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. The Solar System The Solar System There are nine planets in the solar system. The following information can help you learn more about each of them. MERCURY The planet nearest to the sun, Mercury has a cratered surface and almost no atmosphere.

It is the smallest of the inner planets and has no known satellites. It revolves around the sun in about 88 days. VENUS This is the second planet from the sun, and its atmosphere is mainly composed of carbon dioxide. It revolves around the sun in 224.7 days. The entire surface of the planet is covered by a dense layer of clouds. EARTH Earth, the fifth largest planet in the solar system, is the only one known to support life. It revolves around the sun in about 365 days. Estimated to be 4.5 billion to 5 billion years old, this planet has one natural satellite, the moon. MARS The next planet out from Earth, Mars revolves around the sun in about 687 days. It has many volcanoes, including the largest in the solar system, Olympus Mons. JUPITER Jupiter is the largest and most massive of the nine planets. It is the fifth planet away from the sun and has 16 known natural satellites. SATURN Revolving around the sun in about 29.5 years, Saturn is the second largest planet. Its most remarkable feature is its ring system, which is composed of billions of water-ice particles.

URANUS This is the seventh planet from the sun, revolving around it about every 84 years. It was the first planet discovered in modern times with the aid of a telescope. NEPTUNE Almost 3 billion miles from the sun, Neptune revolves around the sun every 164.8 years. Its mass is 17.2 times that of Earth. PLUTO Pluto, with a rather elliptical orbit around the sun, has a diameter of less than half that of Earth. Its surface consists mainly of frozen nitrogen. It has one known satellite, Charon. Planets Song for Kids / Solar System Song - YouTube Planets Song for Kids / Solar System Song Want to watch this again later? Sign in to add this video to a playlist. Need to report the video? Sign in to report inappropriate content. Rating is available when the video has been rented. This feature is not available right now. Please try again later. Published on Mar 9, 2015 This is a song for children about the 8 planets in our solar system. It's colorful, fun and a great learning tool for toddlers, kindergartners and parents. Please sing along and enjoy. Add us on Google+: ...

LYRICS: There are eight planets in our solar system We all revolve around the sun Join us to learn about the different planets Now sing along and have some fun My name is Mercury I’m the second hottest planet The closest one to the sun A year on my surface is 88 days I’m the smallest but I’m lots of fun My name is Venus I’m the hottest planet But the second planet from the sun I’m the brightest planet in our solar system And I’m to hot for any one My name is Earth I’m the planet you live on The third Planet from the sun I’m the only planet with organic life So take care of me because we’re all one My name is mars I am red in color The fourth planet from the sun I have the highest mountain in our solar system A volcano named Olympus Mons There are eight planets in our solar system We all revolve around the sun Join us to learn about the different planets Now sing along and have some fun My name is Jupiter I am covered in clouds I’m the fifth planet from the sun My giant red spot is a raging storm As for size I’m the biggest one My name is Saturn I am brown in color I’m the sixth planet from the sun My outer rings are extremely thin They’re made of

 dust and icy chunks My names Uranus I am blue in color I’m the seventh planet from the sun Humans have named me the icy planet Because I am the coldest one My name is Neptune I am blue in color I’m the eighth planet from the sun I have to many storms in my atmosphere And I’m the furthest planet from the sun There are eight planets in our solar system We all revolve around the sun Join us to learn about the different planets Now sing along and have some fun Category The Solar System The Solar System Our Solar System is an amazing place.

Not only is it home to eight planets, it also hold several dwarf planets, hundreds of moons, and thousands of stars and asteroids. The eight planets in our Solar System are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. These planets orbit the Sun. Here is a funny song to help you remember the planets The Sun The Sun is the center of our Solar System. It is because of the Sun that our solar system gets its name. Sol is the Roman name for Sun. The Sun is the largest object within the Solar System - it makes up more than 95% of all the matter! It is because of the Sun's large size that the rest of the objects within the Solar System orbit around it. The Sun is very, very hot. It is 10,000 degrees Fahrenheit on its surface. Mercury Mercury is the closest planet to the Sun, making it the first planet in the Solar System. It is named after the Roman messenger god. In mythology, the god was known for being quick - Mercury is the fastest of all the planets to orbit the Sun, because it has the smallest route it has to take!

Mercury can occasionally be seen from Earth Mercury is the second densest planet in the Solar System It is mainly made up of iron There is no atmosphere on Mercury Despite its fast trip around the Sun, Mercury's revolution is very slow - one day on Mercury equals about 59 days on Earth! Venus Venus is the second planet from the Sun. It is named after the Roman goddess of beauty and love. Venus is almost the same size as Earth, but other than size the two planets are very different! Fun Facts about Venus Venus appears to be the brightest planet in the sky due to the reflection of the Sun from its clouds, and its closeness to Earth Venus has an incredibly thick atmosphere. This makes it the hottest of the inner planets Venus rotates East to West Venus is covered in volcanoes Venus does not have any moons Earth Earth is the only known planet that has any life forms in the Solar System. It is the third planet from the Sun. It is the only planet that is not named after a god. Most of the Earth is covered in water - 70%!

Earth has a thin atmosphere, that protects us from dangerous gasses and heat in space Earth is the largest of the inner planets Air, water, and land make up the Earth Earth's axis is tilted at 23.5 degrees This a video showing the many different life forms that Earth has Mars Mars is the fourth planet from the Sun. Named for the Roman god of war, it is also known as the "Red Planet". It is the last of the inner planets. Temperatures on Mars are very similar to temperatures on Earth It is believed that Mars once had water on its surface Mars is called the red planet because its surface is covered in red dust The largest volcano in the Solar System is on Mars. It is called Olympus Mons Mars has two small moons Jupiter Jupiter is the first of the outer planets, and the fifth planet from the Sun. It is named for the king of the Roman gods. It is the largest of all of the planets. Fun Facts about Jupiter Jupiter is best known for its "Great Red Spot" - a mass of hurricanes that have been storming for hundreds of years.

The spot is larger in size than that of Earth Jupiter is made up of gasses - there is no solid surface on it Jupiter is so big; Earth could fit inside of it more than 1,000 times! Jupiter has over 50 moons. Four of these moons are as big as planets Jupiter has the most active volcanic activity in the Solar System Learn more about the Great Red Spot! Saturn Saturn is the sixth planet from the Sun. It is named for the Roman god of agriculture. It is most well known for its highly visible rings. Other Features of the Solar System There are many other exciting elements in the Solar System. Here are a few. Asteroids Asteroids are rocky bodies in the Solar System that are too small to be considered actual planets. In between Mars and Jupiter lies the Asteroid Belt, containing well over 90,000 asteroids. Comets Comets are ice and rock that grow tails the closer they get to the sun. The hotter they get, the longer this "tail" becomes. The sunlight makes comets glow, which causes them to be visible on Earth. Dwarf Planets Dwarf planets are bodies that orbit the sun that are not considered large enough to be actual planets.

The most famous dwarf planet is Pluto, which was considered an actual planet up until 2006. Satellites Satellites are machines put into the Solar System by scientists in order to find out more about the Solar System, and what is in it. These machines take pictures, perform experiments, take data, and much, much more. Without them, we wouldn't have the vast amount of information that we have today. Summary The Solar System is a fascinating place. Each of the eight planets have their own unique features, but are also similar in some ways - much like people! It is amazing to think that the Solar System has been around for more than 4.6 billions years. Each of the eight planets orbits the sun in the same direction, and in their own time. We have so much more we can learn about the Solar System, and so much we have learned already today. sit this page to learn more interesting facts about the Solar System! Want to get an idea of how far apart the planets are? Click here! Resources Our Solar System. Retrieved from Solar System. Retrieved from Jupiter - Educational facts and history of the planet Jupiter. Jupiter News Jupiter Facts Jupiter is the fifth planet from the Sun and by far the largest.

Jupiter is more than twice as massive as all the other planets combined (the mass of Jupiter is 318 times that of Earth). Planet Profile orbit : 778,330,000 km (5.20 AU ) from Sun diameter : 142,984 km (equatorial) mass : 1.900e27 kg History of Jupiter Jupiter (a.k.a. Jove; Greek Zeus ) was the King of the Gods, the ruler of Olympus and the patron of the Roman state. Zeus was the son of Cronus (Saturn). Jupiter is the fourth brightest object in the sky (after the Sun , the Moon and Venus ). It has been known since prehistoric times as a bright "wandering star". But in 1610 when Galileo first pointed a telescope at the sky he discovered Jupiter's four large moons Io , Europa , Ganymede and Callisto (now known as the Galilean moons) and recorded their motions back and forth around Jupiter. This was the first discovery of a center of motion not apparently centered on the Earth.

It was a major point in favor of Copernicus 's heliocentric theory of the motions of the planets (along with other new evidence from his telescope: the phases of Venus and the mountains on the Moon ). Galileo's outspoken support of the Copernican theory got him in trouble with the Inquisition . Today anyone can repeat Galileo's observations (without fear of retribution :-) using binoculars or an inexpensive telescope. Jupiter was first visited by Pioneer 10 in 1973 and later by Pioneer 11 , Voyager 1 , Voyager 2 and Ulysses . The spacecraft Galileo orbited Jupiter for eight years. It is still regularly observed by the Hubble Space Telescope . The gas planets do not have solid surfaces, their gaseous material simply gets denser with depth (the radii and diameters quoted for the planets are for levels corresponding to a pressure of 1 atmosphere). What we see when looking at these planets is the tops of clouds high in their atmospheres (slightly above the 1 atmosphere level).

Jupiter is about 90% hydrogen and 10% helium (by numbers of atoms, 75/25% by mass) with traces of methane, water, ammonia and "rock". This is very close to the composition of the primordial Solar Nebula from which the entire solar system was formed. Saturn has a similar composition, but Uranus and Neptune have much less hydrogen and helium. Our knowledge of the interior of Jupiter (and the other gas planets) is highly indirect and likely to remain so for some time. (The data from Galileo 's atmospheric probe goes down only about 150 km below the cloud tops.) Jupiter probably has a core of rocky material amounting to something like 10 to 15 Earth-masses. Above the core lies the main bulk of the planet in the form of liquid metallic hydrogen. This exotic form of the most common of elements is possible only at pressures exceeding 4 million bars , as is the case in the interior of Jupiter (and Saturn). Liquid metallic hydrogen consists of ionized protons and electrons (like the interior of the Sun but at a far lower temperature). At the temperature and pressure of Jupiter's interior hydrogen is a liquid, not a gas.

It is an electrical conductor and the source of Jupiter's magnetic field. This layer probably also contains some helium and traces of various " ices ". The outermost layer is composed primarily of ordinary molecular hydrogen and helium which is liquid in the interior and gaseous further out. The atmosphere we see is just the very top of this deep layer. Water, carbon dioxide, methane and other simple molecules are also present in tiny amounts. Recent experiments have shown that hydrogen does not change phase suddenly. Therefore the interiors of the jovian planets probably have indistinct boundaries between their various interior layers. Three distinct layers of clouds are believed to exist consisting of ammonia ice, ammonium hydrosulfide and a mixture of ice and water. However, the preliminary results from the Galileo probe show only faint indications of clouds (one instrument seems to have detected the topmost layer while another may have seen the second). But the probe's entry point (left) was unusual -- Earth-based telescopic observations and more recent observations by the Galileo orbiter suggest that the probe entry site may well have been one of the warmest and least cloudy areas on Jupiter at that time. Data from the Galileo atmospheric probe also indicate that there is much less water than expected.

The expectation was that Jupiter's atmosphere would contain about twice the amount of oxygen (combined with the abundant hydrogen to make water) as the Sun. But it now appears that the actual concentration much less than the Sun's. Also surprising was the high temperature and density of the uppermost parts of the atmosphere. Jupiter and the other gas planets have high velocity winds which are confined in wide bands of latitude. The winds blow in opposite directions in adjacent bands. Slight chemical and temperature differences between these bands are responsible for the colored bands that dominate the planet's appearance. The light colored bands are called zones; the dark ones belts. The bands have been known for some time on Jupiter, but the complex vortices in the boundary regions between the bands were first seen by Voyager. The data from the Galileo probe indicate that the winds are even faster than expected (more than 400 mph) and extend down into as far as the probe was able to observe; they may extend down thousands of kilometers into the interior. Jupiter's atmosphere was also found to be quite turbulent. This indicates that Jupiter's winds are driven in large part by its internal heat rather than from solar input as on Earth.

The vivid colors seen in Jupiter's clouds are probably the result of subtle chemical reactions of the trace elements in Jupiter's atmosphere, perhaps involving sulfur whose compounds take on a wide variety of colors, but the details are unknown. The colors correlate with the cloud's altitude: blue lowest, followed by browns and whites, with reds highest. Sometimes we see the lower layers through holes in the upper ones. The Great Red Spot (GRS) has been seen by Earthly observers for more than 300 years (its discovery is usually attributed to Cassini , or Robert Hooke in the 17th century). The GRS is an oval about 12,000 by 25,000 km, big enough to hold two Earths. Other smaller but similar spots have been known for decades. Infrared observations and the direction of its rotation indicate that the GRS is a high-pressure region whose cloud tops are significantly higher and colder than the surrounding regions. Similar structures have been seen on Saturn and Neptune . It is not known how such structures can persist for so long. Jupiter radiates more energy into space than it receives from the Sun.

The interior of Jupiter is hot: the core is probably about 20,000 K. The heat is generated by the Kelvin-Helmholtz mechanism, the slow gravitational compression of the planet. (Jupiter does NOT produce energy by nuclear fusion as in the Sun; it is much too small and hence its interior is too cool to ignite nuclear reactions.) This interior heat probably causes convection deep within Jupiter's liquid layers and is probably responsible for the complex motions we see in the cloud tops. Saturn and Neptune are similar to Jupiter in this respect, but oddly, Uranus is not. Jupiter is just about as large in diameter as a gas planet can be. If more material were to be added, it would be compressed by gravity such that the overall radius would increase only slightly. A star can be larger only because of its internal (nuclear) heat source. (But Jupiter would have to be at least 80 times more massive to become a star.) Jupiter has a huge magnetic field, much stronger than Earth's. Its magnetosphere extends more than 650 million km (past the orbit of Saturn!).

(Note that Jupiter's magnetosphere is far from spherical -- it extends "only" a few million kilometers in the direction toward the Sun.) Jupiter's moons therefore lie within its magnetosphere, a fact which may partially explain some of the activity on Io . Unfortunately for future space travelers and of real concern to the designers of the Voyager and Galileo spacecraft, the environment near Jupiter contains high levels of energetic particles trapped by Jupiter's magnetic field. This "radiation" is similar to, but much more intense than, that found within Earth's Van Allen belts. It would be immediately fatal to an unprotected human being. The Galileo atmospheric probe discovered a new intense radiation belt between Jupiter's ring and the uppermost atmospheric layers. This new belt is approximately 10 times as strong as Earth's Van Allen radiation belts. Surprisingly, this new belt was also found to contain high energy helium ions of unknown origin. J upiter has rings like Saturn's, but much fainter and smaller (right). They were totally unexpected and were only discovered when two of the Voyager 1 scientists insisted that after traveling 1 billion km it was at least worth a quick look to see if any rings might be present.

Everyone else thought that the chance of finding anything was nil, but there they were. It was a major coup. They have since been imaged in the infra-red from ground-based observatories and by Galileo . Unlike Saturn's, Jupiter's rings are dark ( albedo about .05). They're probably composed of very small grains of rocky material. Unlike Saturn's rings, they seem to contain no ice. Particles in Jupiter's rings probably don't stay there for long (due to atmospheric and magnetic drag). The Galileo spacecraft found clear evidence that the rings are continuously resupplied by dust formed by micrometeor impacts on the four inner moons , which are very energetic because of Jupiter's large gravitational field. The inner halo ring is broadened by interactions with Jupiter's magnetic field. In July 1994, Comet Shoemaker-Levy 9 collided with Jupiter with spectacular results (left). The effects were clearly visible even with amateur telescopes. The debris from the collision was visible for nearly a year afterward with HST. When it is in the nighttime sky, Jupiter is often the brightest "star" in the sky (it is second only to Venus, which is seldom visible in a dark sky).

The four Galilean moons are easily visible with binoculars; a few bands and the Great Red Spot can be seen with a small astronomical telescope. There are several Web sites that show the current position of Jupiter (and the other planets) in the sky. More detailed and customized charts can be created with a planetarium program . Jupiter's Satellites Jupiter has 67 known satellites : the four large Galilean moons plus many more small ones some of which have not yet been named: Jupiter is very gradually slowing down due to the tidal drag produced by the Galilean satellites. Also, the same tidal forces are changing the orbits of the moons, very slowly forcing them farther from Jupiter. Io, Europa and Ganymede are locked together in a 1:2:4 orbital resonance and their orbits evolve together. Callisto is almost part of this as well. In a few hundred million years, Callisto will be locked in too, orbiting at exactly twice the period of Ganymede (eight times the period of Io). Jupiter's satellites are named for other figures in the life of Zeus (mostly his numerous lovers). Many more small moons have been discovered recently but have not as yet been officially confirmed or named.

The most up to date info on them can be found at Scott Sheppard's site .

Distance Radius Mass Satellite (000 km) (km) (kg) Discoverer Date --------- -------- ------ ------- ---------- ----- Metis 128 20 9.56e16 Synnott 1979 Adrastea 129 10 1.91e16 Jewitt 1979 Amalthea 181 98 7.17e18 Barnard 1892 Thebe 222 50 7.77e17 Synnott 1979 Io 422 1815 8.94e22 Galileo 1610 Europa 671 1569 4.80e22 Galileo 1610 Ganymede 1070 2631 1.48e23 Galileo 1610 Callisto 1883 2400 1.08e23 Galileo 1610 Leda 11094 8 5.68e15 Kowal 19

74 Himalia 11480 93 9.56e18 Perrine 1904 Lysithea 11720 18 7.77e16 Nicholson 1938 Elara 11737 38 7.77e17 Perrine 1905 Ananke 21200 15 3.82e16 Nicholson 1951 Carme 22600 20 9.56e16 Nicholson 1938 Pasiphae 23500 25 1.91e17 Melotte 1908 Sinope 23700 18 7.77e16 Nicholson 1914 Values for the smaller moons are approximate.

Many more small moons are not listed here. Distance Width Mass Ring (km) (km) (kg) ---- -------- ----- ------ Halo 100000 22800 ? Main 122800 6400 1e13 Gossamer 129200 214200 ? (distance is from Jupiter's center to the ring's inner edge) More about Jupiter and its satellites Our Solar System May Have Lost a Fifth Giant Planet - TIME Follow @TIME If you think interplanetary space is a dangerous place now, with asteroids screaming by at close range and comets crashing into planets with the force of multiple H-bombs , be grateful you weren't here 4 billion years ago or so. Back then, our solar system was still getting itself organized. Planets caromed around like so many billiard balls, their gravity whipping smaller objects both outward into deep space and in toward the sun. One consequence of the latter: the Late Heavy Bombardment , which pummeled the inner planets with a barrage of asteroid strikes that gave the moon a goodly fraction of the craters we see today.

(Earth got slammed too, but erosion, submersion in the oceans and plate tectonics have cleaned up most of the scarring.) It wasn't just asteroids that got flung around, however: a paper just published in the Astrophysical Journal Letters argues that the young solar system started out not with four giant planets but five. Once upon a time, says lead author David Nesvorny, of the Southwest Research Institute in Boulder, Colo., Jupiter, Saturn, Uranus and Neptune had another sibling. But that fifth giant, perhaps as big as Neptune, perhaps only half that size, went too close to Jupiter  or Jupiter went close to it  and the nameless world was whipped out into space at a screaming 200,000 m.p.h. (322,000 km/h) or so. "After 4 billion years," says Nesvorny, in a masterpiece of understatement, "it's probably pretty far away." (See photos of an illustrated history of planet Earth.) The evidence for this long-departed planet comes out of complex computer simulations that try to reproduce the solar system as it used to be.

To qualify as plausible, such a model has to set up the conditions that could evolve into the configuration we see today. That means all the planets have to wind up in the right places, as does the Kuiper Belt of comet-like objects out beyond Neptune, and as does a second, bigger bunch of comets  the Oort Cloud  that stretches a good fraction of the way to Proxima Centauri, the nearest star. In 2005, a group of planetary astronomers came up with an idea that worked pretty well: the so-called Nice model (it was named not for its pleasant attributes, but for the French city of Nice, where most of the scientists were based). According to the Nice model, the four familiar giant planets were originally bunched up much closer than they are now. Thanks to millions of small encounters with comets and asteroids, and a few bigger ones with each other, Jupiter ended up moving inward, the other three moved out, the Late Heavy Bombardment happened on schedule, and the Oort Cloud and Kuiper Belt took shape just right. At least that's the way it worked some of the times the model was run.

To be really successful, however, a solar system model has to come up with the right answer over and over again, and when Nesvorny ran his own version of the simulation, it ended up throwing one of the four giant planets out into space. Or else it disrupted the inner planets, so Mars and Earth destroyed each other in a cosmic crash. Not ideal. (See if there is a giant hidden planet in our solar system.) One idea that had been floating around (so to speak) among planetary scientists, however, was the idea of the long-lost fifth giant planet, bunched up with the other four. "This result was not out of the blue sky," Nesvorny says. "Some of my colleagues even mentioned it in passing in the papers they published." Nesvorny added the fifth planet to his simulations  and it worked. More often than not, just as predicted, the extra planet was thrown into the void, a Neptune-size sacrificial lamb that changed things enough to keep Earth safe. If it happened here, of course, it could happen in other solar systems  and in fact, a report last spring claimed the fleeting detection of 10 rogue Jupiters wandering through the depths of the Milky Way.

"There's nothing physically mysterious about this," David Stevenson, a Caltech astronomer, told TIME in May. "It's a perfectly natural outcome." It's so natural, in fact, that Stevenson himself made an even more audacious proposition more than a decade ago. The smaller the planet, the easier it is to fling into deep space, so a planet with the mass of Earth could be catapulted out with little gravitational effort at all. The real planet Earth wouldn't fare well in the frigid darkness between the stars. But, says Stevenson, it's plausible that some Earths could be more like mini-Jupiters, with a thick atmosphere of mostly hydrogen. "Hydrogen," he says, "is well-known to be a greenhouse gas." On Earth, greenhouse gases trap the heat that flows in from the sun. In interstellar space, there is no solar heat. But there would still be heat rising from inside the planet, the result of radioactive minerals deep underground. "A hydrogen atmosphere," Stevenson says, "would trap that heat very efficiently." The result: while the top of the atmosphere would plunge to hundreds of degrees below zero, the surface would warm enough to keep water in liquid form.

Since water is essential to life, it's not impossible that such a vagabond Earth could harbor organisms  even intelligent life, as long as it could handle the noxious air. And since there are so many billions of stars in the Milky Way, there could be at least millions of these nicely insulated planets. Astronomers currently assume that their best bet for finding life in the universe is to look for an earthlike planet basking in the light of its parent star. Maybe they should be looking into the darkness instead. The Planets of the Solar System USOE Our solar system includes many different planets. In the drawing below, click your mouse on each planet to find its name and a short bit of information about that planet. Keep in mind that the following model of the solar system shows the correct order of the planets, but the sizes of the planets or their distances from each other are not drawn to scale. Notice the order of the planets. Make sure you view information for all the planets. Which planet is closest to the sun? Which planet is the farthest away from the sun? A good way to remember the order of the planets is to memorize the following sentence. My Very Educated Mother Just Served Us Nine Pizzas.

Do you get it? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto! Now it is time for you to take The Planet Quiz. Click on your choice for the correct answer. Need a hint, or want more information? Click on the photo of the planet to the left of the question. Photos courtesy of NASA . 1. What is the fourth planet away from the sun? 2. What planet is closest to the sun? 3. What planet is the third as you move away from the sun? 4. What planet is the farthest away from the sun? 5. What is the seventh planet as you move away from the sun? 6. What planet is the fifth as you move away from the sun? 7. What planet is the eighth as you move away from the sun? 8. What is the second planet as you move away from the sun? 9. What planet is the sixth as you go away from the sun? Check your answers by highlighting the box at the right! 1. Mars 2. Mercury 3. Earth 4. Pluto 5. Uranus 6. Jupiter 7. Neptune 8. Venus 9. Saturn

Question: In our solar system which is the 5th planet from the Sun?
Answer: Jupiter