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 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 . Solar System Planets: Order of the 8 (or 9) Planets Solar System Planets: Order of the 8 (or 9) Planets By Robert Roy Britt | January 22, 2016 12:35pm ET MORE The planets of the solar system as depicted by a NASA computer illustration. Orbits and sizes are not shown to scale. Credit: NASA Ever since the discovery of Pluto in 1930, kids grew up learning about the nine planets of our solar system. That all changed starting in the late 1990s, when astronomers began to argue about whether Pluto was a planet. In a highly controversial decision , the International Astronomical Union ultimately decided in 2006 to call Pluto a “dwarf planet,” reducing the list of “real planets” in our solar system to eight.

However, astronomers are now hunting for another planet in our solar system, a true ninth planet , after evidence of its existence was unveiled on Jan. 20, 2016. The so-called "Planet Nine," as scientists are calling it, is about 10 times the mass of Earth and 5,000 times the mass of Pluto. [ Solar System Pictures: A Photo Tour ] If you insist on including Pluto , then that world would come after Neptune on the list; Pluto is truly way out there, and on a wildly tilted, elliptical orbit (two of the several reasons it got demoted). Interestingly, Pluto used to be the eighth planet, actually. More on that below. Terrestrial planets The inner four worlds are called “ terrestrial planets ,” because, like Earth, their surfaces are all rocky. Pluto, too, has a solid surface (and a very frozen one) but has never been grouped with the four terrestrials.

Jovian planets The four large outer worlds — Jupiter, Saturn, Uranus, and Neptune — are known as the “Jovian planets” (meaning “Jupiter-like”) because they are all huge compared to the terrestrial planets, and because they are gaseous in nature rather than having rocky surfaces (though some or all of them may have solid cores, astronomers say). According to NASA , "two of the outer planets beyond the orbit of Mars — Jupiter and Saturn — are known as gas giants; the more distant Uranus and Neptune are called ice giants." This is because, while the first two are dominated by gas, while the last two have more ice. All four contain mostly hydrogen and helium. Dwarf planets The IAU definition of a full-fledged planet goes like this: 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. Yeah, that’s a mouthful.

The problem for Pluto, besides its small size and offbeat orbit, is that it shares its space with lots of other objects in the Kuiper Belt , beyond Neptune. Still, the demotion of Pluto remains controversial . The IAU planet definition puts other small, round worlds in the dwarf planet category, including the Kuiper Belt objects Eris , Haumea , and Makemake . Also now a dwarf planet is Ceres , a round object in the Asteroid Belt between Mars and Jupiter. Ceres was actually considered a planet when discovered in 1801 and then later deemed to be an asteroid. Some astronomers like to consider Ceres as a 10th planet (not to be confused with Nibiru or Planet X ), but that line of thinking opens up the possibility of there being 13 planets, with more bound to be discovered. The planets Below is a brief overview of the eight primary planets in our solar system , in order from the inner solar system outward: Mercury The closest planet to the sun, Mercury is only a bit larger than Earth's moon.

Its day side is scorched by the sun and can reach 840 degrees Fahrenheit (450 Celsius), but on the night side, temperatures drop to hundreds of degrees below freezing. Mercury has virtually no atmosphere to absorb meteor impacts, so its surface is pockmarked with craters, just like the moon. Over its four-year mission, NASA's MESSENGER spacecraft has revealed views of the planet that have challenged astronomers' expectations. Discovery: Known to the ancients and visible to the naked eye Named for: Messenger of the Roman gods Diameter: 3,031 miles (4,878 km) Orbit: 88 Earth days Venus' southern hemisphere, as seen in the ultraviolet. Credit: ESA Venus The second planet from the sun, Venus is terribly hot, even hotter than Mercury. The atmosphere is toxic. The pressure at the surface would crush and kill you. Scientists describe Venus’ situation as a runaway greenhouse effect. Its size and structure are similar to Earth, Venus' thick, toxic atmosphere traps heat in a runaway "greenhouse effect." Oddly, Venus spins slowly in the opposite direction of most planets. The Greeks believed Venus was two different objects — one in the morning sky and another in the evening.

Because it is often brighter than any other object in the sky — except for the sun and moon — Venus has generated many UFO reports. Discovery: Known to the ancients and visible to the naked eye Named for: Roman goddess of love and beauty Diameter: 7,521 miles (12,104 km) Orbit: 225 Earth days An image of the Earth taken by the Russian weather satellite Elektro-L No.1. Credit: NTsOMZ Earth The third planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean. It’s the only world known to harbor life. Earth’s atmosphere is rich in life-sustaining nitrogen and oxygen. Earth's surface rotates about its axis at 1,532 feet per second (467 meters per second) — slightly more than 1,000 mph (1,600 kph) — at the equator. The planet zips around the sun at more than 18 miles per second (29 km per second).

Diameter: 7,926 miles (12,760 km) Orbit: 365.24 days Day: 23 hours, 56 minutes Related: Mars researchers are focusing both Earth-based and planet orbiting sensors to better understand sources of methane on the red planet. Image Credit: Space Telescope Science Institute Mars The fourth planet from the sun, is a cold, dusty place. The dust, an iron oxide, gives the planet its reddish cast. Mars shares similarities with Earth: It is rocky, has mountains and valleys, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms. It snows on Mars. And Mars harbors water ice. Scientists think it was once wet and warm, though today it’s cold and desert-like. Mars' atmosphere is too thin for liquid water to exist on the surface for any length of time. Scientists think ancient Mars would have had the conditions to support life, and there is hope that signs of past life — possibly even present biology — may exist on the Red Planet.

Discovery: Known to the ancients and visible to the naked eye Named for: Roman god of war Diameter: 4,217 miles (6,787 km) Orbit: 687 Earth days Day: Just more than one Earth day (24 hours, 37 minutes) Related: Close-up of Jupiter's Great Red Spot as seen by a Voyager spacecraft. Credit: NASA/JPL-Caltech Jupiter The fifth planet from the sun, Jupiter is huge and is the most massive planet in our solar system. It’s a mostly gaseous world, mostly hydrogen and helium. Its swirling clouds are colorful due to different types of trace gases. A big feature is the Great Red Spot, a giant storm which has raged for hundreds of years. Jupiter has a strong magnetic field, and with dozens of moons, it looks a bit like a miniature solar system.

Discovery: Known to the ancients and visible to the naked eye Named for: Ruler of the Roman gods Diameter: 86,881 miles (139,822 km) Orbit: 11.9 Earth years The shadow of Saturn's moon Mimas dips onto the planet's rings and straddles the Cassini Division in this natural color image taken as Saturn approaches its August 2009 equinox. Credit: NASA/JPL/Space Science Institute Saturn The sixth planet from the sun is known most for its rings . When Galileo Galilei first studied Saturn in the early 1600s, he thought it was an object with three parts. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing — a symbol with one large circle and two smaller ones — in his notebook, as a noun in a sentence describing his discovery. More than 40 years later, Christiaan Huygens proposed that they were rings. The rings are made of ice and rock. Scientists are not yet sure how they formed. The gaseous planet is mostly hydrogen and helium. It has numerous moons .

Discovery: Known to the ancients and visible to the naked eye Named for: Roman god of agriculture Diameter: 74,900 miles (120,500 km) Orbit: 29.5 Earth years Day: About 10.5 Earth hours Related: Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which the planet is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory Uranus The seventh planet from the sun, Uranus is an oddball. It’s the only giant planet whose equator is nearly at right angles to its orbit — it basically orbits on its side. Astronomers think the planet collided with some other planet-size object long ago, causing the tilt. The tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years. Uranus is about the same size as Neptune. Methane in the atmosphere gives Uranus its blue-green tint. It has numerous moons and faint rings.

Discovery: 1781 by William Herschel (was thought previously to be a star) Named for: Personification of heaven in ancient myth Diameter: 31,763 miles (51,120 km) Orbit: 84 Earth years Neptune’s winds travel at more than 1,500 mph, and are the fastest planetary winds in the solar system. Credit: NASA/JPL Neptune The eighth planet from the sun, Neptune is known for strong winds — sometimes faster than the speed of sound. Neptune is far out and cold. The planet is more than 30 times as far from the sun as Earth. It has a rocky core. Neptune was the first planet to be predicted to exist by using math, before it was detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth. Discovery: 1846 Pluto and its moons orbit the sun near the edge of our solar system.

Learn all about Pluto's weirdly eccentric orbit, four moons and more in this Space.com infographic . Credit: SPACE.com/Karl Tate Pluto (Dwarf Planet) Once the ninth planet from the sun, Pluto is unlike other planets in many respects. It is smaller than Earth's moon. Its orbit carries it inside the orbit of Neptune and then way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet — until it was demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years. Pluto’s orbit is tilted to the main plane of the solar system — where the other planets orbit — by 17.1 degrees. It’s a cold, rocky world with only a very ephemeral atmosphere. NASA's New Horizons mission performed history's first flyby of the Pluto system on July 14, 2015.

[Related: New Horizons' Pluto Flyby: Latest News, Images and Video ] Discovery: 1930 by Clyde Tombaugh Named for: Roman god of the underworld, Hades Diameter: 1,430 miles (2,301 km) Orbit: 248 Earth years NASA Solar System Exploration: Dwarf Planets Planet Nine Planet Nine orbits the sun at a distance that is 20 times farther out than the orbit of Neptune. (The orbit of Neptune is 2.7 billion miles from the sun at its closest point.) The strange world's orbit is about 600 times farther from the sun than the Earth's orbit is from the star. Scientists have not actually seen Planet Nine directly . Its existence was inferred by its gravitational effects on other objects in the Kuiper Belt, a region at the fringe of the solar system that is home to icy objects left over from the birth of the sun and planets. Sedna: Planetoid or Planet? Science > Astronomy and Space Exploration > The Solar System Sedna: Planetoid or Planet? On March 15, 2004, astronomers confirmed the discovery of the most distant object ever identified in our solar system.

Twice as far from the Sun as any known object, this red mass has an unusually elliptical orbit that takes a staggering 10,500 years to complete. Officially called 2003 VB12, its discoverers claim that it is the first known object from the long-hypothesized Oort Cloud , believed to be home to billions of frozen comets. The object's surface temperature is about minus 400°F (minus 240°C). Its frigid celestial homeland inspired its informal name, Sedna, the Inuit goddess of the icy northern oceans. In addition to its unparalleled coldness and distance, Sedna also distinguishes itself as the largest object identified since Pluto's discovery in 1930. About three-quarters of the size of Pluto, it is considered a planetoid (a minor planet or asteroid). Thought to be composed of rock and ice, it has a reddish hue similar to that of Mars. In this artist's visualization, the newly discovered planet-like object, dubbed "Sedna," is shown where it resides at the outer edges of our solar system.

The object is so far away that the Sun appears as an extremely bright star instead of the large, warm disc observed from Earth. All that is known about Sedna's appearance is that it has a reddish hue, almost as red and reflective as the planet Mars. In the distance is a hypothetical small moon, which scientists believe may be orbiting this distant body. Photo courtesy of NASA Should Pluto Be a Planet After All? Experts Weigh In Should Pluto Be a Planet After All? Experts Weigh In By Mike Wall, Space.com Senior Writer | November 19, 2010 07:19am ET MORE Artist's impression of Pluto and Charon as seen from one of Pluto's other moons. Credit: David Aguilar/Center for Astrophysics Now that Pluto may have regained its status as the largest object in the outer solar system, should astronomers consider giving it back another former title — that of full-fledged planet? Pluto was demoted to a newly created category, "dwarf planet," in 2006, partly because of the discovery a year earlier of Eris, another icy body from Pluto's neighborhood. Eris was thought to be bigger than Pluto until Nov.

6, when astronomers got a chance to recalculate Eris' size . Now it appears that Pluto reigns — though only by the slimmest of margins (the numbers are so close as to be nearly indistinguishable, when uncertainties are taken into account). The new finding brings renewed attention to Pluto, and to the controversial decision to strip the frigid world of its planet status. Should Pluto be a planet? Should Eris, and many other objects circling the sun beyond Neptune's orbit? Or is the current system, which recognizes just eight relatively large planets, the way to go? [POLL: Should Pluto's Planet Status Be Revisited?] SPACE.com asked some experts to weigh in on this debate, which affects how astronomers view the solar system, as well as how complicated schoolchildren's planet-memorizing mnemonics must be: The background: Pluto's demotion Eris is about 9 billion miles (15 billion kilometers) from the sun at its farthest orbital point, making it about twice as distant as Pluto. Its discovery in 2005 ultimately led astronomers — uncomfortable with the prospect of finding many more planets in the frigid outer reaches of the solar system — to reconsider Pluto's status.

In 2006, the International Astronomical Union 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 orbital space with lots of other objects out in the Kuiper Belt — the ring of icy bodies beyond Neptune — it didn't make the cut . Instead, the IAU rebranded Pluto, and Eris, as "dwarf planets." Dwarf planets are not officially full-fledged planets, so Pluto was stripped of the status it had held since its discovery in 1930. Eight planets officially remain: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The case against Pluto The scientist who discovered Eris, Caltech astronomer Mike Brown, thinks Pluto's demotion was the right move. Pluto, Eris and the many other Kuiper Belt objects are far too different to be lumped in with the eight official planets, he said.

[ Mike Brown: Q & A With the Man Who Killed Pluto ] For one thing, they're much smaller. Pluto is about 1,455 miles (2,342 km) wide. The smallest official planet, Mercury, is more than twice as big at 3,032 miles (4,880 km) across. The dwarfs' orbits tend to be very different, too — much more elliptical and more inclined, relative to the plane of the solar system. And they're made of different stuff, with ices comprising more of their mass. "It just makes no sense from a classification standpoint to take these objects that clearly belong together and pick one — or two, or a dozen — and say, 'Oh, these belong with the very different, large, planet-like things," Brown said. The only reason Pluto was ever deemed a planet, Brown added, is because it was first detected so long ago, before people realized that it was just one of a vast flotilla of objects beyond Neptune's orbit.

The Kuiper Belt — which is now known to host more than 1,000 icy bodies, with many more likely to be discovered — wasn't even discovered until 1992. "It's just a funny historical accident that we found Pluto so early, and that it was the only thing known out there for so long," Brown told SPACE.com. "No one in their right mind would not have called it a planet back then, because we didn't know any better." Astronomers have a much better sense of what Pluto is now, according to Brown. "We have progressed so much further in our understanding of what the solar system is that it's pretty obvious," he said. "We can go back and reassess the mistakes of our ancestors." So Pluto should take its rightful place alongside other Kuiper Belt objects rather than consort with the "real" planets, some astronomers say. "I group Pluto with the other icy bodies in the Kuiper Belt," said Neil deGrasse Tyson, director of New York City's Hayden Planetarium. "I think it's happier there, actually. Pluto has family in the outer solar system." Tyson was one of the first to push for Pluto's demotion.

A decade ago, when he and the Hayden staff redesigned the planetarium's exhibits, they lumped Pluto with the Kuiper Belt objects rather than with the eight official planets. The case for Pluto Yet some astronomers still bristle at the reorganization of the solar system, and not because Pluto's demotion spoiled the popular "My Very Energetic Mother Just Served Us Nine Pizzas" planet-memorizing aid. Rather, the IAU's planet definition is fundamentally flawed, some astronomers say. They take particular issue with the "clearing your neighborhood" requirement, for several reasons. "If you take the IAU's definition strictly, no object in the solar system is a planet," said Alan Stern, a planetary scientist at the Southwest Research Institute in Boulder, Colo. "No object in the solar system has entirely cleared its zone." The definition also sets different standards for planethood at different distances from the sun, according to Stern, who is principal investigator of NASA's New Horizons mission, which is sending a spacecraft to Pluto. The farther away a planet is from the sun, the bigger it needs to be in order to clear its zone.

If Earth circled the sun in Uranus' orbit, it wouldn't be able to clean out its neighborhood and would thus not qualify as a planet, Stern said. "It's literally laughable," he told SPACE.com. In Stern's view, a planet is anything that meets the IAU definition's first two criteria — the bits about orbiting the sun and having enough mass to be roughly spherical, without the "clearing your neighborhood" requirement. So Pluto should be a planet, as should Eris and the dwarf planet Ceres (the largest body in the asteroid belt between Mars and Jupiter), as well as many other objects. Such a definition would greatly expand the list of planets in the solar system. Many astronomers were uncomfortable with this prospect, according to Stern, and that discomfort was a big factor in the decision to demote Pluto. It stemmed from an unscientific desire to keep the numbers low. "Many people think it's special to be a planet," Stern said. But adding a bunch of names to the list wouldn't cheapen the ones that had been there forever, he added. It would simply reflect astronomers' increasing understanding of the solar system. In that understanding, small, icy planets far outnumber big gassy or rocky ones.

"There are a large number of planets, and most of them are small," Stern said. "It's the Earth-like planets and the giant planets that are freakish." For what it's worth, Stern doesn't object to branding Pluto a "dwarf planet" — he said he coined the term — as long as dwarfs are still considered planets. What's in a name? The battle over Pluto's planethood may be more semantic than anything else. But words do matter, because they shape how people classify and understand reality. "You have to be able to sort," Stern said. Tyson said he tries not to use the word "planet" in its traditional, generic sense too much, because it doesn't convey very much meaningful information. It's more revealing to group objects that are similar in size, composition and other properties. "The word 'planet' has far outlived its usefulness," Tyson told SPACE.com. "It doesn't celebrate the scientific richness of the solar system." So Tyson thinks in categories such as gas giants (Jupiter, Saturn, Uranus and Neptune) and terrestrial planets (Mercury, Venus, Earth and Mars) as well as asteroids and Kuiper Belt objects (Pluto, Eris and many others).

For his part, Brown thinks stripping Pluto of its planethood doesn't make the icy body any less interesting or important. "I think that Pluto as an example of a large Kuiper Belt object is so much more interesting than Pluto as this very weird planet at the outer edge of the solar system unlike anything else," Brown said. "We are going to learn so much more about the solar system with our new understanding of what Pluto is." Maybe Stern and other scientists fighting for Pluto's planethood would agree. Or maybe not. Is Pluto A Planet Now? – Starts With A Bang! – Medium BlockedUnblock FollowFollowing The Universe is: Expanding, cooling, and dark. It starts with a bang! #Cosmology Science writer, astrophysicist, science communicator & NASA columnist. Jul 14, 2015 Is Pluto A Planet Now? Now that New Horizons has flown by the Plutonian system, can it be considered a planet after all? Image credit: NASA/JHUAPL/SWRI.

“Words are the source of misunderstandings.” -Antoine de Saint-Exupéry Back in 1930, Clyde Tombaugh was using a blink comparator, where the same region of sky would be photographed on two different nights. While the stars and nebulae would appear in the same exact position, since their motions are far too small to detect at such great distances, objects like comets, asteroids or planets would shift from night-to-night. In the early part of that year, the course of astronomical history would change forever. Image credit: Lowell Observatory Archives. For the first time, a large, non-cometary body orbiting our Sun out beyond Neptune had been discovered. And for 48 years, Pluto reigned alone as the only such object with that status. Finally, in the late 1970s, a second object joined it: Pluto’s giant moon, Charon. Again, for over a decade, the Plutonian system was the only one out there. And yet, Pluto was different from everything else.

It was far smaller than all the other planets (even Mercury or Mars), much smaller than many of the Solar System’s moons, interior to the orbit of Neptune (and almost Uranus) at time, and highly inclined to the ecliptic, unlike any of the other worlds. Montage credit: Emily Lakdawalla, via . The Moon: Gari Arrillaga. Other data: NASA/JPL/JHUAPL/SwRI/UCLA/MPS/IDA. Processing by Ted Stryk, Gordan Ugarkovic, Emily Lakdawalla, and Jason Perry. Finally, in the 1990s (starting in 1992), we discovered other objects out beyond the orbit of Neptune. Not only was Pluto (and its giant moon) out there, along with a large fraction of the known periodic comets, but many other objects as well, such as Eris, Makemake, Quaoar and others. Image credit: © 2015 The Johns Hopkins University Applied Physics Laboratory LLC, via . Clearly, Pluto belonged much more closely to this class of objects than to the other planets of the Solar System.

And just as previously discovered “planets” from the 1800s — Ceres, Pallas, Vesta, Juno and others — were demoted from planetary status when we realized that the asteroid belt was its own entity, Pluto’s status as a planet was not to last forever. In 2006, quite controversially, the International Astronomical Union (IAU) officially defined a planet for the first time, stating that something had to meet three conditions: being in hydrostatic equilibrium (i.e., pulled into a round or spheroidal shape by its own gravity), not orbit another, more massive body (i.e., not be a moon), and to clear its orbit, so that it was the gravitationally dominant body at its distance from its star. Just like that, Pluto was no longer a planet. And yet, it was still an object of tremendous interest and importance. So rather than relying on ground-based images or even Hubble images, we decided to go. From 1930 until July 14, today, 2015, this video (above) showcases just how much better our pictures of Pluto have gotten since we left Earth and traveled to it.

While the data transfer back is incredibly slow — not only does it take over four hours for the light to travel, but the data rate is terribly slow at just 1/8th of a kilobyte per second at such a great distance — we now have unprecedented views and data on Pluto and its neighboring worlds. We’ve discovered surface geology, incredible features, colors and atmospheric properties. At last, Pluto is no longer a “dot” in the sky, but its own world with its own unique cosmic story. Image credit: NASA/JHUAPL/SWRI, of Pluto (and Charon) at New Horizon’s final approach, less than 500,000 miles from the surface. The fly-by today was just the beginning: over the next sixteen months, data will pour in, telling us all sorts of information about Pluto’s surface, a variety of features including the white “heart”, the dark polar patches, the craters, cliffs and “mushrooms,” and a whole host of things we may not even notice today.

The possibilities are tremendous: Frozen water, methane, dry ice or even solid nitrogen (and you thought liquid nitrogen was cold) on the surface, A chance of weather and even snow, Active surface geology, including “black smokers” welling up from inside Pluto’s core, Additional moons, with the capability to detect ones just a single meter in size, and so much more. Already, the discoveries are pouring in. Image credit: NASA/JHUAPL/SWRI. We know the exact size of Pluto, confirming that it’s larger than we thought, but not nearly as large as we once thought, decades ago. We learned there are dark spots on Charon, a wholesale surprise. Moons that small ought to be much more uniform, perhaps evidence that extreme tidal forces (or a violent past) has shaped Pluto’s largest moon. A reddish color to the world overall, whose elemental composition will soon be revealed by New Horizons. And we learned that Pluto does have an atmosphere, but that the lower atmosphere (the troposphere) is much thinner than we once suspected, or possibly even non-existent! Image credit: snapshot from Emily Lakdawalla’s twitter stream, via .

Last year, I argued that there are only eight planets in our Solar System , and that there don’t deserve to be any more than that. But today, as we pass by Pluto and await the first science results, I’m not so sure. You see, Pluto is only the very first (and fittingly, Charon is the second) large Kuiper Belt Object to be studied so well, and studied up close at that. Image credit: Wikimedia commons user Eurocommuter . But the large Kuiper Belt objects — the ones that at least meet the first of the I.A.U.’s planetary criteria — not only exist, but there are probably hundreds of them in our Solar System alone. In other words, for everything we call a planet, there are dozens of Pluto-like worlds out there. And as we’re just starting to discover, for every planet we have in our Solar System, there are between tens to hundreds of thousands of planets without parent stars in the galaxy: rogue planets. Image credit: NASA / JPL-Caltech.

Now, here’s the thing that gets me and makes me think: if Pluto weren’t in our Solar System, we would have no problem calling it a “rogue planet.” So why do we stop calling it a planet because it’s in our Solar System? Perhaps we need a better word that encompasses all the artificial categories we created. Something that includes rocky planets, large moons, gas giants, dwarf planets, rogue planets, large Kuiper belt objects, large asteroids, and so on. Why don’t we just call them what they are: worlds. Image credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute, of Pluto and Charon in false color, so their surface features (and differences) can be more easily seen. These worlds tell the history of how everything in the Universe forms. They tell the elemental and chemical history of everything in our galaxy; they hold the secrets of the origin of the complex molecules, organics and life within them. Pluto and the other Kuiper belt objects, in particular, may hold the keys to understanding where the water on our planet or the nitrogen in our atmosphere came from. And thanks to the New Horizons mission, we’re about to find these secrets out.

In a very meaningful way, including Pluto back in the picture of our Solar System’s planets may be more honest — in terms of understanding what’s really important when it comes to a planetary system — than leaving it out. Image credit: Facebook user Pedro P. Coniconde, via . And at last, you can go back to your MVEMJSUNP acronyms. I’ll be talking about New Horizons’ historic flyby today live on Portland’s KGW NewsChannel 8 at 7 PM (Pacific) tonight, with a permanent video embed to follow below. While currently, that large feature seen on Pluto (at the lower right, above) has been dubbed a “heart” by many, I have a suspicion it’s a lot more. Alan Stern (a.k.a., the guy who killed Pluto and also the principle investigator) thinks it might be snowfall, and a transient feature. Others think it’s a permanent part of the surface geology. Me? I think if you add ears, eyes and a nose, you’ll get your answer. Thank you, internet. The Universe is: Expanding, cooling, and dark. It starts with a bang!

#Cosmology Science writer, astrophysicist, science communicator & NASA columnist. Follow The Universe is out there, waiting for you to discover it. 46 What is a planet? A Place in the Universe The above montage of planetary images was taken by spacecraft managed by NASA's Jet Propulsion Laboratory. Included are (from top to bottom) images of Mercury, Venus, Earth (and moon), Mars, Jupiter, Saturn, Uranus and Neptune. The spacecraft responsible for these images are as follows: 1. The Mercury image was taken by Mariner 10, 2. The Venus image by Magellan, 3. T the Earth image by Galileo, 4. The Mars image by Viking, and 5. The Jupiter, Saturn, Uranus and Neptune were taken images by Voyager.

The International Astronomical Union (IAU) said that the definition for a planet is now officially known as "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 neighborhood around its orbit." At the same time, new moons are also being discovered, both around existing planets and within these mysterious new worlds. Once the existence of a moon is confirmed and its orbit determined, the moon is given a final name by the International Astronomical Union (IAU), the organization that assumed this task since 1919. Relative Sizes of The Planets Since the invention of the telescope, three more planets have been discovered in our solar system: Uranus (1781), Neptune (1846), and Pluto (1930). In addition, there are thousands of small bodies such as asteroids and comets. Most of the asteroids orbit in a region between the orbits of Mars and Jupiter, while the home of comets lies far beyond the orbit of Pluto, in the Oort Cloud.

The Planets The Sun is the center of the solar system.The sun is the largest and most important object in our solar system. It contains 99.8 percent of the solar system's mass (quantity of matter). The sun provides most of the heat, light, and other energy that makes life possible. The Sun SOHO EIT(Extreme ultraviolet Imaging Telescope) Image The sun's outer layers are hot and stormy. The hot gases and electrically charged particles in those layers continually stream into space and often burst out in solar eruptions. This flow of gases and particles forms the solar wind, which bathes everything in the solar system. Planets orbit the sun in oval-shaped paths called ellipses, according to a law of planetary motion discovered by German astronomer Johannes Kepler in the early 1600's. The sun is slightly off to the side of the center of each ellipse at a point called a focus. The focus is actually a point inside the sun -- but off its center -- called the barycenter of the solar system. The inner four planets consist chiefly of iron and rock. They are known as the terrestrial (earthlike) planets because they are somewhat similar in size and composition.

The outer planets, except for Pluto, are giant worlds with thick, gaseous outer layers. Almost all their mass consists of hydrogen and helium, giving them compositions more like that of the sun than that of Earth. Beneath their outer layers, the giant planets have no solid surfaces. The pressure of their thick atmospheres turns their insides liquid, though they may have rocky cores. A distant object called Pluto has been referred to as the ninth planet since its discovery in the 1930's. But Pluto has so many unusual features that some astronomers think it may not be a planet at all. For example, it travels around the sun in an elongated oval path much different from the nearly circular orbits of the other planets. Unlike the other outer planets, Pluto is small and solid. But Pluto contains only 1/500 the mass of Earth. During the 1990's, astronomers discovered dozens of small rocky objects orbiting the sun beyond Neptune and Pluto. Astronomers had long suspected that the outer solar system had such a band of rocky material, called the Kuiper (KY pur) belt. The belt is named for the Dutch-born American astronomer Gerard P. Kuiper, who first predicted its existence.

Pluto may merely be the largest of the objects in the Kuiper belt. Moons orbit all the planets except Mercury and Venus. The inner planets have few moons. Earth has one, and Mars has two tiny satellites. The giant outer planets, however, resemble small solar systems, with many moons orbiting each planet. Jupiter has 25 moons that are at least 6 miles (10 kilometers) in diameter, and many smaller satellites. Jupiter's four largest moons are known as the Galilean satellites because the Italian astronomer Galileo discovered them in 1610 with one of the first telescopes. The largest Galilean satellite -- and the largest satellite in the solar system -- is Ganymede, which is even bigger than Mercury and Pluto. Saturn has 25 moons that are at least 6 miles (10 kilometers) in diameter, and several smaller satellites. The largest of Saturn's moons, Titan, has an atmosphere thicker than Earth's and a diameter larger than that of Mercury or Pluto. Uranus has 21 moons, and Neptune has 11. The giant planets probably have more small moons not yet discovered. Pluto has one moon.

The Planets and Their Largest Satellites: Diameter What is a Planet? Definition, Debate, Dwarf Planets, Kuiper Belt - Windows to the Universe News from NSF: Small Planet, Small Star (6/2/2008) What is a planet? It may surprise you, but astronomers don't really have a good definition of a "planet". Because of this, Pluto is at the heart of a controversy about its status. Is Pluto a planet, or isn't it? Scientists agree that the four rocky or "terrestrial" planets, Mercury , Venus , Earth and Mars , which orbit closest to the Sun , are all indeed planets. Likewise, the larger gas and ice giant planets, Jupiter , Saturn , Uranus and Neptune , are universally considered planets. But when we travel outward beyond the orbit of Neptune, to the icy fringes of our Solar System , the issue of planethood becomes a bit fuzzy. Thousands of huge balls of ice orbit the Sun beyond Neptune. Many of these are in a region of space called the Kuiper Belt . When Pluto was discovered in 1930, scientists had not yet found the Kuiper Belt.

Since, at the time of its discovery, Pluto seemed to be a unique new object, astronomers recognized it as a planet. Since the first detection of a Kuiper Belt Object (KBO) in 1992, hundreds more KBOs have been found . Several are nearly as large as Pluto, and one appears to be even larger than "the ninth planet". Some astronomers believe we may eventually find a dozen KBOs larger than Pluto. If Pluto had been discovered in recent years, it would have been classified as merely another large KBO. So what is a planet? Some astronomers believe all KBOs, including Pluto, should not be considered planets. They point out that even Mercury, the 2nd smallest of the nine planets, has a volume more than eight times as large as Pluto's and is over 26 times as massive. This group believes that KBOs, including Pluto, are too small to be considered planets, and that we thus have only eight true planets in our Solar System. Other scientists think that large KBOs should also be considered planets. If you count only objects as large as Pluto, then the large KBO with the temporary name 2003 UB313 may be the tenth planet .

Over time we may find more KBOs larger than Pluto, possibly pushing the number of planets in our Solar System to 20 or higher. If you consider KBOs that are almost as big as Pluto planets, then we already have several (to dozens or hundreds, depending on how "almost as big" you mean) new planets in our system, and will surely add many more in the future as more KBOs are spotted. Some scientists have proposed calling "planets" of this variety "ice dwarf" planets. Finally, some astronomers think that Pluto should maintain "honorary planet" status for historical reasons, but that other new KBO discoveries should not be considered "true planets". The International Astronomical Union (IAU), the official body charged with deciding such issues, is currently debating the appropriate definition of a planet. Some scientists think that the exact definition isn't important; that recognizing the types of bodies in our Solar System and understanding the history of each type of object is the more important than defining whether a given object is or is not a planet. So what is a planet? There are a couple of points astronomers agree on. A planet must directly orbit a star.

This means that moons , which orbit planets instead of stars, are not themselves planets. Secondly, planets must be large enough that gravity makes them spherical in shape. Small, odd-shaped asteroids are left out of the planet club by this criterion. Beyond these two aspects, the definition of a planet is not universally agreed upon. How big can a planet be? Some of the gas giant planets that have been detected orbiting other stars are very large, much bigger than Jupiter. Low-mass "failed stars", known as brown dwarf stars, have masses similar to those of extremely large gas giant planets. Thus the boundaries of planethood are fuzzy at both the high and low ends of the planetary size scale. Some astronomers feel that the history and evolution of an object within a planetary system is a key to whether the object is or is not a planet. For example, some scientists say that objects which " sweep clean " their orbits of other debris left over from planetary system formation are planets; objects (like those in the Kuiper Belt) that do not clear other debris away are not planets, according to this viewpoint. As you can see, defining "planet" is more difficult than it might first appear.

Until the IAU releases an official pronouncement, competing definitions of "planet" seem likely to remain. Last modified November 30, 2007 by Randy Russell . Shop Windows to the Universe Science Store! The Winter 2010 issue of The Earth Scientist , which includes articles on meteor cratering, classroom glaciers, podcasts in the classroom, and pyro-cumulonimbus clouds, is available in our online store . Windows to the Universe Community News

Question: What object officially joined our solar system on 1st May 1930?
Answer: Pluto