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GENERAL INFORMATION ON MERCURY
Mercury is the planet closest to the Sun in our Solar System. This small, rocky planet has almost no atmosphere. Mercury has a very elliptical orbit and a huge range in temperature. During the long daytime (which lasts 58.65 Earth days or almost an entire Mercurian year, which is 88 days long), the temperature is hotter than an oven; during the long night (the same length), the temperature is colder than a freezer.

Mercury is so close to the Sun that you can only see it near sunrise or sunset.


Craters on the surface of Mercury.Mercury is a heavily cratered planet; its surface is similar to the surface of our Moon. Cratering on Mercury triggered volcanic eruptions that filled much of the surrounding area. Mercury does have a magnetic field (probably generated by a partly-liquid iron core).

SIZE
Mercury is about 3,031 miles (4,878 km) in diameter. It is the smallest planet in our solar system (it used to be comsidered the second-smallest planet, when Pluto was still considered to be a planet). Mercury is a bit over one third of the diameter of the Earth. Mercury is only slightly larger than the Earth's moon.

MASS AND GRAVITY
Mercury's mass is about 3.3 x 1023 kg. This is about 1/20th of the mass of the Earth.

The gravity on Mercury is 38% of the gravity on Earth. A 100 pound person would weigh only 38 pounds on Mercury. To calculate your weight on Mercury, just multiply your weight by 0.38 (or go the planetary weight calculator)
ATMOSPHERE
Mercury's thin atmosphere consist of trace amounts of hydrogen and helium. The atmospheric pressure is only about 1 x 10-9 millibars; this is a tiny fraction (about 2 trillionths) of the atmospheric pressure on Earth.

Since the atmosphere is so slight, the sky would appear pitch black (except for the sun, stars, and other planets, when visible), even during the day. Also, there is no "greenhouse effect" on Mercury. When the sun sets, the temperature drops very quickly since the atmosphere does not help retain the heat.

MERCURY'S ORBIT AND DISTANCE FROM THE SUN
Mercury is closest planet to our Sun and the fastest moving planet in our Solar System.
Mercury is just over a third as far from the sun as the Earth is; it is 0.387 A.U. from the sun (on average). Mercury's orbit is very eccentric; at aphelion (the point in the orbit farthest from the sun) Mercury is 70 million km from the sun, at perihelion Mercury is 46 million km from the sun. There are no seasons on Mercury. Seasons are caused by the tilt of the axis relative to the planet's orbit. Since Mercury's axis is directly perpendicular to its motion (not tilted), it has no seasons. If you were on the surface of Mercury, the Sun would look almost three times as big as it does from Earth!

TEMPERATURE RANGE
Mercury has a huge range in temperatures. Its surface ranges in temperature from -270°F to 800°F (-168°C to 427°C). During the very long daytime (88 Earth-days long), the temperatures are very high (the second-highest in the Solar System - only Venus is hotter); during the long night, the thin atmosphere lets the heat dissipate, and the temperature drops quickly.

MOONS
Mercury has no moons.

MERCURY-EARTH COMPARISONS


SPACECRAFT VISITS
Mercury was visited by NASA's Mariner 10 in 1973 and 1974. Under half of Mercury's surface was mapped by this spacecraft.

MERCURY'S NAME AND SYMBOL

This is the symbol of the planet Mercury.Mercury was named after Mercury, the mythical Roman winged messenger and escort of dead souls to the underworld. It was named for the speedy Mercury because it is the fastest-moving planet.


INTRODUCTION TO VENUS
GENERAL INFORMATION
Venus is the second planet from the sun in our solar system. It is the hottest planet in our Solar System. This planet is covered with fast-moving sulphuric acid clouds which trap heat from the Sun. Its thick atmosphere is mostly carbon dioxide. Venus has an iron core but only a very weak magnetic field.
This is a planet on which a person would asphyxiate in the poisonous atmosphere, be cooked in the extremely high heat, and be crushed by the enormous atmospheric pressure.

Venus is also known as the "morning star" or the "evening star" since it is visible and quite bright at either dawn or dusk. It is only visible at dawn or dusk since it is closer to the sun than we are. Like the moon, Venus' appearance from Earth changes as it orbits around the Sun. It goes from full to gibbous to crescent to new and back.

SIZE
Venus is about 7,521 miles (12,104 km) in diameter. This is about 95% of the diameter of the Earth. Venus is the closest to Earth in size and mass of any of the other planets.

MASS AND GRAVITY
Venus' mass is about 4.87 x 1024 kg. The gravity on Venus is 91% of the gravity on Earth. A 100-pound person would weigh 91 pounds on Venus. The density of Venus is 5,240 kg/m3, slightly less dense than the Earth and the third densest planet in our Solar System (after the Earth and Mercury).

LENGTH OF A DAY AND YEAR ON VENUS
Venus rotates VERY slowly. Each day on Venus takes 243 Earth days. A year on Venus takes 224.7 Earth days. It takes 224.7 Earth days for Venus to orbit the sun once. The same side of Venus always faces Earth when the Earth and Venus are closest together.

VENUS' ORBIT AND DISTANCE FROM THE SUN
Venus is 67,230,000 miles (108,200,000 km) from the sun. Venus has an almost circular orbit. On average, Venus is 0.72 AU, 67,230,000 miles = 108,200,000 km from the sun.

Venus rotates in the opposite direction of the Earth (and the other planets, except possibly Uranus). Looking from the north, Venus rotates clockwise, while the other planets rotate counterclockwise. From Venus, the Sun would seem to rise in the west and set in the east (the opposite of Earth). No one knows why Venus has this unusual rotation.

TEMPERATURE ON VENUS
Venus is the hottest planet in our Solar System. Its cloud cover traps the heat of the sun (the greenhouse effect), giving Venus temperatures up to 480°C. The mean temperature on Venus is 726 K (452°C = 870°F).

MOONS
Venus has no moons.

VENUS-EARTH COMPARISONS


FIRST SPACECRAFT TO REACH VENUS
Venera 3 (from the U.S.S.R.) was the first manmade object to reach Venus. This Soviet spacecraft was launched on November 16, 1965. On March 1, 1966 , the spacecraft arrived at Venus and the capsule parachuted down to the planet, but contact was lost just before entry into the atmosphere.

VENUS' NAME AND SYMBOL


This is the symbol of the planet Venus.
Venus was named after the Roman goddess of love.

INTRODUCTION TO THE EARTH
The Earth is the third planet from the Sun in our Solar System. It is the planet we evolved on and the only planet in our Solar System that is known to support life.

SIZE
The Earth is about 7,926 miles (12,756 km) in diameter. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune).
Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he was very close. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon.


The Earth and the moon. Photo taken by NASA's Galileo mission in 1990.THE MOON
The Earth has one moon. The diameter of the moon is about one quarter of the diameter of the Earth. The moon may have once been a part of the Earth; it may have been broken off the Earth during a catastrophic collision of a huge body with the Earth billions of years ago.

MASS, DENSITY, AND ESCAPE VELOCITY
The Earth's mass is about 5.98 x 1024 kg.

The Earth has an average density of 5520 kg/m3 (water has a density of 1027 kg/m3). Earth is the densest planet in our Solar System.

To escape the Earth's gravitational pull, an object must reach a velocity of 24,840 miles per hour (11,180 m/sec).

LENGTH OF A DAY AND YEAR ON EARTH

Earth rising over the moon. Photo taken by NASA's Apollo 8 mission. Each day on Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once - this is a sidereal day). Each year on Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once).

The Earth's rotation is slowing down very slightly over time, about one second every 10 years.

THE EARTH'S ORBIT


Planet-Sun Orbital Diagram
Label the aphelion (farthest point in orbit) and perihelion (closest point in orbit) of a planet in orbit.
AnswersThe Earth orbits, on average, 93 million miles (149,600,000 km) from the Sun. This distance is defined as one Astronomical Unit (AU). The Earth is closest to the Sun (this is called perihelion) around January 2 each year (91.4 million miles = 147.1 million km); it is farthest away from the Sun (this is called aphelion) around July 2 each year (94.8 million miles = 152.6 million km).



Orbital Eccentricity
The Earth' orbital eccentricity is 0.017; it has an orbit that is close to being circular.

THE EARTH'S AXIS TILT AND THE SEASONS
The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year. This affects the amount of sunlight each receives. For more information on the seasons, click here.

SPEED
At the equator, the Earth's surface moves 40,000 kilometers in 24 hours. That is a speed of about 1040 miles/hr (1670 km/hr). This is calculated by dividing the circumference of the Earth at the equator (about 24,900 miles or 40,070 km) by the number of hours in a day (24). As you move toward either pole, this speed decreases to almost zero (since the circumference at the extreme latitudes approaches zero).

The Earth revolves around the Sun at a speed of about 30 km/sec. This compares with the Earth's rotational speed of approximately 0.5 km/sec (at middle latitudes - near the equator).

For more information on the speed of the Earth, click here.


The size of the atmosphere in this illustration is greatly exaggerated in order to show the greenhouse effect. The Earth's atmosphere is about 300 miles (480 km) thick, but most of the Earth's atmosphere is within 10 miles (16 km) of the Earth's surface.TEMPERATURE ON EARTH
The temperature on Earth ranges from between -127°F to 136°F (-88°C to 58°C; 185 K to 311 K). The coldest recorded temperature was on the continent of Antarctica (Vostok in July, 1983). The hottest recorded temperature was on the continent of Africa (Libya in September, 1922).

The greenhouse effect traps heat in our atmosphere. The atmosphere lets some infrared radiation escape into space; some is reflected back to the planet.

For more information on the greenhouse effect, click here.

ATMOSPHERE
The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases.

The atmosphere was formed by planetary degassing, a process in which gases like carbon dioxide, water vapor, sulphur dioxide and nitrogen were released from the interior of the Earth from volcanoes and other processes. Life forms on Earth have modified the composition of the atmosphere since their evolution.

For more information on the atmosphere, click here.

Earth Activities

• Earth coloring page

MARS
"The Red Planet"
Mars, the red planet, is the fourth planet from the sun and the most Earth-like planet in our solar system. It is about half the size of Earth and has a dry, rocky surface and a very thin atmosphere.

MARS' SURFACE

A Mollweide projection of Mars made from four pictures taken by Hubble Space Telescope in 1999. A storm is visible in the top left near the pole.The surface of Mars is dry, rocky, and mostly covered with iron-rich dust. There are low-lying plains in the northern hemisphere, but the southern hemisphere is dotted with impact craters. The ground is frozen; this permafrost extends for several kilometers.


Olympus Mons, the largest volcano on Mars; it is perhaps the largest volcano in the Solar System. It is 17 miles (27 km) tall and over 320 miles (520 km) across. It is thought to be extinct.The north and south poles of Mars are covered by ice caps composed of frozen carbon dioxide and water.




Scientists have long thought that there is no liquid water on the surface of Mars now, but recent photos from Mars indicate that there might be some liquid water near the surface. The surface of Mars shows much evidence of the effects of ancient waterways upon the landscape; there are ancient, dry rivers and lakes complete with huge inflow and outflow channels. These channels were probably caused by catastrophic flooding that quickly eroded the landscape.

Scientists think that most of the water on Mars is frozen in the land (as permafrost) and frozen in the polar ice caps.

G. Schiaparelli was an Italian astronomer who first mapped Mars (in 1877) and brought attention to the network of "canali" (Italian for canals or channels) on Mars. These "canals" were later found to be dry and not to be canals at all. A Martian impact crater (Crater Schiaparelli, 461 km = 277 mi in diameter) and a hemisphere of Mars have been named after Schiaparelli.

SIZE
Mars is about 4,222 miles (6790 km) in diameter. This is 53% (a little over half) of the diameter of the Earth.

PLANETARY COMPOSITION
Crust and Surface: Mars' surface is composed mostly of iron-rich basaltic rock (an igneous rock). Mars has a thin crust, similar to Earth's.

Mantle: Silicate rock, probably hotter than the Earth's mantle at corresponding depths.

Core: The core is probably iron and sulphides and may have a radius of 800-1,500 miles (1,300-2,400 km). More will be known when data from future Mars missions arrives and is analyzed.

MASS AND GRAVITY
Mars' mass is about 6.42 x 10^23 kg. This is 1/9th of the mass of the Earth. A 100-pound person on Mars would weigh 38 pounds.

LENGTH OF A DAY AND YEAR ON MARS
Each day on Mars takes 1.03 Earth days (24.6 hours). A year on Mars takes 687 Earth days; it takes this long for Mars to orbit the sun once.

MARS' ORBIT
Mars is 1.524 times farther from than the sun than the Earth is. It averages 141.6 million miles (227.9 million km) from the sun. Its orbit is very elliptical; Mars has the highest orbital eccentricity of any planet in our Solar System except Pluto.

ATMOSPHERE
Mars has a very thin atmosphere. It consists of 95% carbon dioxide (CO2), 3% nitrogen, and 1.6% argon (there is no oxygen). The atmospheric pressure is only a fraction of that on Earth (about 1% of Earth's atmospheric pressure at sea level), and it varies greatly throughout the year.
There are large stores of frozen carbon dioxide at the north and south poles. During the warm season in each hemisphere, the polar cap partly melts, releasing carbon dioxide. During the cold season in each hemisphere, the polar cap partly freezes, capturing atmospheric carbon dioxide. The atmospheric pressure varies widely from season to season; the global atmospheric pressure on Mars is 25% different (there is less air, mostly carbon dioxide) during the (northern hemisphere) winter than during the summer. This is mostly due to Mars' highly eccentric orbit; Mars is about 20% closer to the Sun during the winter than during the summer. Because of this, the northern polar cap absorbs more carbon dioxide than the southern polar cap absorbs half a Martian year later. Occasionally, there are clouds in Mars' atmosphere. Most of these clouds are composed of carbon dioxide ice crystals or, less frequently, of frozen water crystals. There are a lot of fine dust particles suspended in Mars' atmosphere. These particles (which contain a lot of iron oxide) absorb blue light, so the sky appears to have little blue in it and is pink/yellow to butterscotch in color.

TEMPERATURE RANGE
Mars' surface temperature averages -81 °F (-63 °C). The temperature ranges from a high of 68° F(20° C) to a low of -220° F(-140° C). Mars is much colder than the Earth.

MARS' MOONS
Mars has 2 tiny moons, Phobos and Deimos. They were probably asteroids that were pulled into orbit around Mars.

SPACECRAFT VISITS
Mariner 4 was the first spacecraft to visit Mars (in 1965). Two Viking spacecraft landed in 1976. Mars Pathfinder landed on Mars on July 4, 1997, broadcasting photos. For more on the Mars missions, click here.

THE FACE ON MARS
This photograph of the Cydonia Mense region of Mars was taken by NASA's Mars Global Surveyor in 1998. It is a coincidental alignment of rocks and other geologic formations that happens to look like a human face from this angle.

DISCOVERY OF MARS
Mars has been known since ancient times.

MARS' NAME AND SYMBOL

This is the symbol of the planet Mars.Mars was named after the Roman god of war.

JUPITERGeneral
DescriptionAtmosphere and Planetary CompositionGreat Red SpotBelts and ZonesJupiter's RingsJupiter's MoonsStatisticsActivities, Web Links
Jupiter is the fifth and largest planet in our solar system. This gas giant has a thick atmosphere, 39 known moons, and a dark, barely-visible ring. Its most prominent features are bands across its latitudes and a great red spot (which is a storm).

Jupiter is composed mostly of gas. This enormous planet radiates twice as much heat as it absorbs from the Sun. It also has an extremely strong magnetic field. It is slightly flattened at its poles and it bulges out a bit at the equator.

SIZE
Jupiter's diameter is 88,700 miles (142,800 km). This is a little more than 11 times the diameter of the Earth. Jupiter is so big that all the other planets in our Solar System could fit inside Jupiter (if it were hollow).



MASS AND GRAVITY
Jupiter's mass is about 1.9 x 1027 kg. Although this is 318 times the mass of the Earth, the gravity on Jupiter is only 254% of the gravity on Earth. This is because Jupiter is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).

A 100-pound person would weigh 254 pounds on Jupiter.

LENGTH OF A DAY AND YEAR ON JUPITER
It takes Jupiter 9.8 Earth hours to revolve around its axis (this is a Jovian day). It takes 11.86 Earth years for Jupiter to orbit the sun once (this is a Jovian year).

Jupiter is made up of gases and liquids, so as it rotates, its parts do not rotate at exactly the same velocity. It rotates very rapidly, and this spinning action gives Jupiter a large equatorial bulge; it looks like a slightly-flattened sphere (it is oblate)

JUPITER'S ORBIT
Jupiter is 5.2 times farther from than the Sun than the Earth. On average, it is 480,000,000 miles (778,330,000 km) from the sun.

At aphelion (the place in its orbit where Jupiter is farthest from the Sun), Jupiter is 815,700,000 km from the Sun. At perihelion (the place in its orbit where Jupiter is closest to the Sun), Jupiter is 749,900,000 km from the Sun.
Jupiter has no seasons. Seasons are caused by a tilted axis, and Jupiter's axis is only tilted 3 degrees (not enough to cause seasons).

JUPITER'S MOONS
Jupiter has four large moons and dozens of smaller ones (there are 39 moons known so far). More moons are being found all the time.

Galileo first discovered the four largest moons of Jupiter, Io (which is volcanically active), Europa, Ganymede (the largest of Jupiter's moons, pictured at the left), and Callisto in 1610; these moons are known as the Galilean moons. Ganymede is the largest moon in the Solar System.

For more information on Jupiter's moons, click here.

RINGS
Jupiter has faint, dark rings composed of tiny rock fragments and dust. These rings were discovered by NASA's Voyager 1 in 1980. The rings were investigated further when Voyager 2 flew by Jupiter. The rings have an albedo of 0.05; they do not reflect very much of the sunlight that they receive.

For more information on Jupiter's rings, click here.

TEMPERATURE RANGE
The cloud-tops average 120 K = -153°C = -244°F.

DISCOVERY OF JUPITER
Jupiter has been well-known since ancient times. It is the third-brightest object in the night sky (after the moon and Venus).

COMET SL9 HITS JUPITER

An SL-9 impact site on Jupiter, July 6, 1994. Photo by Hubble Space Telescope.Shoemaker-Levy 9 (SL-9) was a short-period comet that was discovered by Eugene and Carolyn Shoemaker and David H. Levy. As the comet passed close by Jupiter, Jupiter's gravitational forces broke the comet apart . Fragments of the comet collided with Jupiter for six days during July, 1994, causing huge fireballs in Jupiter's atmosphere that were visible from Earth.

SPACECRAFT VISITS
Jupiter was first visited by NASA's Pioneer 10, which flew by Jupiter in 1973. Later fly-by visits included: Pioneer 11, Voyager 1, Voyager 2, Ulysses, and Galileo.

JUPITER'S NAME AND SYMBOL

This is the symbol of the planet Jupiter.Jupiter was named after the Roman primary god, Jupiter.

JUPITER ACTIVITIES
Jupiter coloring page

SATURNGeneral
DescriptionInside Saturn and the AtmosphereRingsMoonsActivities, Web Links
GENERAL INFORMATION ON SATURN

Saturn is the sixth planet from the sun in our solar system. It is the second-largest planet in our solar system (Jupiter is the largest). It has beautiful rings that are made mostly of ice chunks (and some rock) that range in size from the size of a fingernail to the size of a car. Saturn is made mostly of hydrogen and helium gas.

Saturn is visible without using a telescope, but a low-power telescope is needed to see its rings.

SIZE AND SHAPE
Saturn is about 74,898 miles (120,536 km) in diameter (at the equator at the cloud tops). This is about 9.4 times the diameter of the Earth. 764 Earths could fit inside a hollowed-out Saturn.

Saturn is the most oblate (flattened) planet in our Solar System. It has a equatorial diameter of 74,898 miles (120,536 km) (at the cloud tops) and a polar diameter of 67,560 miles (108,728 km). This is a difference of about 10%. Saturn's flattened shape is probably caused by its fast rotation and its gaseous composition.

RINGS
Saturn's beautiful rings are only visible from Earth using a telescope. They were first observed by Galileo in the 17th century.

Saturn's bright rings are made of ice chunks (and some rocks) that range in size from the size of a fingernail to the size of a car. Although the rings are extremely wide (almost 185,000 miles = 300,000 km in diameter), they are very thin (about 0.6 miles = 1 km thick).

For more information on Saturn's rings, click here.

MASS, GRAVITY AND DENSITY
Saturn's mass is about 5.69 x 1026 kg. Although this is 95 times the mass of the Earth, the gravity on Saturn is only 1.08 times the gravity on Earth. This is because Saturn is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).

A 100 pound person would only weigh 108 pounds on Saturn.

Saturn is the only planet in our Solar System that is less dense than water. Saturn would float if there were a body of water large enough!

LENGTH OF A DAY AND YEAR ON SATURN
Each day on Saturn takes 10.2 Earth hours. A year on Saturn takes 29.46 Earth years; it takes 29.46 Earth years for Saturn to orbit the sun once.

ORBIT AND DISTANCE FROM THE SUN
Saturn is 9.539 AU, on average, from the sun, about 9 and a half times as far from the Sun as the Earth is.

At aphelion (the place in its orbit where Saturn is farthest from the Sun), Saturn is 1,503,000,000 km from the Sun. At perihelion (the place in its orbit where Jupiter is closest to the Sun), Saturn is 1,348,000,000 km from the Sun.

TEMPERATURE ON SATURN
The mean temperature on Saturn (at the cloud tops) is 88 K (-185° C; -290° F).

MOONS
Saturn has dozens of moons (33 discovered as of August, 2004). It has 18 named moons. including Titan (the largest), Rhea, Iapetus, Dione, Tethys, Enceladus, Mimas, Hyperion, Phoebe, Janus, Epimetheus, Pandora, Prometheus, Helene, Telesto, Atlas, Calypso, and Pan (the smallest named moon of Saturn). At least a dozen others have been noted (but not named yet).
SPACECRAFT VISITS
Saturn has been visited by Pioneer 11 (in 1979) and by Voyager 1 and Voyager 2. Cassini, a spacecraft named for the divisions in Saturn's rings, is on the way and will arrive in 2004.

SATURN-EARTH COMPARISON



SATURN'S NAME AND SYMBOL


This is the symbol of the planet Saturn.
Saturn was named for the Roman god of agriculture.

Saturn Activities

GENERAL INFORMATION ON URANUS

Uranus is the seventh planet from the sun in our solar system. This huge, icy planet is covered with clouds and is encircled by a belt of 11 rings and 22 known moons. Uranus' blue color is caused by the methane (CH4) in its atmosphere; this molecule absorbs red light.

ROTATIONAL AXIS
Uranus' rotational axis is strongly tilted on its side (97.9°). Instead of rotating with its axis roughly perpendicular to the plane of its orbit (like all the other planets in our Solar System), Uranus rotates on its side (along its orbital path). This tipped rotational axis gives rise to extreme seasons on Uranus. For more information on Uranus' extreme seasons, click here.
Because of its almost-perpendicula r axis orientation, there is a debate over which of Uranus' poles is its north pole. This debates leads to yet another: Is Uranus spinning in a retrograde orbit (like Venus) or not (like the other planets)?

SIZE
Uranus is about 31,690 miles (51,118 km) in diameter. This is about 4 times the diameter of the Earth.

This gas giant is the third-largest planet in our Solar System (after Jupiter and Saturn).

MASS AND GRAVITY
Uranus' mass is about 8.68 x 1025 kg. This is about 14 times the mass of the Earth. The gravity on Uranus is only 91% of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).

A 100-pound person on Uranus would weigh 91 pounds.

LENGTH OF A DAY AND YEAR ON URANUS
Each day on Uranus takes 17.9 Earth hours. A year on Uranus takes 84.07 Earth years; it takes 84.07 Earth years for Uranus to orbit the sun once.

URANUS' ORBIT AND DISTANCE FROM THE SUN
Uranus is over 19 times as far from the Sun as the Earth is; it averages 19.18 A.U.

At aphelion (the farthest point in its solar orbit) it is 1,850,000,000 miles (3,003,000,000 km) from the Sun. At perihelion (the closest point in its solar orbit) it is 1,700,000,000 miles (2,739,000,000 km) from the Sun.

TEMPERATURE
The mean temperature on the surface of Uranus' cloud layer is -350°F (59 K). Uranus radiates very little heat in comparison with the other gas giants, Jupiter, Saturn and Neptune).

PLANETARY COMPOSITION AND ATMOSPHERE
Uranus is a frozen, gaseous planet with a molten core. Uranus' atmosphere consists of 83% hydrogen, 15% helium and 2% methane.

For more information on Uranus' composition, click here.

RINGS

Uranus and its rings photographed by an infrared camera.Uranus has a belt of 11 faint, narrow rings composed of rock and dust. They circle Uranus is very elliptical orbits. These rings are only a fraction of the size of Saturn's rings, and were only discovered in 1977.

For more information on Uranus' rings, click here.



MOONS

Oberon, the largest moon of Uranus. Photo taken by NASA's Voyager mission in 1986.Uranus has 5 large moons (2 were discovered by Wm. Herschel in 1781, 2 were discovered by Wm. Lassell in 1851, and one by G. Kuiper in 1948) and many small moons (which were discovered much later).

For more information on Uranus' moons, click here.

DISCOVERY OF URANUS
Uranus was discovered by the British astronomer William Herschel on March 13, 1781. Herschel also discovered two of the moons of Uranus (Titania and Oberon) and some of the moons of Saturn.

URANUS-EARTH COMPARISON


URANUS' NAME AND SYMBOL


This is the symbol of the planet Uranus.
This planet was originally named in 1781 by the British astronomer William Herschel - he called it Georgium Sidus (meaning "the Georgian planet") to honor the King George III of England. The name was later changed to Uranus, the ancient mythological god of the sky, Ouranos. The name Uranus was suggested by the German astronomer Johann Elert Bode.

GENERAL INFORMATION ON NEPTUNE Neptune is the eighth planet from the sun in our solar system. This giant, frigid planet has a hazy atmosphere and strong winds. This gas giant is orbited by eight moons and narrow, faint rings arranged in clumps. Neptune's blue color is caused by the methane (CH4) in its atmosphere; this molecule absorbs red light.
Neptune cannot be seen using the eyes alone. Neptune was the first planet whose existence was predicted mathematically (the planet Uranus's orbit was perturbed by an unknown object which turned our to be another gas giant, Neptune).

SIZE
Neptune is about 30,775 miles (49,528 km) in diameter. This is 3.88 times the diameter of the Earth. If Neptune were hollow, it could hold almost 60 Earths.

Neptune is the fourth largest planet in our Solar System (after Jupiter, Saturn, and Uranus).

MASS AND GRAVITY
Neptune's mass is about 1.02 x 1026 kg. This is over 17 times the mass of the Earth, but the gravity on Neptune is only 1.19 times of the gravity on Earth. This is because it is such a large planet (and the gravitational force a planet exerts upon an object at the planet's surface is proportional to its mass and to the inverse of its radius squared).

A 100-pound person would weigh 119 pounds on Neptune.

LENGTH OF A DAY AND YEAR ON NEPTUNE
Each day on Neptune takes 19.1 Earth hours. A year on Neptune takes 164.8 Earth years; it takes almost 165 Earth years for Neptune to orbit the sun once. Since Neptune was discovered in 1846, it has not yet completed a single revolution around the sun.

NEPTUNE'S ORBIT AND DISTANCE FROM THE SUN
Neptune is about 30 times farther from the sun than the Earth is; it averages 30.06 A.U. from the sun. Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

At aphelion (the point in Neptune's orbit farthest from the sun) Neptune is 4,546,000,000 km from the sun, at perihelion (the point in Neptune's orbit closest from the sun) Neptune is 4,456,000,000 km from the sun.

Neptune's rotational axis is tilted 30 degrees to the plane of its orbit around the Sun (this is few degrees more than the Earth). This gives Neptune seasons. Each season lasts 40 years; the poles are in constant darkness or sunlight for 40 years at a time.

TEMPERATURE
The mean temperature is 48 K.

DISCOVERY OF NEPTUNE
Neptune's existence was predicted in 1846, after calculations showed perturbations in the orbit of Uranus. The calculations were done independently by both J.C. Adams and Le Verrier. Neptune was then observed by J.G. Galle and d'Arrest on September 23, 1846.

SPACECRAFT VISITS
Neptune was visited by NASA's Voyager 2 in August, 1989. Before this visit, virtually nothing was known about Neptune.

NEPTUNE-EARTH COMPARISON


NEPTUNE'S NAME AND SYMBOL


This is the symbol of the planet Neptune.
Neptune was named after the mythical Roman god of the seas. Neptune's symbol is the fishing spear.


PLUTO

GENERAL INFORMATION
Pluto is a dwarf planet that usually orbits past the orbit of Neptune. It was classified as a dwarf planet in 2006; before that it was considered to be a planet, the smallest planet in our solar system. Pluto is smaller than a lot of the other planets' moons, including our moon. Pluto is the only "planet" in our solar system that has not been visited by our spacecraft yet. We only have blurry pictures of its surface; even the Hubble Space Telescope orbiting the Earth can only get grainy photos because Pluto is so far from us. In 2015, a spacecraft called New Horizons (launched by NASA in 2006) will visit Pluto.


SIZE
Pluto is about 1,413 miles (2274 km) in diameter. This is about 1/5 the diameter of the Earth. Pluto is smaller than the 8 planets in our Solar System.

MASS AND GRAVITY
Pluto's mass is about 1.29 x 1022 kg. This is about 1/500th of the mass of the Earth. The gravity on Pluto is 8% of the gravity on Earth.

Pluto is the least massive planet in our Solar System (and is now classified as a dwarf planet).

A 100 pound person on Pluto would weigh only 8 pounds.

LENGTH OF A DAY AND YEAR ON PLUTO
Each day on Pluto takes 6.39 Earth days. Each year on Pluto takes 247.7 Earth years (that is, it takes 247.7 Earth years for Pluto to orbit the Sun once).

PLUTO'S ORBIT
Pluto is 39 times farther from than the sun than the Earth is. Pluto ranges from 2.8 to 4.6 billion miles (4.447 billion to 7.38 billion km) from the Sun. From Pluto, the sun would look like a tiny dot in the sky. Occasionally, Neptune's orbit is actually outside that of Pluto; this is because of Pluto's highly eccentric (non-circular) orbit. During this time (20 years out of every 248 Earth years), Neptune is actually the farthest planet from the Sun (and not Pluto). From January 21, 1979 until February 11, 1999, Pluto was inside the orbit of Neptune. Now and until September 2226, Pluto is outside the orbit of Neptune.

Orbital Eccentricity
Pluto has a very eccentric orbit; that means that its distance from the sun varies a lot during its orbit around the sun. Sometimes it is even closer to the Sun than the planet Neptune (it was that way from January 1979 to February 11, 1999)! Pluto also rotates about its axis in the opposite direction from most of the other planets.

Orbital Inclination
Pluto's orbit is tilted from the plane of the ecliptic. This angle, its orbital inclination, is 17.15°. This is the largest inclination of any of the planets.

TEMPERATURE ON PLUTO
Pluto is VERY, VERY cold. Its temperature may range from between -396°F to -378°F (-238°C to -228°C, or 35 K to 45 K). The average temperature is -393°F (-236°C = 37 K).

PLANETARY COMPOSITION
Pluto's composition is unknown. It is probably made up of about 70% rock and 30% water. This is determined from density calculations; Pluto's density is about 2,000 kg/m3. There may be methane ice together with frozen nitrogen and carbon dioxide on the cold, rocky surface.

ATMOSPHERE
Not much is known about Pluto's atmosphere. It is probably mostly nitrogen with a little carbon monoxide and methane - definitely not breatheable by humans. The atmospheric pressure is probably very low. The atmosphere forms when Pluto is closest to the Sun and the frozen methane is vaporized by the solar heat. When it is farther from the Sun, the methane freezes again. From Pluto, the sky would appear black, even when the Sun (the size of a star) is up.

PLUTO'S MOONS
Pluto has one large moon, named Charon; two minscule moons were discovered in 2005. Although Charon is small, about 1,172 km (728 miles) in diameter, it about half of the size of Pluto itself. Charon orbits about 19,640 km from Pluto on average. It may be covered by water ice and probably has no atmosphere. Charon is in a synchronous orbit around Pluto. That is, Charon is always over the same spot on Pluto; Charon's orbit takes exactly one Pluto day.

Charon was discovered by Jim Christy in 1978. Charon was named after the mythological demon who ferried people across the mythological river Styx into Hades. The two tiny moons are from 30 and 100 miles (45 to 160 km) in diameter, and orbit Pluto about 27,000 miles (44,000 km) from Pluto, more than twice as far as the orbit of Charon.

DISCOVERY OF PLUTO
Pluto was discovered after the 8 planets and was originally considered a planet itself (until 2006). In the early 1900s, Planet "X" was the temporary name given to the then-unknown planet beyond Neptune that disturbed the orbits of Uranus and Neptune. Percival Lowell calculated the rough location of Planet "X's" orbit, but died in 1916 before it was found. This planet was eventually found by the American astronomer Clyde W. Tombaugh in 1930 and named Pluto. He did his observations at the Lowell Observatory in Arizona.

PLUTO-EARTH COMPARISON


PLUTO'S NAME

This is the symbol of Pluto.Pluto was named after the Roman god of the underworld, Pluto.
Its symbol is the combined letters "P" and "L," either for Percival Lowell or for Pluto. The name Pluto was suggested by Venetia Burney of England, who was 11 years old at the time. She suggested the name to her grandfather, who was Librarian at Oxford. He passed her idea to the astronomers who were trying to name the newly-discovered planet.

PLUTO: PLANET, ASTEROID, OR TNO
Pluto's unusual orbit makes some scientists think that Pluto is not a regular planet. but a "dwarf planet," "minor planet" or a Trans Neptunian Object (TN [Kuiper Belt objects left over from the formation of the solar system]. In 2006 Pluto was re-classified as a dwarf planet. Alternatively, Pluto could have been be listed as an asteroid (it would have been given the asteroid number 10,000). It also may be the first TNO - it will also still be considered a dwarf planet.

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