely elongated shape, with its longest dimension being about 1,218 miles long (1,960 km).

Haumea rotates very rapidly and has the shortest day of all the dwarf planets, only 3.9 hours. Orbiting 43.1 times farther from the sun than Earth does, Haumea takes nearly 282 Earth years to complete one orbit. Haumea has two moons, Hi’iaka and Namaka. Makemake , discovered in 2005, has no known moons. Makemake orbits at 45.3 times Earth’s distance and takes more than 305 years to complete a circuit of the sun. Its day is 22.5 hours. Makemake’s average diameter is 882 miles (1,420 km). Ceres , first spotted by astronomers in 1801, was first called a planet and later an asteroid. In 2006 it was reclassified as a dwarf planet. Ceres is the closest dwarf planet to Earth, orbiting at only 2.8 times Earth’s distance from the sun. Its year takes 4.6 Earth years and its day is 9.1 hours. Ceres has no known moons.

Five Years Later Artist's impression of Pluto and Charon as seen from one of Pluto's other moons. Credit: David Aguilar/Center for Astrophysics Five years ago today, the solar system lost a planet. On Aug. 24, 2006, Pluto — which had been known as the ninth planet since its 1930 discovery — was demoted to the newly created category of "dwarf planet." The decision was controversial, rankling some scientists who disagreed with the reasoning behind it. It also upset and confused many laypeople, who had regarded the nine planets as permanent fixtures in the sky — key touchstones for their understanding of the cosmos, and their place in it. But Pluto's reclassification shows that our knowledge of the world around us is always changing, that scientific truths aren't handed down from on high. And that reminder may be the greatest legacy of the longstanding debate about Pluto's planet status . "This debate shows people, especially kids, that science is always evolving, and it's exciting," said planetary scientist Scott Sheppard of the Carnegie Institution of Washington, who hunts for faraway dwarf planets.

"And you should get involved in science, because there's a lot more to learn out there." [ Photos of Pluto and Its Moons ] Pluto is not alone When Pluto was discovered in 1930, it was viewed as an oddball, and that perception stuck for the next six decades. Pluto, after all, is much smaller than the eight "traditional" planets, and its highly elliptical orbit is considerably out of whack with theirs. Pluto is also much farther away, zooming around the sun at an average distance of 3.65 billion miles (5.87 billion kilometers). [ Infographic: Pluto, A Dwarf Planet Oddity ] But in the 1990s, astronomers started realizing that Pluto isn't such an oddball after all. They began finding many other large objects in Pluto's domain, the ring of icy bodies beyond Neptune known as the Kuiper Belt . The new discoveries prompted some scientists to re-examine their basic understanding of the solar system's structure, including the way they'd classified its large objects. Just what should we call these icy, faraway bodies? If Pluto is a planet, then should they all be planets, too?

This rethink really got a jump-start in 2005, when Caltech astronomer Mike Brown announced the discovery of Eris, a Kuiper Belt object that seemed to be even larger than Pluto. And shortly thereafter, the International Astronomical Union (IAU) stepped in. Artist's rendering of Eris, announced in July 2005 by Mike Brown of Caltech. It is more massive than Pluto. The sun is in the background. Credit: NASA/JPL/Caltech Pluto's demotion In 2006, the IAU came up with the following official 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 the Kuiper Belt with lots of other big objects, it failed to meet the third criterion and didn't make the cut.

Instead, the IAU rebranded Pluto and Eris as "dwarf planets." [ Meet the Solar System's Dwarf Planets ] Some planetary scientists generally agreed with the new classification, saying that Pluto, Eris and other big Kuiper Belt bodies don't belong in the same category as the eight "original" planets. The faraway icy objects, they say, are just too different — in their sizes, compositions, orbits and evolutionary histories. "It's classification that matters. Classification is the first step toward understanding," Brown told SPACE.com. "If you mess up the most basic classification of the whole solar system by not noticing the difference between the eight large planets and everything else — if you miss that, you've kind of missed the solar system." Further, the word "planet" is so fundamental, and so central to any attempt to teach the public about the solar system, that defining it properly is crucial, Brown added. "If you said that there are eight planets and then a kajillion dwarf planets, that's a pretty good description of what the solar system's actually like," Brown said. "If you said, 'Wow, we've got this planetary system and there are 20,000 planets.

Oh yeah, and eight of them are significantly bigger than all the rest of them' — it's not a very useful classification." Dissent and disagreement Other scientists, however, aren't at all happy with the IAU's planet definition, calling it flawed and unscientific. "I think the IAU really embarrassed themselves with this," said Alan Stern of the Southwest Research Institute in Boulder, Colo. Stern leads NASA's New Horizons mission , which is sending a spacecraft to study Pluto up close. "They created a problem for themselves and for astronomy. It [the definition] created an unworkable algorithm for deciding what's a planet and what's not." [ Cosmic Definitions: What Is a Planet? ] Stern particularly objects to the "clearing your neighborhood" criterion. "In no other branch of science am I familiar with something that absurd," Stern told SPACE.com. "A river is a river, independent of whether there are other rivers nearby. In science, we call things what they are based on their attributes, not what they're next to." Further, Stern said, the criterion sets different standards for planethood at different distances from the sun. That's because the farther away a planet is from the sun, the bigger it needs to be to clear its zone.

If Earth circled the sun in Pluto's orbit , for example, it wouldn't qualify for planethood in the IAU's eyes. "I would say any definition that produces a result where Earth is not a planet under any circumstance is immediately indicted as ridiculous, because one thing we all agree is a planet is planet Earth," Stern said. Stern doesn't have a problem with the term "dwarf planet." He just thinks dwarfs should be deemed "true" planets like the terrestrial planets and gas giants. Excluding the dwarfs from the official list amounts to an effort to keep the number of planets down to a manageable size, Stern added. "That's not a very scientific way of going about it, since we have countless numbers of stars, galaxies, asteroids and everything else," Stern said. Images of the dwarf planet Pluto taken by NASA's Hubble Space Telescope. Credit: NASA, ESA, and M. Buie (Southwest Research Institute) A better understanding of the solar system While scientists such as Brown and Stern may differ on the question of Pluto's planethood, they agree that the last two decades have brought about a sea change in our understanding of the solar system.

For example, we now know that the solar system's frigid outer reaches are packed full of big, interesting objects. The first big Kuiper Belt body other than Pluto was discovered just in 1992, but Brown said up to 2,000 or so dwarf planets might actually exist out there. And many more big objects may lurk even farther away. Right now, astronomers know of one mysterious probable dwarf planet, called Sedna, from the region beyond the Kuiper Belt . But that distant zone — which is very difficult to survey with today's instruments — may harbor up to 20,000 dwarf planets, Brown said. So Pluto is now known to be just one of many big, icy and quite diverse bodies orbiting the sun from far away. "We have gone from nearly complete ignorance about these sorts of bodies to an incredible richness, and trying to understand how they function as little worlds of their own," Brown said. Stern voiced similar sentiments, stressing that recent finds are showing planets — dwarf and otherwise, in our solar system and beyond — are fantastically diverse. "That's what we're finding in planetary science, as a result of not just the Kuiper Belt but also discoveries about extrasolar planets," Stern said.

"There are planets that have densities like balsa wood, and planets that are darker than coal, and hot Jupiters and super-Earths . I suspect we're just scratching the surface of the diversity — both in the Kuiper Belt and around other stars." The debate over Pluto's planethood is a result of this vast increase in knowledge, this revolution in how we view our cosmic neighborhood. And it serves as a reminder that science is not an ironclad set of facts but rather a process, a way to learn as much as possible about ourselves and our surroundings. "It shows that we're constantly expanding our knowledge of the solar system and our place in the universe," Sheppard told SPACE.com. "Science is always evolving, and we're always discovering new things." You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall . Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook . Editor's Recommendations Did our sun steal 'Planet 9' from another star? | Fox News Did our sun steal 'Planet 9' from another star?

Facebook 0 Twitter 0 livefyre Email Print (Lund University) There are eight planets in our solar system, and have been officially ever since Pluto was reclassified as a dwarf planet in 2006. But what if there was a ninth planet, billions of miles past Neptune? Earlier this year, researchers from CalTech announced that they had found signs of the planet, which is referred to a “Planet 9,” through modeling and computer simulations. If a ninth planet were out there, it would be a big one — ten times the mass of Earth— and very, very far away, completing just one orbit around the sun as slowly as perhaps every 10,000 to 20,000 years. Related: Scientists may have just found a ninth planet and it's massive Now, scientists from Lund University in Sweden have used computer simulations to propose a new theory about how Planet 9— if it exists— came to be a member of the solar system. They propose that it was stolen by our sun from another star about 4.5 billion years ago.

“What we were arguing was that you could create this [Planet 9] around another star, and then the sun could capture it, in a close encounter,” Alexander Mustill, a researcher in the department of astronomy and theoretical physics at Lund University, explained in a video about the theory. Related: NASA identifies 1,284 new exoplanets, most ever announced at once “We argue that this is how you could put this planet on a wide orbit around the sun,” he added. “You first create it around another star, and then the sun captures it.” The researchers argue that this would make this planet an exoplanet, which is the term scientists use to describe planets in other star systems beyond our own. Just last month, NASA announced that they had added over 1,200 new exoplanets to the official roster, all of them discoveries from the Kepler spacecraft that had been validated through a new statistical method. Related: Planet discovery fuels interest in mythical world of deep space “It’s very exciting to this that there might be an extrasolar planet in our own solar system,” Mustill said. The study proposing the new theory about Planet 9 was published online in the Monthly Notices of the Royal Astronomical Society in April.

The Astrology of Pluto The Astrology of Pluto Discovered in 1930, and originally classified as the ninth planet in our solar system, in 2006 the planet Pluto was re-classified as a dwarf planet. In spite of its re-classification, Pluto’s astrological importance is not thought to be diminished, especially when considered as a representative of the overall influence of the celestial bodies in the Kuiper belt (see Pluto Facts). Pluto was the Roman god of the underworld, and was also associated with wealth, specifically the wealth that the earth provided. A son of Saturn , and the brother of Jupiter and Neptune , Pluto’s Greek counterpart was Hades. His wife was Proserpina (Greek Persephone). Pluto’s influence is characterized not only by the rulership of its namesake deity, but also by the gods Bacchus and Vulcan. Positive Influences: Pluto Definition and Meaning plutonium PLZT PMOS pn pn junction Pluto noun the smallest and most distant planet of the solar system , reclassified as a dwarf planet in 2006 A dwarf planet, discovered by Tombaugh in 1930.

It was originally thought to be the ninth planet of the solar system , but in 2006 it was officially re-classified as a dwarf planet and part of the Kuiper Belt. Pluto’s exact mass is a matter of debate, although it seems to be about 0.25 per cent that of the Earth. It has not visited by space probes, so our knowledge of it is limited. Pluto’s largest moon Charon was discovered in 1978 and orbits 19,000km from Pluto. It is the largest satellite in the solar system relative to the body it orbits, being about 1300km across to Pluto’s 2300km. Observations of the interaction between Pluto and Charon have allowed our information on the dwarf planet to be enhanced. It has a density of about 2 gm/cc, which is consistent with a mix of rock and ice. Solid nitrogen appears to dominate its solid surface. Pluto’s other moons Nix and Hydra were discovered by the Hubble Space Telescope in 2005.

Pluto’s orbit has an eccentricity of 0.25, so that it spends part of its time nearer to the Sun than Neptune, including the period from 1979 to 1999. Pluto’s mean distance from the Sun is 39AU, and it takes 248 years to orbit the Sun. Pluto’s minute size and its strange orbit formerly encouraged the idea that it is an escaped satellite of Neptune. Some satellites, including Neptune’s satellite Triton, are bigger than Pluto. A Hubble Space Telescope view in 1996 finally revealed that Pluto, which rotates on its axis in 6.4 days, has a surface with dark and bright spots and linear markings, whose nature is at present unknown. A ‘Pluto Express’ plan, for a mission which could visit Pluto after a 10–12 year spaceflight, was drawn up by US and Russian scientists but cancelled for budgetary reasons. Not what you were looking for? © 2015 Dictionary of Engineering All Rights Reserved. Unauthorized duplication, in whole or in part, is strictly prohibited. Why is Pluto no longer a planet?

(Everyday Mysteries: Fun Science Facts from the Library of Congress) Why is Pluto no longer a planet? The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet. Essentially Pluto meets all the criteria except one- it “has not cleared its neighboring region of other objects.” In August 2006 the International Astronomical Union (IAU) downgraded the status of Pluto to that of "dwarf planet." This means that from now on only the rocky worlds of the inner Solar System and the gas giants of the outer system will be designated as planets. The “inner Solar System” is the region of space that is smaller than the radius of Jupiter’s orbit around the sun. It contains the asteroid belt as well as the terrestrial planets, Mercury, Venus, Earth, and Mars. The “gas giants” of course are Jupiter, Saturn, Neptune, and Uranus. So now we have eight planets instead of the nine we used to have. What is a Dwarf Planet?

A “dwarf planet,” as defined by the IAU, is a celestial body in direct orbit of the Sun that is massive enough that its shape is controlled by gravitational forces rather than mechanical forces (and is thus ellipsoid in shape), but has not cleared its neighboring region of other objects. So, the three criteria of the IAU for a full-sized planet are: It is in orbit around the Sun. It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape). It has "cleared the neighborhood" around its orbit. Pluto meets only two of these criteria, losing out on the third. In all the billions of years it has lived there, it has not managed to clear its neighborhood. You may wonder what that means, “not clearing its neighboring region of other objects?” Sounds like a minesweeper in space! This means that the planet has become gravitationally dominant -- there are no other bodies of comparable size other than its own satellites or those otherwise under its gravitational influence, in its vicinity in space. So any large body that does not meet these criteria is now classed as a “dwarf planet,” and that includes Pluto, which shares its orbital neighborhood with Kuiper belt objects such as the plutinos.

History of Pluto The object formerly known as the planet Pluto was discovered on February 18, 1930 at the Lowell Observatory in Flagstaff, Arizona, by astronomer Clyde W. Tombaugh, with contributions from William H. Pickering. This period in astronomy was one of intense planet hunting, and Pickering was a prolific planet predictor. In 1906, Percival Lowell, a wealthy Bostonian who had founded the Lowell Observatory in Flagstaff, Arizona in 1894, started an extensive project in search of a possible ninth planet, which he termed "Planet X." By 1909, Lowell and Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted the search until his death in 1916, to no avail. Unknown to Lowell, on March 19, 1915, his observatory had captured two faint images of Pluto, but they were not recognized for what they were. Lowell was not the first to unknowingly photograph Pluto. There are sixteen known pre-discoveries, with the oldest being made by the Yerkes Observatory on August 20, 1909.

The search for Planet X did not resume until 1929, when the job was handed to Clyde Tombaugh, a 23-year-old Kansan who had just arrived at the Lowell Observatory. Tombaugh's task was to systematically image the night sky in pairs of photographs taken two weeks apart, then examine each pair and determine whether any objects had shifted position. Using a machine called a blink comparator, he rapidly shifted back and forth between views of each of the plates to create the illusion of movement of any objects that had changed position or appearance between photographs. On February 18, 1930, after nearly a year of searching, Tombaugh discovered a possible moving object on photographic plates taken on January 23 and January 29 of that year. After the observatory obtained further confirmatory photographs, news of the discovery was telegraphed to the Harvard College Observatory on March 13, 1930. The discovery made headlines across the globe.

The Lowell Observatory, which had the right to name the new object, received over 1,000 suggestions from all over the world; the name Pluto was proposed by Venetia Burney, an eleven-year-old schoolgirl in Oxford, England. Venetia was interested in classical mythology as well as astronomy, and considered the name for the god of the underworld appropriate for such a presumably dark and cold world. She suggested it in a conversation with her grandfather Falconer Madan, a former librarian at the University of Oxford's Bodleian Library. Madan passed the name to Professor Herbert Hall Turner, who then cabled it to colleagues in the United States. Pluto officially became Pluto on March 24, 1930. The name was announced on May 1, 1930, and Venetia received five pounds (£5) as a reward. Related Web Sites International Astronomical Union (IAU): Pluto and the Developing Landscape of our Solar System - A discussion about Pluto from IAU that includes a history, references to how a planet is defined and a link to the report on the final resolution. Also included are questions and answers about Planets, Dwarf Planets, and Small Solar System Bodies.

The Girl Who Named a Planet - This is an article about Venetia (Burney) Phair, the girl who named the planet Pluto. NOVA: The Pluto Files - Watch the PBS program which features Neil deGrasse Tyson exploring the rise and fall of America’s favorite planet. Solar System Exploration: Pluto - NASA provides an abundance of information about Pluto such as facts, images, headline news, and a video. Further Reading Boyle, Alan. The case for Pluto: how a little planet made a big difference. Hoboken, N.J, John Wiley & Sons, c2010. 258 p. Doressoundiram, Alain, and Emmanuel Lellouch. At the edge of the solar system: icy new worlds unveiled. [Translator, Bob Mizon]. Berlin, New York, Springer Verlag; Chichester, U.K,: Published in association with Praxis, c2010. 205 p. Original French edition: Aux confin de système solaire Hoyt, William Graves. W. H. Pickering's planetary predictions and the discovery of Pluto. Isis, v. 67, Dec.1976: 551-564.

Pickering, W.H. Trans-Neptunian Planet. Popular astronomy, v. 38, June-July1930: 341-344. Putnam, Roger Lowell, and V.M. Slipher. Searching out Pluto, Lowell’s trans-Neptunian planet X. Scientific monthly, v. 34, Jan. 1932: 5-21. Tyson, Neil deGrasse. The Pluto files: the rise and fall of America’s favorite planet. New York, W.W. Norton, c2009. 194 p. For more print resources... Search on "Kuiper Belt" "Pluto (dwarf planet)," "Planets," or "Solar System" in the Library of Congress Online Catalog .

Question: Which original ninth planet of our solar system was re-classified as a dwarf planet in 2006?
Answer: PLUTO