Uranus, Neptune and Pluto Uranus, Neptune and Pluto How are Uranus and Neptune different from Jupiter and Saturn? How are they similar? Who discovered Uranus? What are the satellites of Uranus and Neptune like? What are Pluto's characteristics? What are the other objects found beyond Neptune like? Sir William Herschel (1738-1822). Image ï¿½ Copyright Royal Astronomical Society. The first of the planets to be "discovered" was named after the muse of Astronomy (Urania). Uranus is also the name of Saturn's father. Uranus was discovered by William Herschel in 1781, though it had been mistakenly mapped in the past as a star. It is possible under the correct conditions to see Uranus with your just eye (no telescope), though it is pretty difficult to do, and you would need to have good eyesight and know exactly where to look to see it. Of course, you have to pronounce the name correctly. If you say something that sounds like "yer-anus," I don't think you'll make too many friends that way, unless they are proctologists.

You should instead say the name as "yer-ah-nis" - not much of an improvement, but it may help. Uranus is so far away that we can't get a great deal of information on it from this distance. A few things we can see are its tilt and rotation rate. Uranus is pretty much tipped over on its side. You know that the Earth is tilted over 23 1/2 degrees; Uranus is tilted over 98 degrees! This means it is on its side (since being 90 degrees would be exactly on its side). Due to this the seasons on Uranus are pretty extreme, especially if you are located at one of the poles. It takes Uranus about 84 years to go around the Sun, so the seasons last for about 21 years. If you were located at one of the poles, you would have 21 years of constant daylight during summer and 21 years of continual darkness during the winter. The temperatures would be pretty extreme. Not only is the planet tilted over, but the rings and satellites are also in tilted orbits about Uranus.

It is likely that early in its history the planet was knocked over by an impact and as the system formed (as the satellites and rings formed), they also ended up in tilted orientations. Figure 1. The orbit and tilt of Uranus are shown. Uranus is tilted pretty much on its side, so there are times when only certain parts of it will get any sunlight. This is especially true for the polar regions. There are several very thin rings around Uranus. The rings were discovered from the Earth by accident in 1977. Astronomers were trying to study the atmosphere of the planet by watching how the light of a star would change (get absorbed by the atmosphere) as the planet moved in front of it. Before the star was anywhere close to the planet, the star's brightness faded several times. This happened again when the star was located on the other side of the planet. Now it is possible that the star could have been eclipsed by some moons, but that would have required some very precise alignments of the moons with the distant star. Rings were the cause for the dimming of the starlight!

The rings are rather similar to Jupiter's ring in that they are made of darker material as well as small microscopic particles - stuff like dust. Uranus actually has 11 main rings, far fewer than Saturn. Figure 2. The dimming of a star's light as it approached Uranus indicated that there are rings around Uranus. Viewing Uranus from the Earth doesn't provide much information, so let's send a spacecraft out there. Actually, we have - the only spacecraft to visit Uranus was Voyager 2 in January of 1986. The other Voyager spacecraft did not visit any other planets after its encounter with Saturn, while Voyager 2 continued on to Uranus and Neptune. All of the up close images we have of them are from that one spacecraft. What did it see? When Voyager 2 flew by Uranus, there were no cloud features (spots) as had been seen on Jupiter and Saturn. In fact, there are no markings at all; the atmosphere of Uranus appeared to be very dull. This was really surprising, since at the time that the Voyager spacecraft flew by Uranus, one of the poles was pointed toward the Sun.

Astronomers thought that this would have caused the atmosphere to heat up a great deal and therefore produce a bunch of storms. It was a bit confusing when all the Voyager images were so dull. When we look at Uranus today with the Hubble Space telescope, there are cloud features visible. The atmosphere appears to change as it orbits the Sun. Figure 3. The view of Uranus as seen by the Hubble Space Telescope. This differs from the Voyager view from 1986, which showed no cloud features. The clouds seen here as the pinkish features have been observed since the mid-1990s. Image credits: Erich Karkoschka (University of Arizona) and NASA. What is the inside of Uranus like? (You'll want to make sure you pronounce Uranus correctly if you ever ask anyone that question.) The discussions of Jupiter and Saturn showed how similar those two objects are to one another. The same is true for Uranus and Neptune, so most of the stuff I say here about Uranus also applies to Neptune. From the observations of the Voyager spacecraft we know that Uranus is similar to Jupiter and Saturn in some respects, though there are some drastic differences.

First of all, while Saturn and Jupiter are 100s of times more massive than the Earth, Uranus (and Neptune) are only about 15 times more massive than the Earth and their radii are only about 5 times greater than the Earth's. Their internal structures are also different. Just check out Figure 4. Figure 4. The internal structure of Uranus is shown. The main difference between Uranus (and Neptune) and Jupiter (and Saturn) is the large liquid water ocean that exists under the upper atmosphere. Uranus is rather cool; unlike Jupiter and Saturn, there is not any excess heat coming from its interior. Remember, Jupiter and Saturn are both hot because they haven't cooled off. Uranus, being much smaller, has had the time to cool down completely. When you look at the top of the atmosphere of Uranus, it has a rather nice blue-green color. This is due to methane in the upper atmosphere. While this is only a small part of the overall composition of these layers, it is enough to provide the color.

It is sort of interesting that most of the inside of Uranus is not made up of just regular hydrogen but is comprised of hydrogen compounds (remember, water, methane and ammonia all have hydrogen in them). Is there a magnetic field? Yes, but it is very strange. The magnetic axis does not go through the center of the planet and it is not located anywhere near the rotation axis. It is both off center and off axis. This produces a rather unusual wobble in the magnetic field as the planet rotates once every 16 hours. In terms of strength, the field is about 50 times stronger than the Earth's. Check out this animation to see what is happening. Figure 5. The alignments of the Earth's and the jovian planets' magnetic fields are shown. The top row shows each planet with no axis tilt - so that their rotation poles are all oriented the same. The bottom row shows each planet with its normal tilt. The magnetic axes are indicated by the red lines, while the rotation axes are shown with green lines. There is only about a 10 degree difference between these two axes for the Earth and Jupiter, while Saturn's axes are completely in line.

Both Uranus and Neptune have a huge difference in the alignments of their magnetic and rotation axes. Also, note how the "magnet" doesn't go through the center of Uranus or Neptune but is off to the side. Of course, there really are no giant magnets in the cores of planets, this just shows the alignment of the magnetic fields with the rotation axis. What produces the magnetic fields of Uranus and Neptune? Unlike the terrestrial planets, there are not large amounts of iron and metal, and unlike Jupiter and Saturn there aren't layers of liquid metallic hydrogen. Take a look at the image showing the interior of Uranus - what material in there could conduct electricity? If you know that you aren't supposed to have electrical appliances near water, then you should guess that the water would be the most likely one to produce the magnetic field. It appears that the magnetic field is produced by electrically charged water in the huge water layer. The high pressures on the water cause it to be electrically charged. Satellites - Like other Jovian planets, most of the satellites around Uranus fall into the puny category (less than 300 km in diameter). They are, for the most part, dark, small, icy worlds.

The ices seen on their surfaces are in various forms including water, ammonia, and methane ice. As has been seen on other satellites, they have quite a few craters and there are some ice flow features. Voyager flew by when one of the poles of Uranus was pointed toward the Sun, so the satellites where also in that same orientation. Therefore we have pictures of only 1/2 the surfaces of these worlds. Five satellites were known of before the Voyager 2 probe flew by. These were Miranda, Ariel , Umbriel , Titania , and Oberon . The Voyager 2 spacecraft discovered 10 more, and they were named Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Puck, and Belinda. In case you haven't figured it out, the moons are named after characters from William Shakespeare and Alexander Pope. Twelve more moons have been discovered since the Voyager mission bringing the total to 27. It's a good thing that Shakespeare and Pope had a lot of characters or else we'd run out of names. Figure 6. The surface of Miranda. There is always one satellite that is unusual.

In the case of Uranus, the unusual satellite is Miranda. While this is not a very large object, only about 470 km wide, it does show some very unusual surface features. These include some large circular patterns on its surface and abrupt color variations. There is also a very large cliff visible (Verona Chasm, 5 km high). Why is it so screwy? It is possible that after the moon formed, it was broken up and then reformed in a jumbled mess. It is also possible that there is a great deal of tidal heating and this may have rearranged the features on Miranda, or at least allowed them to be a bit mobile. It should be noted that Miranda is the closest of the major moons to Uranus, so it would have experienced the most tidal heating. Several of the other moons are a bit mysterious. Ariel and Umbriel both appear to be the same size, yet Ariel's surface shows evidence of ice volcanism and tectonics, while Umbriel's surface is just full of craters. Titania and Oberon also appear to be very similar in size, and Titania appears to have had much more activity on its surface.

It was thought that worlds of similar size should have similar natures, yet this small sample of worlds shows quite drastic differences - just more of those annoying mysteries we have yet to solve. Neptune Neptune Symbol The last of the jovian planets was named after the god of the oceans. It was discovered in 1846 by people using mathematics. What? You mean math is actually good for something? Of course it is. Any ways, here's what happened - once Uranus was discovered, people started to observe it and plot its orbit. They noticed that its motion wasn't exactly correct; that it deviated slightly from Kepler's and Newton's laws of how things move. This would happen if there was another object out there pulling on it. This led John Adams, an English astronomer, to calculate where a planet should be based upon the unusual motions of Uranus. A little bit later, Urbain Le Verrier, a French astronomer, did pretty much the same calculations, and his results were published (Adams's calculations were pretty much ignored). The published results of Le Verrier were used by Johann Galle, a German astronomer, who used the information from Le Verrier to search for the planet. He found it (quite easily)!

Who actually discovered Neptune? Credit for the discovery of the planet is given to both Adams and Le Verrier, since without their work Galle would have never seen it. Left to right, John Couch Adams (1819-1892), Urbain Jean Joseph Le Verrier (1811-1877) and Johann Gottfried Galle (1812-1910). Adams and Le Verrier are credited with the discovery of Neptune. It is also possible that Galileo saw it hundreds of years before Galle, since some of his diagrams indicate an object where Neptune would have been at the location he was observing. He would have probably mistaken it for a star. The only spacecraft to go by Neptune was Voyager 2, which flew by in August 1989. Neptune is quite a long ways out there and the fact that the spacecraft lasted long enough to get there was quite amazing (it was launched in 1977). Also, the path that it was on as it flew by Neptune was very precise.

If you were to scale the flight path down to a golf putt, the accuracy of the path was the equivalent of sinking a putt from 3630 km (2260 miles) away. That's accurate! The images from the Voyager 2 spacecraft look a little strange, in part because the spacecraft is traveling so fast and it has to take fairly long exposures, since there is not a lot of sunlight out at that distance. Now the spacecraft is on its way to the outer parts of our solar system, which it won't get out of for a few thousand more years (even though it is traveling at about 38,500 mph) While Neptune is pretty similar to Uranus in terms of its composition, structure and density, there are some subtle differences. One area where it is quite different is in the atmospheric features observed by Voyager 2. The Great Dark Spot was observed on Neptune. This is a storm like feature about the size of the Earth. It appears to have been similar in many respects to the Great Red Spot on Jupiter (proportionally about the same size).

Current observations by the Hubble Space Telescope indicate that the Great Dark Spot is no longer present (I guess it wasn't so great) and other spots have appeared. It appears that Neptune's atmosphere is much more active compared to Uranus's. Figure 7. To the left is an image of the Great Dark Spot as photographed by the Voyager 2 spacecraft in 1989. On the right is an image of Neptune obtained by the Hubble Space Telescope in 1996. The Great Dark Spot is no longer visible, but other spots appear that weren't seen by Voyager. Images from NASA and Lawrence Sromovsky (University of Wisconsin-Madison). The Voyager images also showed other higher altitude clouds as well as smaller spots. Many of these can't be seen from the Earth with the Hubble Space Telescope, but large changes in the atmosphere of Neptune are visible. As with Uranus, the color of Neptune is due to methane in its atmosphere, even though the main components of the atmosphere are hydrogen and helium. As mentioned previously, it appears that the internal structure is much like that of Uranus, and the magnetic field is also like that of Uranus (tilted 47 degrees from the rotation axis and off center).

The strength is a bit weaker than Uranus's, though, only about 35x the Earth's magnetic field strength. You have to remember that Neptune itself is not tilted over like Uranus. A rather major difference between Uranus and Neptune is that Neptune has an internal heat source (Uranus doesn't). This heat may be left over from the planet's formation, but that seems unlikely considering that Neptune is not that large (like Jupiter and Saturn). We are still not quite sure why there is still so much heat in Neptune and none in Uranus, but the heat in Neptune does help to explain the greater activity in its atmosphere, since heat helps to produce turbulence in the atmosphere, which leads to the formation of storms and spots. One rather surprising feature of Neptune's atmosphere was the high wind speed observed - it can go up to 2000 km/h (1200 mph). Like the other jovian planets, Neptune has rings. This wasn't known for certain until Voyager 2 went out there. Neptune isn't tilted like Uranus, so the rings could not be seen in the same way they were around Uranus.

However, when astronomers did try to find rings using the variation of star light around Neptune, they tended to get conflicting results. Sometimes it looked rings were there; other times it looked as if there were no rings. This inconsistent result is due to the presence of not only rings but also ring arcs, which are thicker parts of rings. The ring arcs would block light very well, so they would at times give the indication that there were rings, but since they didn't extend all of the way around, the expected ring effects (dimming of background stars) weren't seen everywhere. Like the rings of Uranus, the rings of Neptune are made up of dark material. Figure 8. The rings of Neptune are seen above in a very long exposure image. The rings are made up of dark material, so it is easier to see them in over exposed pictures - but you have to remove Neptune from the pictures. The image on the right shows an up close view with the ring arcs clearly visible. Image from NASA. Neptune has the fewest number of satellites amongst the jovian planets. Before the Voyager 2 spacecraft visited, we knew of two satellites Triton, and Nereid.

Voyager 2 discovered six new ones, which are named Naiad, Galatea, Thalassa, Larissa, Proteus, and Despina. Since then, five more were discovered but have yet to be named. Either way, this gives us a grand total of 13 satellites. The theme here is to name them after water deities (which fits in well with the whole ocean god thing). As usual, most are pretty dull - just small, icy worlds with lots of craters. As usual, there is an exception - one strange world. Figure 9. Triton, the largest of Neptune's satellites, is shown (left). Note the strange color of its surface features. An up close view of the surface is seen on the right. Note the dark streaky parts that are thought to be due to ice geysers. Image from NASA. The largest of the Neptunian satellites is Triton . Not only is it large, but it doesn't act normally. It is in a rather bizarre orbit, since it actually goes clockwise (rather than counterclockwise) around Neptune, which is usually only seen in satellites that are small, puny and thought to be captured.

To have a large satellite traveling in the wrong direction is a bit of a mystery. The really freaky thing about Triton, though, is its surface. There are not very many large craters and there does appear to be evidence of resurfacing. One of the more interesting items observed by Voyager 2 were cracks and streaks in the ice, possibly produced by ice volcanoes - ice volcanoes?! It seems that there are active volcanoes (or perhaps we should say geysers) that are erupting mainly nitrogen and methane gas and also methane ice onto the surface. The very thin atmosphere of Triton causes these particles to drift down wind and leave long, streaky marks on the surface. Triton's thin atmosphere is composed mainly of nitrogen and methane. Triton also has the coldest surface temperature yet measured on a solar system object, only 37 K (-393 degrees F). What is Triton's problem? Why does it have activity? What do you need to have such action? HEAT! For the most part, little worlds shouldn't be hot unless something is heating them. It is thought that the heating could be due to the tidal forces exerted by Neptune.

Triton's orbit is not only backwards but also rather elliptical. This means that it gets pretty close to Neptune and gets pulled pretty strongly by Neptune's tidal forces. This can cause some major internal problems in poor little Triton. Of course, when it gets far from Neptune, the heating is diminished so that it freezes up its surface again until the next heating episode. As with many things, this is just a theory. Beyond Neptune In 1930 an object was discovered beyond Neptune and was named Pluto. In 1951, Gerard Kuiper proposed that there was a region beyond Neptune where objects that could become comets may originate from (we'll get to comets in the next set of notes, so don't get all excited just yet). In 1992 the first object to fit Kuiper's theory was found in what is now called the Kuiper Belt. Some could argue that Pluto was the first Kuiper belt object, but initial findings found that the objects discovered there were much smaller than Pluto, so Pluto was still considered a planet. To date, about 1000 of these objects (sometimes also refered to as Trans-Neptunian Objects) have been found.

It is thought that there may be millions of these objects out there, but most are pretty small. The Kuiper Belt is thought to extend from about 30 AU (the distance of Neptune) out to several hundred AUs. Most KBOs found so far are within the 30-40 AU range, and appear to be rather similar to the satellites of the outer solar system. The orbits of KBOs is not at circular as that of the planets and the orbits tend to be a bit more tilted relative to the ecliptic. Clyde Tombaugh (1906-1997) with the telescope he built. This was before he was hired to look for a planet beyond Neptune. There are some very famous KBOs, and of course the most well known one is Pluto, which was named after the god of the Underworld. Pluto is so far from the Sun that it takes about 249 years to complete one orbit. It was discovered in 1930 by Clyde Tombaugh. At the time people thought that Neptune's orbit was being altered by another planet and Clyde Tombaugh was hired by Lowell observatory to search for the unseen planet.

As you'll see, Pluto is too small to alter Neptune's orbit, and the unusual motions have been explained without the need for another planet. Pluto is so far from us that it wasn't until 1978 that its largest satellite was discovered. The satellite is named after the ferryman of the underworld, Charon. And over the years four more satellites were discovered. For the story of Pluto's discovery, follow this link . Not much was known about Pluto due to its great distance. But that all changed in 2015 when the New Horizons spacecraft successfully flew past Pluto and obtained our first high resolution images of this small world and its moons. This mission provided images of not only Pluto but also Charon. The final results of the fly-by have yet to be released, but a few things were obvious - Pluto appears to have ice flow features. Other areas, such as large "heart" shaped region seem to indicate that the surface is geologically active (remember, few crater indicates a young surface). There are large icy mountains on Pluto, similar in elevation to the Rocky Mountains, which is rather unexpected given the small size of the world. The atmosphere of Pluto was measured, and it is rather surprising that it even has an atmosphere.

Pluto's low mass prevents it from being able to hold on to the atmosphere very effectively, since the solar winds are able to deplete it. It is not yet known how the atmosphere remains at the level it is at currently. New Horizons was able to detect the atmosphere at distances of up to 1600 km above the surface. The atmosphere's composition includes nitrogen, methane, and other hydrocarbons like ethylene and acetylene. It is likely that these gases will freeze at the surface and become part of the icy layers. Charon has some very extreme geological features, which is surprising given the fact that it is even smaller than Pluto. The size and mass of Pluto was measured very accurately, which may make it the largest known object beyond Neptune (since the size of Eris isn't as well measured), but that may still be debatable. Pluto is actually rather "red" in appearance, looking very similar in color to Mars, which may surprise many people. This is due to the presence of various hydrocarbons on the surface. Pluto in true-color (click for a larger version. Charon in true-color (click for a larger version.

Images from NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Prior to this flyby, we knew that the Plutonian system is tilted over like the Uranian system - Pluto is on its side and its satellites are in a tilted orbit. This is actually a good thing, since not too long ago Pluto and Charon were orientated in such a way that they would eclipse each other. Why is that important? The sizes (radii) of the objects can be learned from eclipses, and using Kepler's laws, we can get their masses. This information can give clues as to the densities and likely compositions of not only Pluto and Charon, but also of other Kuiper Belt Objects. We know that Pluto and Charon are very similar in terms of their sizes. Pluto is only eight times the mass of Charon (for comparison, the Earth is 80 times the mass of the Moon). Pluto is also only about two times wider than Charon. Some people have referred to this system as a binary system, since the two objects are relatively close in size, though with the other satellites, this isn't really the case. They are rather different in density, though.

Charon's density is a bit lower than Pluto's, indicating that perhaps it formed in a similar manner to how our Moon formed, as a big impact event. Figure 10. Hubble Space Telescope image of Pluto and its family of satellites. Image credit: NASA, ESA, and M. Showalter (SETI Institute) A rather nifty aspect of Pluto-Charon is that both objects are tidally locked - one side of Pluto faces Charon and one side of Charon faces Pluto. This also has the rather nifty aspect that the time for a month on Pluto (the time for the satellite to orbit) is equal to a day on Pluto (the time for the planet to rotate). This is about 6.4 Earth days. If you were to stand on the surface of Pluto, the view of Charon would be great - it would be about 10 times wider in appearance than our Moon - very romantic! Odds are the other satellites are also locked to Pluto, but since they are so small it is difficult to determine if that is the case. The densities of Pluto and Charon are similar to the ice/rock mixture that is seen in the moons of the outer planets.

Pluto has a very thin atmosphere made up mainly of nitrogen and methane. The atmosphere is only present around the time of perihelion. This is when it is close enough to the Sun that some of the surface ices can melt and become gaseous. Most of the time, Pluto doesn't have an atmosphere. And it is also pretty cold, on average around 40 K. It appears that there is also some wind in this atmosphere which may effect the surface features on Pluto. Currently, the largest Kuiper Belt object known is Eris (formerly known as 2003 UB313), which is about 2600 km wide. Pluto, in comparison, is about 2,370 km wide - large, but not larger. Eris is really the one that got Pluto kicked out of the planet club. Not only does Eris have a larger diameter, but observations of its moon reveal that its mass is about 27% greater than Pluto. So if Pluto is a planet, some people thought, then Eris should also be a planet. But where does one draw the line?

That's why there was all of that debate in the summer of 2006 when the definition of of a planet came about. So if you are upset that Pluto is no longer officially a planet, blame it on Eris - who is appropriately named after the goddess of discord and rivalry. Other very large KBOs have been discovered and given fun names like Quaoar , and Orcus (formerly known as 2004 DW). Observations of the surface of Quaoar indicate that the surface is icy, but that the ice is in a form that was created due to a heating and freezing mechanism. It is possible that something like an impact may have caused the heating, or perhaps some tidal forces, or maybe....who knows? The presence of this form of ice also indicates that whatever formed it happened fairly recently (only a few million years ago), so that's sort of exciting. In 2004 the discovery of an object called Sedna was announced. This is an object that is located beyond the Kuiper Belt, in a region that's called the "inner" Oort Cloud (the animation shows a zoom-out from the inner solar system, to Sedna's orbit and finally to the inner Oort Cloud).

You'll learn more about the Oort cloud in the next section, so stay tuned for that. Sedna's orbit is much more elliptical than a Kuiper Belt object and it is located well beyond the edge of the Kuiper Belt. It is larger than Quaoar though. But there are even more distant objects that we've seen. The one with the largest orbit is 2006 SQ372 (gotta love the name), which has an aphelion distance of 1570 AU, while it only gets about 24 AU from the Sun at perihelion. 2006 SQ372 is a relatively small object, only 60 or so km in size. Figure 11. To scale view of various objects, including the largest Kuiper Belt Object (Eris), as well as other objects in the outer solar system, along with the Earth for comparison. Only the image the Earth is accurate, while the others are artist's renditions. The known satellites of these objects are also shown. So are Kuiper Belt Objects planets? If they are, or if some of them are, where do you draw the line?

Pluto was considered a planet for more than 60 years and some still consider it a planet, but if you include it, then you would have to include a whole bunch of other KBOs. If you look back at the characteristics of Terrrestrial and Jovian planets you'd see that none of the KBOs fit into that classification scheme. Sort of as a compromise, the category of dwarf planet was invented. These are objects that meet the first two criteria of "planet-hood" - they orbit the Sun and are large enough to be fairly spherical. In this group we have Pluto, Eris, two other KBOs (Haumea and Makemake), and the asteroid Ceres. So you could say we have 8+5=13 planets which can be further broken down into "regular" and "dwarf" groups, or you could just say we have 8 planets and a bunch of small stuff. Since the two types of planets, Terrestrials and Jovians, only work with 8 of these, perhaps that's the best way to distinguish those in the planet club from those not in the club. Another name you may hear is plutoid.

That refers to a dwarf planet that is located beyond Neptune, which means Eris, Makemake, Haumea and Pluto are in that group. Sort of silly, but I guess it will make the people who want to return Pluto to "full planet status" sort of happy, or maybe not. In the next (and last) section of notes you'll learn about the rest of the stuff in the solar system. This is mainly small stuff, but as you'll see, it can have a big impact. Now that you've read this section, you should be able to answer these questions.... What are the season on Uranus like? What is unusual about Uranus' and Neptune's magnetic fields? How are the internal structures of Uranus and Neptune different from Jupiter and Saturn? How do the ring systems of Uranus and Neptune compare to that of Jupiter and Saturn? What is unusual about Triton? What is a possible cause for Triton's unusual behavior? What characteristics of Pluto make it unlike the planets in the outer solar system? What are the Kuiper Belt Objects? Pluto: Facts & Information About the Dwarf Planet Pluto A wide view of Pluto taken by the New Horizons space probe in July 2015.

Credit: NASA/APL/SwRI Pluto, once considered the ninth and most distant planet from the sun, is now the largest known dwarf planet in the solar system. It is also one of the largest known members of the Kuiper Belt , a shadowy disklike zone beyond the orbit of Neptune thought to be populated by hundreds of thousands of rocky, icy bodies each larger than 62 miles (100 kilometers) across, along with 1 trillion or more comets. In 2006, Pluto was reclassified as a dwarf planet , a change widely thought of as a demotion. The question of Pluto's planet status has attracted controversy and stirred debate in the scientific community , and among the general public , since then. American astronomer Percival Lowell first caught hints of Pluto's existence in 1905 from odd deviations he observed in the orbits of Neptune and Uranus, suggesting that another world's gravity was tugging at these two planets from beyond. Lowell predicted the mystery planet's location in 1915, but died without finding it. Pluto was finally discovered in 1930 by Clyde Tombaugh at the Lowell Observatory, based on predictions by Lowell and other astronomers.

Pluto got its name from an 11-year-old Venetia Burney of Oxford, England, who suggested to her grandfather that the new world get its name from the Roman god of the underworld. Her grandfather then passed the name on to Lowell Observatory. The name also honors Percival Lowell, whose initials are the first two letters of Pluto. Physical characteristics Since Pluto is so far from Earth, little was known about the dwarf planet's size or surface conditions until 2015, when NASA's New Horizons space probe made a close flyby of Pluto. New Horizons showed that Pluto has a diameter of 1,473 miles (2,370 km), less than one-fifth the diameter of Earth, and only about two-thirds as wide as Earth's moon. Observations of Pluto's surface by the New Horizons spacecraft revealed a variety of surface features , including mountains that reach as high as 11,000 feet (3,500 meters), comparable to the Rocky Mountains on Earth. While methane and nitrogen ice cover much of the surface of Pluto, these materials are not strong enough to support such enormous peaks, so scientists suspect that the mountains are formed on a bedrock of water ice.

[ Photos of Pluto and Its Moons ] Pluto's surface is also covered in an abundance of methane ice, but New Horizons scientists have observed significant differences in the way the ice reflects light across the dwarf planet's surface. Another distinct feature on Pluto's surface is a large heart-shaped region known unofficially as Tombaugh Reggio (after Clyde Tombaugh). The left side of the reggio (an area that takes on the shape of an ice cream cone) is covered in carbon monoxide ice. Other variations in the composition of surface materials have been identified within the "heart" of Pluto. In the center left of Tombaugh Reggio is a very smooth region unofficially known by the New Horizons team as "Sputnik Planum," after Earth's first artificial satellite, Sputnik. This region of Pluto's surface lacks craters caused by meteorite impacts, suggesting that the area is, on a geologic timescale, very young — no more than 100 million years old. It's possible that this region is still being shaped and changed by geologic processes . These icy plains also display dark streaks that are a few miles long, and aligned in the same direction.

It's possible the lines are created by harsh winds blowing across the dwarf planet's surface. NASA's Hubble Space Telescope has also revealed evidence that Pluto's crust could contain complex organic molecules . Pluto's surface is one of the coldest places in the solar system, at roughly minus 375 degrees Fahrenheit (minus 225 degrees Celsius). When compared with past images, pictures of Pluto taken by the Hubble Space Telescope revealed that the dwarf planet had apparently grown redder over time, apparently due to seasonal changes. Orbital characteristics Pluto's highly elliptical orbit can take it more than 49 times as far out from the sun as Earth. Since the dwarf planet's orbit is so eccentric, or far from circular, Pluto's distance from the sun can vary considerably. The dwarf planet actually gets closer to the sun than Neptune is for 20 years out of Pluto's 248-Earth-years-long orbit, providing astronomers a rare chance to study this small, cold, distant world.

As a result of that orbit, after 20 years as the eighth planet (in order going out from the sun), in 1999, Pluto crossed Neptune's orbit to become the farthest planet from the sun (until it was demoted to the status of dwarf planet). When Pluto is closer to the sun, its surface ices thaw and temporarily form a thin atmosphere, consisting mostly of nitrogen, with some methane. Pluto's low gravity, which is a little more than one-twentieth that of Earth's, causes this atmosphere to extend much higher in altitude than Earth's. When traveling farther away from the sun, most of Pluto's atmosphere is thought to freeze and all but disappear. Still, in the time that it does have an atmosphere, Pluto can apparently experience strong winds. A close-up view of the surface of Pluto, taken by the New Horizons space probe in July, 2015, revealed the presence of icy mountains on the dwarf planet's surface. Credit: NASA-JHUAPL-SwRI Composition & structure Atmospheric composition : Methane, nitrogen.

Observations by New Horizons show that Pluto's atmosphere extends as far as 1,000 miles (1,600 km) above the surface of the dwarf planet. Magnetic field: It remains unknown whether Pluto has a magnetic field, but the dwarf planet's small size and slow rotation suggest it has little to no such field. Chemical composition: Pluto probably consists of a mixture of 70 percent rock and 30 percent water ice. Internal structure: The dwarf planet probably has a rocky core surrounded by a mantle of water ice, with more exotic ices such as methane, carbon monoxide and nitrogen ice coating the surface. Orbit & rotation Pluto's rotation is retrograde compared to the solar systems' other worlds; it spins backward, from east to west.

Average distance from the sun: 3,670,050,000 miles (5,906,380,000 km) — 39.482 times that of Earth Perihelion (closest approach to the sun): 2,756,902,000 miles (4,436,820,000 km) — 30.171 times that of Earth Aphelion (farthest distance from the sun): 4,583,190,000 miles (7,375,930,000 km) — 48.481 times that of Earth Pluto's moons Pluto has five moons: Charon, Styx, Nix, Kerberos and Hydra, with Charon being the closest to Pluto and Hydra the most distant. In 1978, astronomers discovered that Pluto had a very large moon nearly half the dwarf planet's own size. This moon was dubbed Charon, after the mythological demon who ferried souls to the underworld in Greek mythology.

Because Charon and Pluto are so similar in size, their orbit is unlike that of most planets and their moons. Both Pluto and Charon orbit a point in space that lies between them, similar to the orbits of binary star systems , For this reason, scientists refer to Pluto and Charon as a double dwarf planet, double planet or binary system. Pluto and Charon are just 12,200 miles (19,640 km) apart, less than the distance by flight between London and Sydney. Charon's orbit around Pluto takes 6.4 Earth days, and one Pluto rotation — a Pluto day — also takes 6.4 Earth days. This is because Charon hovers over the same spot on Pluto's surface, and the same side of Charon always faces Pluto, a phenomenon known as tidal locking. While Pluto has a reddish tint, Charon appears more grayish. In its early days, the moon may have contained a subsurface ocean , though the satellite probably can't support one today. Compared with most of the solar system's planets and moons, the Pluto-Charon system is tipped on its side in relation to the sun.

Observations of Charon by New Horizons have revealed the presence of canyons on the moon's surface. The deepest of those canyons plunge downward for up to 6 miles (9.7 km). A long swatch of cliffs and troughs stretches for 600 miles (970 km) across the middle of the satellite. A section of the moon's surface near one pole is covered in a much darker material than the rest of the planet. Similar to regions of Pluto, much of Charon's surface is free of craters — suggesting the surface is quite young and geologically active . In 2005, as scientists photographed Pluto with the Hubble Space Telescope in preparation for the New Horizons mission and discovered two other tiny moons of Pluto , now dubbed Nix and Hydra. These satellites are two and three times farther away from Pluto than is Charon. Based on measurements by New Horizons, Nix is estimated to be 26 miles (42 km) long and 22 miles (36 km) wide, while Hydra is estimated at 34 miles (55 km) long and 25 miles (40 km) wide.

It is likely that Hydra's surface is coated primarily in water ice. Scientists using Hubble discovered a fourth moon, Kerberos, in 2011. This moon is estimated to be 8 to 21 miles (13 to 34 km) in diameter. On July 11, 2012, a fifth moon , Styx, was discovered (with an estimated width of 6 miles or 10 km), further fueling the debate about Pluto's status as a planet. The four newly spotted moons may have formed from the collision that created Charon. Their orbits have been found to be highly chaotic . Research & exploration NASA's New Horizons mission is the first probe to study Pluto, its moons and other worlds within the Kuiper Belt up close. It was launched on January 2006, and successfully made its closest approach to Pluto on July 14, 2015. It will take 16 months to download all the data that the spacecraft collected during its flyby of Pluto and its moons. Mission leaders say they would like to send New Horizons to study another object in the Kuiper belt, but NASA has not yet approved an extended mission.

The New Horizons probe carries some of the ashes of Pluto's discoverer , Clyde Tombaugh. The limited knowledge of the Pluto system created unprecedented dangers for the New Horizons probe. Prior to the mission's launch, scientists knew of the existence of only three moons around Pluto. The discovery of Kerberos and Styx during the spacecraft's journey fueled the idea that more satellites could orbit the dwarf planet, unseen from Earth . Collisions with unseen moons, or even small bits of debris, could have seriously damaged the spacecraft. But the New Horizons design team equipped the space probe with tools to protect it during its journey. Pluto's formation & origins The leading hypothesis for the formation of Pluto and Charon is that a nascent Pluto was struck with a glancing blow by another Pluto-size object. Most of the combined matter became Pluto, while the rest spun off to become Charon, this idea suggests. [ Infographic: Pluto: A Dwarf Planet Oddity ] Additional reporting by staff writer Calla Cofield and Nola Taylor Redd, Space.com contributor. Additional resources The International Astronomical Union makes its case for demoting Pluto to a dwarf planet. Track the New Horizons spacecraft as it it continues on its journey into the Kuiper Belt.

Learn more about Pluto at NASA's Dwarf Planets page . Related: Seeing in the Dark . Astronomy Topics . Planets and Pluto | PBS For Families What's a Planet and Why Is Pluto Not in the Planet Club Anymore? by Andrew Fraknoi Although Seeing in the Dark doesn't directly discuss Pluto, it does celebrate the joy of observing planets. Viewers who watch the show may wonder what happened to Pluto in 2006 and whether any of the planets discussed in the show might lose their status next. So we thought it might be useful to tell the story of Pluto briefly and put it in context. To understand what happened to Pluto, we must actually go back to the late 1700's, when a number of astronomers thought that there was something odd to the spacing of the planets orbiting the Sun. They thought Mars and Jupiter were too far apart and that there should be a planet between them. A group of German astronomers organized themselves into a Missing Planet Bureau and undertook a detailed search of likely locations for such a planet. Giuseppe Piazzi In 1801, an Italian astronomer named Giuseppe Piazzi beat them to the punch when he found a dim object between Mars and Jupiter.

He named it Ceres, after the patron goddess of Sicily, where he lived. The disappointed Germans kept looking and in 1802 found another one, which was soon called Pallas. A third dim object was found in 1804 (called Juno) and a fourth in 1807 (called Vesta). By 1890 more than 300 of these objects had been discovered and it was clear that they were not planets in the traditional sense. When astronomers realized that Ceres had many "relatives" in roughly the same region of the solar system, they began calling them asteroids or minor planets. But as you can see in the table at the bottom, for a while, schoolkids had to memorize Ceres, Juno, Pallas, and Vesta as full-fledged members of our planetary system. Today, when tens of thousands of minor planets have been discovered (most residing in an "asteroid belt" between Mars and Pluto), not even the meanest of teachers would require students to learn all their names. We bring up this history because it is directly related to the story of Pluto.

In 1846, Neptune, the fourth of the giant planets beyond the asteroid belt, was discovered—thirty times farther from the Sun than our Earth. With 8 planets now known, the thoughts of astronomers naturally turned to the region beyond Neptune. Were any more distant worlds out there, waiting to be found? Two Sides of Pluto from the Hubble telescope In 1930, a systematic search revealed what seemed to be a 9th planet, although measurements eventually showed it to be much smaller than Jupiter, Saturn, Uranus, and Neptune. In many ways it more nearly resembled Triton, a moon of Neptune's, than any of the planets in the outer solar system. Nevertheless, because it orbited the Sun like a planet, it was classified as a planet and named Pluto, after the god of outer darkness in Roman mythology. So things stood for 65 years. Although Pluto's orbit was more elongated than the other planets' (and even crossed the orbit of Neptune—not the kind of thing you would expect a decent planet to do), it seemed alone in its region of space, much as Ceres had seemed before the other asteroids were discovered.

But then, starting in 1995, astronomers with better instruments began to find Pluto's nearby relatives. Soon, they had discovered several worlds almost as big as Pluto and in 2003 they identified an object which has turned out to be bigger than Pluto. (The object is now called Eris.) Over a thousand objects are now known to orbit the Sun beyond Neptune, and once again we can't imagine torturing school children by adding even a dozen of the biggest ones to the list of planets. The region where these bodies orbit is now called the Kuiper (pronounced Chi-per) Belt, after one of the astronomers who hypothesized its existence. Eris, artist's concept To honor and distinguish the largest members of the Kuiper Belt—objects such as Pluto and Eris—the International Astronomical Union in 2006 recommended that they be called "dwarf planets"—much as asteroids are sometimes called "minor planets." While some people mourned the passing of Pluto from mainstream planet to this new classification, other Pluto fans thought it was pretty cool to be the first and defining member of a whole new category. Having heard the whole story, what do you think?

A Brief History of the Number of Planets Being Taught Time Period Pluto - Educational facts and the history of Pluto. Pluto News Pluto Facts Pluto orbits beyond the orbit of Neptune (usually). It is much smaller than any of the official planets and now classified as a "dwarf planet". Pluto is smaller than seven of the solar system's moons (the Moon , Io , Europa , Ganymede , Callisto , Titan and Triton ). Planet Profile mass : 1.303e22 kg History of Pluto In Roman mythology, Pluto (Greek: Hades ) is the god of the underworld. The planet received this name (after many other suggestions ) perhaps because it's so far from the Sun that it is in perpetual darkness and perhaps because "PL" are the initials of Percival Lowell . Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune , based on the motions of Uranus and Neptune. Not knowing of the error, Clyde W. Tombaugh at Lowell Observatory in Arizona did a very careful sky survey which turned up Pluto anyway.

After the discovery of Pluto, it was quickly determined that Pluto was too small to account for the discrepancies in the orbits of the other planets. The search for Planet X continued but nothing was found. Nor is it likely that it ever will be: the discrepancies vanish if the mass of Neptune determined from the Voyager 2 encounter with Neptune is used. There is no Planet X. But that doesn't mean there aren't other objects out there, only that there isn't a relatively large and close one like Planet X was assumed to be. In fact, we now know that there are a very large number of small objects in the Kuiper Belt beyond the orbit of Neptune, some roughly the same size as Pluto. Until 2015 even the Hubble Space Telescope was able to resolve only the largest features on its surface (left and above). On 14 July 2015 the New Horizons spacecraft did a flyby of Pluto after being launched. Pluto has five moons: Charon, Hydra, Nix, Kerberos, Styx.

Charon - 1,208 km Hydra - 55 km x 40 km Nix - 42 km x 36 km Kerberos - 12 km x 4.5 km Styx - 7 km x 3 km Family of Moons Until the visit by New Horizons the individual masses of Pluto and Charon could not be determined with great accuracy. Pluto and Charon size comparisons by New Horizons: Diameter Mass Density Pluto 2372 km 1.303e22 kg 1.860 +/- 0.013 g/cm Charon 1208 km 1.586e21 kg 1.702 +/- 0.021 g/cm The Pluto-Charon pair orbit about each other around a common center of mass called the barycenter. Pluto has an atmosphere consisting of mainly nitrogen extending to 1,600 km above the surface. Methane is another constituent of the atmosphere and it is likely caused by sunlight breaking down methane gas particles into ethylene and acetylene, which were also discovered by New Horizons.

As the ethylene and acetylene sink into the atmosphere they condense and create a haze. In fact two distinct layers of haze surround the planet, one starting at about 80 km above the surface and extending to 130 km, the other is lower at an altitude of 50 km. New Horizons captured this image of the haze layers. The ultraviolet sunlight also acts on the haze converting it to tholins which are dark hydrocarbons and gives Pluto its characteristic color. Pluto has a heart shaped region called Sputnik Planum. The region is composed of nitrogen, carbon monoxide and methane ices. These ices flow like glaciers in the minus 234 C environment. The glaciers are thought to flow just as glaciers here on Earth do, although water ices on Pluto is very hard and virtually immovable. New Horizons sent back images from the edge of Sputnik Planum showing the tell-tale signs of glacial flow. Question: Pluto has a surface pressure over 10,000 times less than Earth, it has a atmosphere consisting of 98 percent Nitrogen.

It has been determined Pluto is losing Nitrogen from its atmosphere at a rate in the order of tons per hour, yet we have nitrogen glaciers. Where does all the nitrogen come from? Could cometary impacts be replenishing the nitrogen directly or possibly excavating enough nitrogen ices for replenishment? It could also be enough residual heat internal to Pluto and geologic activity is releasing nitrogen from within Pluto itself. As more data from New Horizons is downloaded we may find out. Pluto is the second most contrasty body in the Solar System (after Iapetus ). There has been controversy about the classification of Pluto. It was classified as the ninth planet shortly after its discovery and remained so for 75 years. But on 2006 Aug 24 the IAU decided on a new definition of "planet" which does not include Pluto. Pluto is now classified as a "dwarf planet", a class distinct from "planet". While this may be controversial at first (and certainly causes confusion for the name of this website) it is my hope that this ends the essentially empty debate about Pluto's status so that we can get on with the real science of figuring out its physical nature and history.

Pluto has been assigned number 134340 in the minor planet catalog. Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets. Pluto is locked in a 3:2 resonance with Neptune; i.e. Pluto's orbital period is exactly 1.5 times longer than Neptune's. Its orbital inclination is also much higher than the other planets'. Thus though it appears that Pluto's orbit crosses Neptune's, it really doesn't and they will never collide. (Here is a more detailed explanation .) Like Uranus , the plane of Pluto's equator is at almost right angles to the plane of its orbit. The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). The "warmer" regions roughly correspond to the regions that appear darker in optical wavelengths. The unusual nature of the orbits of Pluto and of Triton and the similarity of bulk properties between Pluto and Triton suggest some historical connection between them.

It was once thought that Pluto may have once been a satellite of Neptune's, but this now seems unlikely. A more popular idea is that Triton, like Pluto, once moved in an independent orbit around the Sun and was later captured by Neptune. Perhaps Triton, Pluto and Charon are the only remaining members of a large class of similar objects the rest of which were ejected into the Oort cloud . Like the Earth 's Moon , Charon may be the result of a collision between Pluto and another body. Pluto can be seen with an amateur telescope but it is not easy. There are several Web sites that show the current position of Pluto (and the other planets) in the sky, but much more detailed charts and careful observations over several days will be required to reliably find it. Suitable charts can be created with many planetarium programs . Charon ) is Pluto's largest satellite: orbit : 19,500 km from Pluto(average) diameter : 1208 km mass : 1.586e21 kg Charon is named for the mythological figure who ferried the dead across the River Acheron into Hades (the underworld).

(Though officially named for the mythological figure, Charon's discoverer was also naming it in honor of his wife, Charlene. Thus, those in the know pronounce it with the first syllable sounding like 'shard' ("SHAHR en"). Charon was discovered in 1978 by Jim Christy . Prior to that it was thought that Pluto was much larger since the images of Charon and Pluto were blurred together. Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon ). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon. Charon is a little over half the diameter of Pluto (50.9 percent) and has a very different surface composition than Pluto. Pluto is mainly nitrogen, methane and carbon monoxide ices, Charon is more water ice. Charon's radius is not well known. JPL's value of 586 has an error margin of +/-13, more than two percent. Its mass and density are also poorly known.

Pluto and Charon are also unique in that not only does Charon rotate synchronously but Pluto does, too: they both keep the same face toward one another. (This makes the phases of Charon as seen from Pluto very interesting.) Charon's composition is unknown, but its low density (about 2 gm/cm3) indicates that it may be similar to Saturn 's icy moons (i.e. Rhea ). Its surface seems to be covered with water ice. Interestingly, this is quite different from Pluto. Unlike Pluto, Charon does not have large albedo features, though it may have smaller ones that have not been resolved. Charon has a massive "canyon" across more than 1,600 km its equatorial region almost as if the moon was nearly split in two. It has been proposed that Charon was formed by a giant impact similar to the one that formed Earth's Moon The New Horizons team also discovered the plains south of the canyon called the Vulcan Plaum have noticeably fewer craters than the north so they are younger and the smooth plains along with the grooves and faint ridges are tell-tale signs of wide scale resurfacing. Could this be explained by cryovolcanism?

Possibly further data from New Horizons will shed light on this mystery. Mission:Science Science Mission Directorate. "What is a Planet?" Mission:Science. 2009. National Aeronautics and Space Administration. What is the definition of a planet? Before telescopes, scientists relied on their naked eye and careful observations to catalog the night sky. The five planets easily visible with the naked eye have been observed for all of human history: Mercury, Venus, Mars, Jupiter and Saturn. Technically, there was never a scientific definition of the term Planet before 2006. When the Greeks observed the sky thousands of years ago, they discovered objects that acted differently than stars. These points of light seemed to wander around the sky throughout the year. We get the term "planet" from the Greek word "Planetes" - meaning wanderer. Telescopes add to our knowledge In the 1600's scientists began to use telescopes to view our solar system. As technology got better, scientists discovered more planets orbiting our Sun such as Uranus in 1781, Neptune in 1846 and Pluto in 1930.

Then, in 1991, advances in telescope technology enabled scientists to discover many more objects in a disk-shaped cloud beyond Pluto called the Kuiper (KYE per) Belt. These objects were classified as Kuiper Belt Objects (KBO) partly because they are smaller than Pluto. This classification of KBO seemed to work fine as long as objects weren't bigger than Pluto. Clyde Tombaugh discovered Pluto in 1930. Pluto was added to the list of planets and was our 9th planet until August 2006. Eris, the Goddess of Discord It was the recent discovery of an object larger than Pluto within the Kuiper Belt that changed everything. Is this object, now named Eris, our 10th planet since it is larger than Pluto? This discovery and the naming of this new object prompted the IAU to discuss a scientific definition for the term planet. What if Eris is given planet status? Then our solar system could grow to dozens of planets as more and more Kuiper Belt Objects are discovered. Try remembering all those planet names. But if Eris is not a planet, then is Pluto still a planet?

The new definition – Part 1 and 2 Astronomers of the International Astronomical Union (IAU) voted on and passed the first scientific definition of a planet in August 2006. According to this new definition, an object must meet three criteria in order to be classified as a planet. First, it must orbit the Sun. Second, it must be big enough for gravity to squash it into a round ball. Some scientists believe that if Pluto remains classified as a planet, then the dozens of Kuiper Belt Objects (KBO's) orbiting our Sun would also be classified as planets. Our solar system would have the 9 original planets, an additional 43 KBO's, and more as they are cataloged. Part 3 – The Ongoing Debate And third, it must have cleared other objects out of the way in its orbital neighborhood. To clear an orbit, a planet must be big enough to pull neighboring objects into the planet itself or sling-shot them around the planet and shoot them off into outer space. According to the IAU, Pluto does not meet this third requirement but is now in a new class of objects called "dwarf planets." It is this third part of the definition that has sparked debate.

To distinguish a planet from a round asteroid in the asteroid belt, a planet must be massive enough to clear smaller objects – like asteroids – from their own orbit. The gravity of the planet would pull in smaller objects which would become part of the new planet. The problem for Pluto The problem for Pluto is the fact that its orbit is in the Kuiper Belt along with 43 other known Kuiper Belt Objects (KBOs). There are possibly billions of objects in the Kuiper Belt that have not been cataloged yet. Scientists have even found 8 KBOs between Neptune and Pluto. Some scientists view the new definition as unclear. Exactly how much does Pluto have to "clear" from its neighborhood to be considered a planet? And how much has Pluto already influenced its own neighborhood since the planet formed? These and other questions have been raised in response to the IAU's definition of a planet. Consider this: Pluto crosses into Neptune's orbit, but Neptune is still classified a planet. This is because of the orbits of Pluto and Neptune and that they never get closer to each other than 17AU (AU=distance from Earth to the Sun).

Pluto may cross orbits with many other Kuiper Belt Objects, but how close do these objects get to Pluto? How close to objects have to get to Pluto to be considered "in" Pluto's neighborhood? Diagram of the planet orbits in our solar system, including Pluto, distinctly shows the cross over of Neptune's and Pluto's orbits. Journey to the edge NASA's New Horizon spacecraft is speeding toward the edge of the solar system on its mission to Pluto. Launched in January 2006, it will not be until July 2015 that we will reach Pluto. It will swing past Jupiter for a gravity boost and scientific studies in February 2007, and reach Pluto and its moon, Charon, in July 2015. Then, as part of an extended mission, the spacecraft would head deeper into the Kuiper Belt to study one or more of the icy mini-worlds in that vast region, at least a billion miles beyond Neptune's orbit. Sending a spacecraft on this long journey will help us answer basic questions about the surface properties, geology, interior makeup and atmospheres on these bodies. New Horizon’s Space Craft will reach Pluto in 2015.

Peculiar Pluto :: NASA Space Place Tweet Peculiar Pluto One of the final images taken before New Horizons made its closest approach to Pluto on 14 July 2015. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute Which sentence best describes Pluto? Pluto is actually closer to the sun than Neptune for about 8% of its orbit. Pluto is just one of many icy objects in a distant area of our solar system. Pluto and its large, orbiting companion object Charon, are tipped on their sides. All of the above. Well, just pick the answer you like best, because they are all true! Pluto is a dwarf planet that lies in the Kuiper Belt. It's an area full of icy bodies and other dwarf planets at the edge of our solar system. Because Pluto is the biggest object in this region, some call it "King of the Kuiper Belt." One thing is certain. Pluto and its neighborhood are very peculiar. If scientists could unravel some of their mysteries, we would know more about how our solar system formed. More Fun Facts About Pluto: Pluto is only about half the width of the United States.

Charon is about half the size of Pluto. Charon is the largest moon compared to the body it orbits (whether planet or dwarf planet) of any moon in the solar system. Almost all the planets travel around the sun in nearly perfect circles. But Pluto does not. It takes an oval-shaped path with the sun nowhere near its center. What's more, its path is quite tilted from the nice, orderly plane where all the planets orbit. (Mercury has a slightly lop-sided orbit, although not nearly so much as Pluto's.) In the picture above, the arrows show the direction the planets and Pluto rotate. Notice Pluto's spin goes the opposite direction of all the others except Venus and Uranus. In the picture above, the arrows show which direction the planets' and Pluto's axes of rotation point. Notice Pluto's and Uranus' point along the same plane as their orbits, instead of more or less "up and down." Compared to most of the planets and their moons, the whole Pluto-Charon system is tipped on its side. Like the planets, Pluto's spin axis stays pointed in the same direction as it orbits the sun. But unlike all planets except Uranus, Pluto is tipped on its side.

The planets' axes of rotation stand more or less upright from the plane of their orbits. If you lived on Pluto, you'd have to live 248 Earth years to celebrate your first birthday in Pluto-years. If you lived on Pluto, you would see Charon from only one side of the planet. Charon's orbit around Pluto takes about six and one-half Earth days. Pluto's day (that is, one complete rotation) takes exactly the same amount of time. So, Charon always "hovers" over the same spot on Pluto's surface, and the same side of Charon always faces Pluto. At Pluto's current distance from the sun, the temperature on its surface is about 400 degrees below zero Fahrenheit! It will get even colder as it moves farther from the sun. From Pluto, the sun looks like just a bright dot in the sky, the brightest star visible. The light from the sun is as bright on Pluto as the light from the full Moon is on Earth. If you weigh 100 pounds on Earth, you would weigh only 7 pounds on Pluto! Pluto orbits in a far-out region of the solar system called the Kuiper (rhymes with viper) Belt.

There are lots of icy, rocky objects out there. But they are so far from the sun they are really hard to see, even with powerful telescopes. Let's Go There! We finally got to visit Pluto, Charon, and the Kuiper Belt! On January 19, 2006, NASA launched a robot spacecraft on the long journey. This mission is called New Horizons. The spacecraft arrived at Pluto in July 2015, and will continue to study other objects in the Kuiper Belt from about 2018 to 2022. With New Horizons, we will visit and learn about the objects at the very edge of our solar system. They may help us understand how our solar system formed. article last updated February 3, 2016 Why Pluto is *not* a planet A while back, Pluto was the ninth planet from the sun. It was also the smallest planet. But not any more. Poor Pluto. Just how did it get "kicked out" of our family of planets? And who are its "real" family members? Astronomers have already named three other objects in the solar system that are about the same small size as Pluto.

They are Ceres [SEAR-ees], Makemake (MAH-kee-MAH-kee], and Eris (AIR-iss]. These objects, along with Pluto, are much smaller than the "other" planets. Ceres orbits in the asteroid belt between Mars and Jupiter. Makemake, like Pluto, is part of the Kuiper [KI-per] Belt, which is a region of trillions of icy objects orbiting beyond Neptune. Eris' orbit is even farther out. If they're not planets, then what are they? Astronomers have put these objects into a new family called dwarf planets. This picture shows the sizes of dwarf planets Pluto, Ceres, Eris, and Makemake as compared to Earth and Earth's moon, here called "Luna." None of the distances between objects are to scale.) If you liked this, you may like: Why Pluto is *not* a planet A while back, Pluto was the ninth planet from the sun. It was also the smallest planet. But not any more. Poor Pluto. Just how did it get "kicked out" of our family of planets? And who are its "real" family members?

Astronomers have already named three other objects in the solar system that are about the same small size as Pluto. They are Ceres [SEAR-ees], Makemake (MAH-kee-MAH-kee], and Eris (AIR-iss]. These objects, along with Pluto, are much smaller than the "other" planets. Ceres orbits in the asteroid belt between Mars and Jupiter. Makemake, like Pluto, is part of the Kuiper [KI-per] Belt, which is a region of trillions of icy objects orbiting beyond Neptune. Eris' orbit is even farther out. If they're not planets, then what are they? Astronomers have put these objects into a new family called dwarf planets. This picture shows the sizes of dwarf planets Pluto, Ceres, Eris, and Makemake as compared to Earth and Earth's moon, here called "Luna." None of the distances between objects are to scale.)

Question: What planet orbits between Uranus and Pluto?
Answer: Neptune