The solar system we call home is located in an outer spiral arm of the vast Milky Way galaxy. The Universe is a very big place, and we occupy a very small corner of it. Known as the Solar System, our stomping grounds are not only a tiny fraction of the Universe as we know it, but is also a very small part of our galactic neighborhood (aka. the Milky Way Galaxy). When it comes right down to it, our world is just a drop of water in an endless cosmic sea.

The solar system consists of a central star, the Sun, and all of the smaller celestial bodies that continuously travel around it. The smaller bodies include eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, which are themselves orbited by more than 140 moons. (Only Mercury and Venus have no moons.) In addition, the solar system contains millions of rocky asteroids and billions of icy comets. All of these objects are held together in a group by the Sun’s gravity.

Mercury, Venus, Mars, Jupiter and Saturn can be seen with the unaided eye. That is how they were discovered by ancient civilizations. Uranus and Neptune were discovered using telescopes.

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.

“Cleared the neighborhood” means that the “planet” has to be the dominant gravitational body in their orbit around the sun.  This means that the “planet” has to cruise its orbit while consuming or slinging away smaller objects in its orbital path.  Pluto is only .007 times the mass of the other objects in its orbit.  Earth, in contrast, has 1.7 million times the mass of the other objects in its orbit.  Pluto simply isn’t big enough to “clear its neighborhood” properly.

Inner Solar System:

In the inner Solar System, we find the “Inner Planets” – Mercury, Venus, Earth, and Mars – which are so named because they orbit closest to the Sun. In addition to their proximity, these planets have a number of key differences that set them apart from planets elsewhere in the Solar System.

Composition Of The Solar System

The Sun contains 99.85% of all the matter in the Solar System. The planets, which condensed out of the same disk of material that formed the Sun, contain only 0.135% of the mass of the solar system. Jupiter contains more than twice the matter of all the other planets combined. Satellites of the planets, comets, asteroids, meteoroids, and the interplanetary medium constitute the remaining 0.015%. The following table is a list of the mass distribution within our Solar System.

Sun: 99.85%

Planets: 0.135%

Comets: 0.01% ?

Satellites: 0.00005%

Minor Planets: 0.0000002% ?

Meteoroids: 0.0000001% ?

Interplanetary Medium: 0.0000001% ?


The eight bodies officially categorized as planets are often further classified in several ways:

By composition:

terrestrial or rocky planets: Mercury, Venus, Earth, and Mars

The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.

jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:

The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.


By size:

  • small planets: Mercury, Venus, Earth, Mars.
  • The small planets have diameters less than 13000 km.


  • giant planets: Jupiter, Saturn, Uranus and Neptune.
  • The giant planets have diameters greater than 48000 km. The giant planets are sometimes also referred to as gas giants.


By position relative to the Sun:

inner planets: Mercury, Venus, Earth and Mars.

outer planets: Jupiter, Saturn, Uranus, Neptune.


The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.


By positionrelative to Earth:

  • inferior planets: Mercury and Venus.
  • closer to the Sun than Earth.
  • The inferior planets show phases like the Moon’s when viewed from Earth.
  • Earth.


  • superior planets: Mars thru Neptune.
  • farther from the Sun than Earth.
  • The superior planets always appear full or nearly so.


By history:

  • classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
  • known since prehistorical times
  • visible to the unaided eye
  • in ancient times this term also referred to the Sun and the Moon; the order was usually specified as: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, based on the time for them to go “all the way round” the sphere of the “fixed” stars).


  • modern planets: Uranus, Neptune.
  • discovered in modern times
  • visible only with optical aid
  • Earth.


The IAU decided that “classical” should refer to all eight planets (Mercury thru Neptune, including Earth but not Pluto). This is contrary to historical usage but makes some sense from a 21st century perspective.

The Solar System formed 4.568 billion years ago from the gravitational collapse of a region within a large molecular cloud composed of hydrogen, helium, and small amounts of heavier elements fused by previous generations of stars. As the region that would become the Solar System (known as the pre-solar nebula) collapsed, conservation of angular momentum caused it to rotate faster.

The center, where most of the mass collected, became increasingly hotter than the surrounding disc. As the contracting nebula rotated faster, it began to flatten into a protoplanetary disc with a hot, dense protostar at the center. The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies.

The Solar System will remain roughly as we know it today until the hydrogen in the core of the Sun has been entirely converted to helium. This will occur roughly 5 billion years from now and mark the end of the Sun’s main-sequence life. At this time, the core of the Sun will collapse, and the energy output will be much greater than at present.

The outer layers of the Sun will expand to roughly 260 times its current diameter, and the Sun will become a red giant. The expanding Sun is expected to vaporize Mercury and Venus and render Earth uninhabitable as the habitable zone moves out to the orbit of Mars. Eventually, the core will be hot enough for helium fusion and the Sun will burn helium for a time, after which nuclear reactions in the core will start to dwindle.

At this point, the Sun’s outer layers will move away into space, leaving a white dwarf – an extraordinarily dense object that will have half the original mass of the Sun, but will be the size of Earth. The ejected outer layers will form what is known as a planetary nebula, returning some of the material that formed the Sun to the interstellar medium.


The Sun

  • The sun is 92% hydrogen, 7.8% helium.
  • Our sun is only 4.5 billion years old and is only half way through its life cycle of 10 billion years. That means, in. about 5 billion years, the sun would grow to such an enormous extent that it would even en – gulf Earth.
  • The sun’s core temperature is over 15 million degrees Celsius.
  • The sun has a diameter of 863,700 miles ( 1,390,000km ). Also, it is over 2.5 million miles around at its equator.
  • The sun also rotates, but different parts of the sun rotate at different rates. The equator of the sun completes one rotation in 25 days while the poles take 36 days for a full rotation.
  • The Sun does not explode or collapse in on itself, because the gravitational pressure form its mass is proportional to the outward pressure of the expanding gases.
  • Every eleven years the magnetic poles of the sun switch. This cycle is called Solarmax.
  • The sun is so large it takes several thousand years for the energy released from the fusion process in the core, to teach the surface as light or radiation. It takes only 8 minutes for sunlight to travel from the sun to the earth, which also means that, when the sun sets, it actually set 8 minutes ago. In Spit Bergen, Norway at one time of the year the sun shines continuously for three and a half months. More than 1 million earths would fit inside the sun.
  • The sun is 330,330 times larger than the earth.
  • The force of Gravity is so much greater on the sun that a man Weighing 150 pounds on earth would weight 2 tons on the sun. The Sun is also the source of energy ( as in light and heat ) for Earth. The Sun provides our planet with 126,000,000,000,000 horsepower of energy every day.
  • The sun generates energy through fusion of its hydrogen atoms. We know that the sun is made of hydrogen because every element has a distinct spectrum and on earth we know the spectrum of Hydrogen. Through observations of the sun with a spectrograph, we could conclude that the sun is made of Hydrogen.
  • No solar eclipse can last longer than 7 minutes 58 seconds because of the speed at which the sun moves.
  • The Sun is the closest star to Earth and is the center of our solar system. A giant, spinning ball of very hot gas, the Sun is fueled by nuclear fusion reactions. The light from the Sun heats our world and makes life possible. The Sun is also an active star that displays sunspots, solar flares, erupting prominences, and coronal mass ejections. These phenomena impact our near – Earth space environment and determine our space weather.
    Solar atmosphere : The visible solar atmosphere consists of three regions : the photosphere, the chromosphere, and the solar corona. Most of the visible ( white ) light comes from the photosphere. This is the part of the Sun we actually see. The chromosphere and corona also emit white light, and can be seen when the light from the photosphere is blocked out, as occurs in a solar eclipse.
  • The sun emits electromagnetic radiation at many other wavelengths as well. Different types of radiation ( such as radio, ultraviolet, X – rays, and gamma rays ) originate from different parts of the sun. Scientists use special instruments to detect this radiation and study different parts of the solar atmosphere.
  • The solar atmosphere is so hot that the gas is primarily in a plasma state, i.e., electrons are no longer bound to atomic nuclei, and the gas is made up of charged particles ( mostly protons and electrons ).
  • In this charged state, the solar atmosphere is greatly influenced by the strong solar magnetic fields that thread through it. These magnetic fields, and the outer solar atmosphere ( the corona ) extend out into interplanetary space as part of the solar wind.
  • Rising above the Sun’s chromosphere, the temperature jumps sharply from a few tens of thousands degrees Kelvin to as much as a few million degrees in the Sun’s outer atmosphere, the solar corona.
  • Understanding the reason, why the Sun’s corona is so hot, is one of the many challenges facing solar physicists today.


  • The Moon
  • The moon is the only satellite of the earth.
  • The Moon keeps one side permanently turned towards Earth.
  • The moon orbits the Earth at an average speed of 2,300 miles an hour ( 3,700 kilometers an hour ).
  • The Moon is the only surface in the Solar System ( other than Earth ) to have been stepped on by human beings.
  • Footprints on the Moon will remain there for millions of years.
  • The Moon is the fourth biggest moon in the Solar System ( after Ganymede and Callisto of Jupiter, and Titan of Saturn ).
  • It takes about 1.25 seconds for moonlight to reach the Earth.
  • The Moon is also larger than the dwarf planets Pluto and Eris. It is the 14th largest known object in the Solar System.
  • Selenophobia is the name given for the persistant and irrational fear of the Moon.
  • The Moon is Earth’s natural satellite. The Moon, unlike the Earth, has no atmosphere and has a weaker gravitational force because its mass is only about 1% of Earth’s mass.
  • The surface of the Moon is covered with craters, basins, and cratered highlands, called terrae.
  • The soil that covers the Moon is called regolith, and is composed of rock fragments and fine dust grains.
  • The maria ( Latin for seas ) are lowland plains that were once craters that were later flooded by lava. From Earth the terrae appear bright and the maria are dark.
  • Exploration : The Moon has been visited by several spacecrafts since 1959. The first Moon landing was made by Apollo 11, on July 20,1969 ( Apollo 11 was launched on 16th July 1969 ).
  • Neil Armstrong and Buzz Aldrin were the first human beings to walk on an extraterrestrial body.
  • The last manned moon landing was three years later on 11th December 1972 ( Apollo 17 was launched on 7th December 1972 ). A series of 12 Americans have so far landed on the moon.
  • Our Moon is bigger than Ruto. And at roughly one-fourth the diameter of Earth, some scientists think the Moon is more like a planet. They refer to the Earth – Moon system as a “double planet.” Pluto and its moon Charon are also called a double – planet system by some.
  • Same face : Moon takes the same amount of time for its rotation and revolution, about 27.3 days. This means that the moon always shows, the same face to the Earth.
  • The surface gravity of the moon is only one – sixth that of the Earth. The force gravity exerts, on a person determines the person’s weight. Even though your mass would be the same on Earth and the moon, if you weigh 132 pounds ( 60 kilograms ) on Earth, you would weight about 22 pounds ( 10 kilograms ) on the moon.
  • The Moon has no atmosphere. It is through atmosphere that light is dispersed ( spread ) and sound waves travel. This means that no sound can be heard on the Moon. The sky is always black because light cannot be spread, unlike on Earth where light spreads to give the sky a blue colour and on Mars where it spreads through its thin atmosphere to give the sky a pinkish / reddish colour.


  • Planets
  • Planets, meaning wanderers, are named after Roman deities : Mercury, messenger of the gods; Venus, the god of love and beauty; Mars, the god of war; Jupiter, king of the gods; Saturn, father of Jupiter and god of agriculture; Uranus, god of sky; Neptune, god of the sea.
  • The three most recently discovered planets were Uranus in 1781, Neptune in 1846, and Pluto in 1930. Venus and Uranus are the only planets that rotate opposite to the direction of their orbit.
  • All the planets in the solar system rotate anticlockwise, except Venus and Uranus. Both are the only planet that rotates clockwise.
  • International Astronomical Union ( IAU ) members, gathered at the 2006 General Assembly, agreed that a ‘planet’ is defined 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. .


  • Mercury
  • It is the closest planet to the sun.
  • Mercury is also one the brightest planets visible to us on earth. However, since it is so close to the sun it is only visible just before sunrise and sunset.
  • At its closest point to the sun ( perihelion ) it is 29 million miles or 47 million kilometers away. The aphelion, the farthest position of orbit, is 44 million miles ( 71 million km ).
  • Mercury does not have an atmosphere but does contain a thin atmospheric layer of sodium and helium.
  • An asteroid approximately 62 miles wide struck mercury and created an impact basin about 810 miles wide. The basin is called the Caloris Basin and was once covered with molten lava.
  • The surface temperature on Mercury varies more than 630 degrees Celsius ( 1,130°F ). Before sunrise, Mercury is at – 300°F ( – 184°C ) and at noon, it is almost 800°F ( 427°C ).
  • Since Mercury has a highly elliptical orbit, the view of the sun would be strange. The sun would seem to stop moving east to west and reverse directions, then reverse again.
  • Mercury is the innermost and smallest planet in the solar system ( before Pluto was renamed as a dwarf planet, it was the second smallest planet of our solar system ), orbiting the Sun once every 88 days. The Romans named the planet after the Roman god Mercurius, equated to the Greek Hermes.
  • Natural satellites : Mercury has no moon. Exploration : Mercury was visited by the Mariner 10 space – craft that returned images on March 29, 1974.


  • Venus
  • Venus is the brightest planet of our solar system. Venus is the closest planet to earth and is also similar in size.
  • It is one of the most hostile planets in the solar system. The surface temperature can reach over 900 degrees F ( 480 C ).
  • Venus is the hottest planet in the solar system because the atmosphere in Venus consists of dense gases and mostly Carbon dioxide ( 96% ) which traps the heat inside.
  • Venus is covered by an atmosphere of sulphur dioxide, which is why it has yellow color. It even rains sulphuric acid.
  • Venus takes more time in rotation than in revolution.
  • There is a very low probability of life existing on Venus because of the extreme heat. Not only would the heat prevent life from developing, the atmospheric pressure on Venus is so strong, it would easily crush any living organism found on earth. Also water is only found as vapor on Venus.
  • Venus is the second – closest planet to the Sun. It is the brightest natural object in the night sky, except for the Moon. It is sometimes called Earth’s ‘sister planet’, for the two are similar in size, gravity, and bulk composition. It’s named after the Greek goddess of love, Venus.
  • Natural satellites : Venus has no moon. Exploration : The Soviet Union managed to send several satellites called the Venera series into Venus. Some were destroyed easily and the Venera 4 was the first to send back data before succumbing to Venus’s hostile environment.
  • The Venera 7 actually landed on the surface and sent the first spectacular images of Venus’ surface. The United States sent the Mariner series of satellites to explore Venus and also managed to send back data of its atmospheric contents.
  • The Magellan spacecraft, launched in August 1990 was of America’s most successful ventures to Venus. It revealed many features of Venus with high – resolution radar images and orbited Venus over 15,000 times and mapped over 98% of the surface. The Magellan on October 12, 1994 finally succumbed to its death as it burned up in Venus’ atmosphere.


  • Earth
  • The only planet in the solar system to support life.
  • On its trip around the sun, the earth travels over a million and a half miles per day.
  • Earth moves along its orbit at 30 km per second, covering its own diameter in 7 minutes.
  • A speed of 6.95 miles per second is required to escape the Earth’s gravitational pull.
  • Earth is the densest planet in the solar system and the only one not named after a god.
  • One year on earth is 365.26 days long. One day is 23 hours, 56 minutes, and 4 seconds long.
  • The extra day in a leap year was introduced to compensate for the discrepancy in the Gregorian calendar.
  • Earth is slowing down – in a few million years there won’t be a leap year.
  • There is zero gravity at the centre of earth.
  • Almost two – thirds of the earth’s surface is covered by water. If the earth were flat, water would cover everything in a layer two miles deep.
  • Earth is the third planet from the Sun and is the largest of the terrestrial planets in the Solar System in both diameter and mass. Earth, as seen from satellite images is a beautiful, bright blue planet filled with patches of white water vapor ( clouds ) that cover about 40% of the earth.
  • Scientists theorize that earth was formed 4.5 billion years ago when many other planets formed. Earth takes about 365 days to orbit the sun and completes one rotation in approximately 24 hours.
  • Earth’s orbit is somewhat elliptical with only a 3% difference from a circular orbit. The tilt of the earth’s axis is 23.5 degrees, which accounts for the seasons on earth. The tilt causes certain parts of the planet to be heated differently thus causing seasonal changes.


  • Mars
  • Many scientists believe there could be a possibility of life on Mars, but not complex, just microorganisms.
  • Beneath its thin atmosphere, Mars is barren, covered with pink soil and boulders.
    It is the most similar planet to earth. Mars completes one rotation in 24.6 hours. Its axis is tilted 25.2°, while earth’s is 23.5°.
  • Mars is also very different from earth. Mars’ orbit is much more elliptical than earth’s. As a matter of fact, Mars’ orbit is the third most elliptical.
  • Unlike Earth’s atmosphere, Mars’ atmosphere is made up of 95.3% carbon dioxide and only 0.2% oxygen.
  • Mars is the fourth planet from the Sun in the Solar System. It is named after Mars, the Roman god of war it is also referred to as the ‘Red Planet’ because of its reddish appearance as seen from Earth.
  • Natural satellites : Mars has two moons, Phobos and Deimos.
  • Exploration : Three probes which landed on Mars are : the Viking 1, which landed on Mars on July 20, 1976. The Viking 2 also landed successfully in September 1976.
  • Mars Pathfinder landed in 1977. Mars is currently host to three functional orbiting spacecraft – Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter. This is more than any planet except Earth. The surface is also home to the two Mars Exploration Rovers – Spirit and Opportunity.


  • Jupiter
  • An unusual phenomenon of Jupiter is the speed of its rotation. It completes one rotation in only 9 hours and 55 minutes.
  • A distinctive feature of Jupiter is the Great Red Spot found on the Southern Hemisphere of the planet. It has been found that this great red, spot is in fact a giant hurricane that rotates counterclockwise.
  • Jupiter’s moon Ganymede is the largest moon in the Solar System, and is larger than the planets Mercury and Pluto.
  • Jupiter is the fifth planet from the Sun and the largest planet within the solar system. Over 1000 earths could fit inside Jupiter. It is two and a half times as massive as all of the other planets in our solar system combined. It is named after the Roman god, Jupiter, the god of light and sky.
  • Natural satellites : Jupiter has 17 known satellites ( moons ) : 10, Europa, Ganymede, Callisto, Metis, Adrastea, Amalthea, Thebe, Leda, Himalia, Lysithea, Elara, Ananke, Carme, Pasiphae, Callirrhoe, Sinope. The four largest moons are called Galilean Satellites: Ganymede, Callisto, 10 and Europa, in the order. Some scientists claim that Jupiter has 63 satellites, including the recent findings.
  • Exploration : The Pioneer 10 sent back images in December, 1972. Pioneer 11 also flew by Jupiter and sent back data on its atmospheric composition and information about Jupiter’s moons. The Voyager missions also sent back data from Jupiter. The Galileo spacecraft reached Jupiter in December 1995 and was the first to orbit Jupiter.


  • Saturn
  • Titan is the largest of Saturn’s moons. It is the second largest moon in the solar system. In fact, it is larger than both Mercury and Pluto.
  • Of all the satellites, Titan has an atmosphere and is believed to contain organic molecules. The atmosphere is 80% nitrogen and contains common galsses found on earth such as methane, ethane, argon, and hydrogen.
  • Saturn is the sixth planet from the Sun and the second largest planet in the Solar System after Jupiter. It is named after the Roman god Saturn us, equated to the Greek Kronos ( the Titan father of Zeus ). Even though Saturn is so large, it has the lowest density in the solar system.
  • The most prominent feature of Saturn is its rings. Saturn also rotates extremely, fast which causes the bulges at its equator. Saturn rotates once in 10 hours and 40 minutes.
  • Natural satellites : Saturn has about 30 discovered moons, though some scientists put the figure at 60. Some of them are : Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, lapetus, Pan, Atlas, Prometheus, Pandora, Epimetheus, Janus, Calypso, Telesto, Helene, Phoebe, Daphnis, Methone, Pallene, Poly deuces, Paaliaq and Ymir.
  • Exploration : The Pioneer 11 was the first spacecraft to reach Saturn and it sent back some images of Saturn on September 1, 1979. Both Voyagers photographed Saturn. The next spacecraft, to study Saturn and its moons was Cassini.


  • Uranus
  • Not unlike Jupiter and Saturn, Uranus also has rings.
  • Uranus is covered in a light blue color because of the methane clouds on Uranus atmosphere.
    Uranus axis is at 97 degrees. It means that it orbits on its side. ( Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus axis is almost ( parallel to the ecliptic. ) )
  • Uranus, the seventh planet from the Sun, is the third largest and fourth most massive planet in the solar system. It is named after the ancient Greek deity of the sky Uranus. Uranus was discovered by William Herschel, a British astronomer in 1781. He also discovered the largest moons of Uranus, Oberon, and Titania.
  • Natural satellites : Uranus has seventeen discovered satellites, however, once again, some claim the number to be 27. 17 moons are: Miranda, Ariel, Umbriel, Titania, Oberon, Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, Puck, Caliban, Sycorax.
    Many of the names are borrowed from names of the characters in William Shakespeare’s, A Midsummer Nights Dream.
  • Exploration : Uranus was visited only by the voyager 2 in 1986.


  • Neptune
  • A Great dark spot has been found on Neptune, a gaint storm where wind blows at 2000 km / h.
  • It has a counter clockwise rotation.
  • Neptune emits about 2.3 times more energy than it receives from the sun.
    Triton is its largest moon.
  • Neptune is the eighth planet from the Sun in the Solar System. It is named after the Roman god of the sea. It is also the fourth largest planet by diameter, and the third largest by mass.
  • Neptune was officially discovered by Johann Gottfried in 1846 after the planet’s mass and location were already calculated by John Couch Adams and Urbain Leverrier. They had managed to calculate the mass and location of Neptune because of the gravitational effect Neptune has on Uranus.
  • Natural satellites : It has 9 satellites ( some claim 13 ) : Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, Nereid and Rsamathe. Of these, Triton is the largest. Another oddity of Triton is the fact that it orbits Neptune clockwise while all the other moons around Neptune orbit counterclockwise.
  • Neptune completed its first orbit around the sun in 165 years after its discovery on 13 July 2011.


  • Pluto
  • Pluto was discovered by Clyde Tombough. NASAs spacecraft, New Horizon, started its journey towards Pluto on 19 January, 2006, carrying the bones of Clyde Tombough to Pluto, and is to study Charon, the moon of Pluto.
  • From the time of its discovery in 1930 until 2006, Pluto was counted as the Solar System’s ninth planet. In the late 20th and early 21st centuries, however, many objects similar to Pluto, were discovered in the outer solar system, notably the scattered disc object Eris which is 27% more massive than Pluto.
  • On August 24, 2006 the IAU ( International Astronomical Union ) defined the term ‘planet’ for the first time.
  • This definition excluded Pluto, and reclassified it as a member of the new category of dwarf planets along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340.
  • Pluto is the second – largest known dwarf planet in the Solar System ( after Eris ) and the tenth – largest body observed directly orbiting the Sun. Pluto is now recognized as the largest member of a distinct region called the Kuiper belt.
  • Like other members of the belt, it is composed primarily of rock and ice – and is relatively small – approximately a fifth the mass, of the Earth’s Moon and a third of its volume.
  • It has an eccentric orbit that takes it from 30 to 49 AU ( Astronomical Unit ), i.e., 4.4 to 7.4 billion km from the Sun, and is highly inclined with respect to the planets. As a result, Pluto occasionally comes closer to the Sun than the planet Neptune does.
  • Pluto and its largest moon, Charon, are often treated together as a binary system. The IAU ( with headquarters at Prague ) has yet to formalize a definition for binary dwarf planets, and until it passes such a ruling, Charon is classified as a moon of Pluto. Pluto has two more known smaller moons, Nix and Hydra, discovered in 2005.