solar system

The Solar System

Our solar system is the group of planets, moons and other space debris circling our Sun. There are also at least 120 moons circling the planets, and millions of asteroids in a belt between Mars and Jupiter. At the outer edge of the Solar system, far beyond the orbit of distant Pluto, is a halo of icy comets called the Oort cloud.

The Sun

The Sun has the mass of 333 000 Earths; its massive gravity is what holds the solar system together. As stars go, the Sun is medium-sized. It generates almost all of the light in the solar system.

We see the planets and moons because they reflect sunlight

The Planets

There are eight planets in our solar system. The four inner planets-Mercury, Venus, Earth and Mars-are small, dense and rocky and are called the "terrestrial" planets. Four of the outer planets-Jupiter, Saturn, Uranus and Neptune-are mostly gas and are called the "gas giants." The Dwarf planet, Pluto and brothers, does not fit into either category: they are small, icy world, more similar to comets than the other planets so it was calssified in new catagory called Dwarf Planets in 2007 by IAU.

Our planet, Earth, is third from the Sun. Earth takes about 365 days to circle the Sun- or one year. The length of a year on each planet depends on its orbit around the Sun; the closer a planet is to the Sun, the faster the planet moves. Mercury, the innermost planet, races around the Sun in only 88 Earth days, while Pluto, takes 248 Earth years to complete one orbit.

More about the planets:

Mercury
Venus
Earth
Mars
Saturn
Uranus
Neptune
Dwarf Planets: Pluto

The Sun

The Sun is a star, just like the stars we see at night. The Sun is much closer to us than the other stars, so it appears much bigger and brighter. The Sun's visible disk is called the photosphere. Surrounding the photosphere is a thin layer called the chromosphere, which is only seen during a total solar eclipse. The outer atmosphere of the Sun is called the corona.

Eclipses of the sun inspire creative works and cultural rituals. One rock-climbing performance group from Canada performed a piece during the solar eclipse of May 1994

The basics:

Period of revolution (around centre of galaxy): 220 000 000 years
Period of rotation 27 days on average
Diameter: 1 392 000 km
Surface gravity: 27.9 times greater than Earth's
Composition: 75% hydrogen, 25% helium, with traces of heavier elements
Temperature: 5 500 degrees Celsius at surface; 15 000 000 degrees Celsius at core

Appearance in Earth's sky:

The Sun appears as a blindingly bright disk of light. In fact, the Sun is bright enough to cause serious eye damage, so it's important never to look at it directly. When the Sun is in the sky, the atmosphere of the Earth scatters the blue component of the Sun's light, turning the sky blue. This blue sky is bright enough that we cannot see the other stars in the sky. (The stars are still there, however, and can be seen in the daytime during a solar eclipse.)

Telescopic appearance:

WARNING! NEVER LOOK DIRECTLY AT THE SUN!
The Sun is so bright that looking at it through a telescope for even an instant can cause permanent blindness. The best way to observe the Sun is by using a pinhole camera or another indirect viewing method. The most obvious features of the solar disk are sunspots. These are cooler regions of the Sun's surface and are sometimes visible as dark spots. The Sun rotates relatively slowly, once every 27 days.

Mercury

Mercury is the densest planet (5.4 times the density of water) and the closest planet to the Sun. Mercury was last visited by the Mariner 10 spacecraft in 1974.

The basics:

Order (outwards from the Sun): 1
Average distance from the Sun: 57 900 000 km (0.39 astronomical units)
Period of revolution (length of year): 87.95 days
Period of rotation (length of day): 58.6 days
Diameter: 4 879 km
Surface gravity: 0.38 times greater than Earth's
Atmosphere: Virtually none
Satellites: None
Composition: Basaltic rocks and dust; nickel-iron core
Surface temperature: Average 227 degrees Celsius during the day, -173 degrees Celsius at night

Appearance of surface:

Rocky and covered with meteorite craters. Appearance similar to Earth's Moon.

Appearance in Earth's sky:

Mercury is visible to the unaided eye, and appears as a bright star-like object. In the northern hemisphere, Mercury stays close to the glow of dusk and dawn, either very low in the west after sunset, or very low in the east before sunrise. In the southern hemisphere, Mercury is high enough above the horizon that it can be viewed in total darkness several times a year.

Telescopic appearance:

Mercury is hard to observe because it is usually close to the horizon, where it's obscured by atmospheric turbulence. Because it is closer to the Sun than the Earth, Mercury shows phases, like the Moon. It is unlikely you will see any surface details on Mercury.

Venus

Venus is the hottest planet in the solar system

Venus is the hottest planet in the solar system because the constant cloud cover traps heat like a greenhouse. In 1975, the Soviet Venera probes landed on Venus, sending back TV pictures of the surface. In 1992, NASA's Magellan went into a Venus orbit and produced a complete radar picture of the surface.

Venus spins in the opposite direction of every other planet. This means that, on Venus, the Sun rises in the west and sets in the east.

The basics:

Order (outwards from the Sun): 2
Average distance from the Sun: 108 200 000 km (0.72 astronomical units)
Period of revolution (length of year): 224.70 days
Period of rotation (length of day): 243.02 days
Diameter: 12 104 km
Surface gravity: 0.91 times greater than Earth's
Atmosphere: Carbon dioxide, hydrogen sulphide, sulphuric acid, 90 times thicker than Earth's atmosphere. Dense clouds completely cover the planet. Constant lightning.
Satellites: None
Composition: Basaltic and granitic (hard volcanic) rocks
Surface temperature: 460 degrees Celsius both day and night, due to greenhouse effect caused by heavy cloud cover

Appearance of surface:

Landscapes of rocky rubble or flat rock outcrops. The rocks are partially molten and flow slowly. Light level similar to an overcast day on Earth.

Appearance in Earth's sky:

A brilliant star-like object seen as a "morning star" or "evening star." Always appears close to the Sun in the sky. Venus outshines every other object in the sky except the Sun and Moon. It is often misidentified as an aircraft or an unidentified flying object (UFO) because of its striking brilliance.

Telescopic appearance:

Thick clouds make it impossible to see the surface of Venus, but the phases (similar to Mercury's and the Moon's) are easily seen with any telescope. Even in a thin crescent phase, Venus is large enough that the phase can be seen with binoculars.

Earth

Earth is unique in the solar system. It is the only planet that has both liquid water and an oxygen-nitrogen atmosphere, two features essential to life as we know it. It is also the only planet known to harbour living creatures. There are over six billion people on Earth now, and countless animals and plants.

As the Earth orbits the Sun it also rotates on its axis. When we're facing the Sun it's daytime, and when we're facing away it's night. Because the Earth rotates on its axis as it orbits the Sun, parts of the Earth that face the Sun experience daytime while parts that face away experience night. Though the Earth rotates once every 23 hours 56 minutes, the period from one sunrise to the next is actually 24 hours. That's because the Earth changes position from one day to the next as it moves around the Sun. As a result, it must rotate for an additional 4 minutes to bring the Sun back to the same position in the sky.

The basics:

Order (outwards from the Sun): 3
Average distance from the Sun: 149 600 000 km (1.00 astronomical unit)
Period of revolution (length of year): 365.25 days
Period of rotation (length of day): 23 hours 56 minutes
Diameter: 12 756 km
Atmosphere: 78% nitrogen, 21% oxygen, traces of carbon dioxide, water vapour.
Satellites: 1 (the Moon), plus numerous man-made satellites
Composition: Basaltic rock continents with an iron core. Liquid water covers about two-thirds of the Earth's surface
Surface temperature: Range of -89 degrees Celsius to 55 degrees Celsius, depending on season and location.

Appearance of surface:

Mostly covered with liquid water. Rocky continents with a wide variety of geologic formations. Green plants cover large areas of the land. Effects of human population (farming, mining, nighttime lighting of cities) visible from orbit.

The Moon

The Moon is our closest neighbour in space. It orbits the Earth at an average distance of 384,000 km and takes 27.3 days to complete one revolution. As it orbits the Earth the Moon is illuminated from different angles by the Sun, which produces a quarter lunar phase roughly each week. Since the Moon is only one quarter the Earth's diameter, it has much less mass and only 1/6th the Earth's gravity.

Because the Moon takes the same amount of time to rotate on its axis as it does to revolve around the Earth, one side of the Moon always faces us. Long ago, the Moon used to rotate more quickly but the earth's gravity gradually slowed it down.

Scientists believe the Moon formed 4.6 billion years ago when a Mars-sized asteroid tore a huge chunk out of the Earth. This chunk later solidified in orbit around the Earth and became the Moon. Lunar samples, gathered by the Apollo astronauts, show that the Moon consists of exactly the same minerals as parts of the Earth.

Mars

In August 1996, scientists announced the discovery of a 1.9 kg (4.2 Ib) rock, determined to have originated on Mars and showing evidence of what appears to be the fossil remains of microscopic bacteria that lived on Mars 3.6 billion years ago. If confirmed, this discovery will be one of the most significant in astronomy.

The Mars Global Surveyor camera has sent back photos showing recent flow patterns, areas which look like dry riverbeds, that may have been caused by water.

The basics:

Order (outwards from the Sun): 4
Average distance from the Sun: 227 900 000 km (1.524 astronomical units)
Period of revolution (length of year): 686.95 days
Period of rotation (length of day): 24 hours 37 minutes
Diameter: 6 794 km
Surface gravity: 0.38 times greater than Earth's
Atmosphere: Thin carbon dioxide
Satellites: 2 small moons (Phobos and Deimos)
Composition: Basaltic rock
Surface temperature: As low as -140 degrees Celsius, but occasionally as high as 27 degrees Celsius

Appearance of surface:

Cold desert world covered with rocky, dusty terrain. Ice caps of frozen carbon dioxide at the north and south poles. Craters, valleys and dormant volcanoes, much larger than those found on Earth, dot the surface.

Appearance in Earth's sky:

Mars changes its appearance dramatically over the course of its year. At its closest approach to Earth it can be brighter than any of the stars, and quite obviously reddish-coloured. When farthest away, it is a dim red point in the sky.

Telescopic appearance:

Mars is the only planet whose surface is easily visible from Earth.

Mars is the only planet whose surface is easily visible from Earth. When Mars is at its closest point to Earth (every two years or so), a telescope can show a reddish disk, as well as the white polar ice caps and some of the dark surface markings. At other times, Mars appears so small that little detail can be seen even with a large telescope.

Jupiter

In July 1994, the fragments of Comet Shoemaker-Levy collided with Jupiter, producing the largest explosions ever observed in our solar system. The fragments left behind dark patches in the clouds of Jupiter, allowing astronomers to observe the way Jupiter's atmosphere works.

Jupiter rotates on its axis every 9 hours and 55 minutes.

In December 1995, the Galileo spacecraft went into orbit around Jupiter and began returning close-up images of Jupiter's atmosphere and moons.

The basics:

Order (outwards from the Sun): 5
Average distance from the Sun: 778 300 000 km (5.20 astronomical units)
Period of revolution (length of year): 11.86 years
Period of rotation (length of day): 9 hours 55 minutes at the equator
Diameter: 142 980 km at the equator
Surface gravity: 2.54 times greater than Earth's
Composition: Scientists do not agree on whether Jupiter has a solid rocky core or if it consists only of gases that condense in the middle to form liquids. The interior ocean is made primarily of hydrogen and helium, with a few other gases
Atmosphere: Hydrogen, helium, some methane and ammonia
Satellites: 61 moons and one faint ring
Surface temperature: 10 000 degrees Celsius in the interior ocean; -140 degrees Celsius at the cloud tops; 25 000 degrees Celsius at the centre. Therefore, Jupiter radiates more heat than it receives

Appearance of surface:

The visible cloud tops of Jupiter are divided into multicoloured cloud belts and zones, stretched out by Jupiter's rapid rotation. The Great Red Spot, easily visible with small telescopes, is the largest example of the many violent storms in Jupiter's atmosphere. There is a vast liquid hydrogen ocean under the visible cloud tops.

Appearance in Earth's sky:

Jupiter is easily visible for about 10 months of the year, appearing as a very bright, star-like object.

Telescopic appearance:

Jupiter offers a wealth of detail to telescope users. The cloud bands, the Great Red Spot, and the four Galilean moons are all easily visible.

Saturn

Saturn is best known for its beautiful system of rings. The ring system is only about 10 kilometres thick and is made up of particles of ice and dust. Most of the rings are less than one metre across, but a few are as large as 100 kilometres across. During 1995, Saturn's rings were visible almost edge-on, making them appear to vanish as viewed from Earth.

The basics:

Order (outwards from the Sun): 6
Average distance from the Sun: 1 429 000 000 km (9.56 astronomical units)
Period of revolution (length of year): 29.42 years
Period of rotation (length of day): 10 hours 40 minutes at the equator
Diameter: 120 540 km at the equator; rings are 275 000 km across
Surface gravity: 1.08 times greater than Earth's
Composition: Scientists are not sure if Saturn is made completely of gases or if it has a solid rocky core. The entire planet has a density less than water- if you could put it in a big enough bathtub, it would float
Atmosphere: Hydrogen, helium, some methane and ammonia
Satellites: 31 moons and a ring system of 23 major rings
Surface temperature: -190 degrees Celsius at the cloud tops. Like Jupiter, Saturn radiates more heat than it receives from the Sun

Appearance of surface:

Saturn has cloud structures similar to Jupiter, but not as pronounced. There is probably a liquid hydrogen ocean under the visible cloud tops.

Appearance in Earth's sky:

Saturn can be seen in the sky as a bright star-like object. It moves very slowly against the background stars, and several weeks may be required to see it change its position relative to the stars.

Telescopic appearance:

A good pair of binoculars will reveal that Saturn is not quite round,due to its ring system. Almost any telescope that magnifies over 30 times will show the rings of Saturn and perhaps a faint cloud band or two on the disk of the planet.

The view of Saturn through a telescope is one of the most beautiful sights in astronomy.

Uranus

Uranus was discovered by amateur astronomer William Herschel in 1781 using a six-inch telescope. The planet is tilted on its side relative to its orbit around the Sun. The north pole of Uranus sometimes points almost directly towards the Sun, and sometimes almost directly away. This means that the "daytime" and "nighttime" on Uranus can be over 40 years long!

"Daytime" and "nighttime" on Uranus can be over 40 years long!

Uranus was visited by the Voyager 2 robotic spacecraft in January 1986. Voyager 2 increased our knowledge of Uranus tremendously by providing close-up pictures of the planet and its moons and ring system.

The basics:

Order (outwards from the Sun): 7
Average distance from the Sun: 2 875 000 000 km (19.22 astronomical units)
Period of revolution (length of year): 83.75 years
Period of rotation (length of day): 17 hours 14 minutes at the equator
Diameter: 51 120 km at the equator
Surface gravity: 0.91 times greater than Earth's
Composition: Possibly has an inner rocky core, overlaid with ice (frozen methane, water and ammonia) covered with an ocean of liquid hydrogen
Atmosphere: Hydrogen, helium, methane
Satellites: 22 moons, and 9 dark rings
Surface temperature: -195 degrees Celsius at the cloud tops

Appearance of surface:

Probably a liquid surface under the visible cloud tops.

Appearance in Earth's sky:

Uranus is technically visible to the unaided eye, but it is near the limit of detection. It appears as an extremely faint greenish star.

Telescopic appearance:

A large telescope reveals that Uranus is not a star but a tiny blue-green disk. The planet is so far away that cloud features are not visible in small telescopes.

Neptune

Neptune's existence was predicted by both English mathematician John Adams and French mathematician U.J.J. Leverrier, based on irregularities in Uranus' orbit. The German astronomer J.G. Galle began searching for a new planet and found it very near the predicted position.

In August 1989, the robotic spacecraft Voyager 2 flew past Neptune, radioing back close-up pictures of the planet and its family of moons. Voyager also discovered a large storm system, similar to Jupiter's Great Red Spot, which is called The Great Dark Spot.

Due to the elliptical shape of Pluto's orbit, Neptune is the furthest planet from the Sun for about 20 years out of every 248 years. This was the case from about 1979 to 1999.

The basics:

Order (outwards from the Sun): 8 or 9
Average distance from the Sun: 4 504 400 000 km (30.11 astronomical units)
Period of revolution (length of year): 163.73 years
Period of rotation (length of day): 16 hours 3 minutes at equator
Diameter: 49 530 km at the equator
Surface gravity: 1.19 times greater than Earth's
Composition: Hydrogen and helium ice
Atmosphere: Hydrogen, helium, methane
Satellites: 11 moons and 5 incomplete ring arcs
Surface temperature: -205 degrees Celsius at the cloud tops

Appearance of surface:

Perhaps a rocky core surrounded by massive layers of ice. May be covered by a deep ocean of liquid hydrogen under the visible cloud tops.

Appearance in Earth's sky:

Neptune is so far away it cannot be seen without the aid of a telescope. Neptune revolves so slowly that it spends years in basically the same area of the sky.

Telescopic appearance:

Even in large telescopes, Neptune appears as a small pale blue speck, with no detail visible.

Dwarf Planets: Pluto

Pluto's orbit is very different from that of the other planets. Pluto orbits in a highly oval path called an ellipse, which is angled to the other planets' orbits by about 17.1 degrees. Pluto's distance from the Sun varies because of this oval path, so that sometimes it is closer to the Sun than Neptune.

In 1976, James Christie discovered Pluto's moon, Charon. Charon is about half as big as Pluto, which makes Pluto and Charon more like a double planet than a planet and moon.

The basics:

Order (outwards from the Sun): 9 or 8
Average distance from the Sun: 5 915 800 000 km (39.55 astronomical units)
Period of revolution (length of year): 248.03 years
Period of rotation (length of day): 6 days 9 hours 17 minutes
Diameter: 2 300 km
Surface gravity: 0.05 times greater than Earth's
Composition: Probably rock covered with methane ice
Atmosphere: A thin atmosphere of nitrogen, methane, and carbon monoxide that is produced only when Pluto is closest to the Sun
Satellites: 1 moon (Charon, diameter = 1 200 to 1 300 km)
Surface temperature: -215 degrees C

Appearance of surface:

Scientists suppose that the rocky surface of Pluto is covered by ice frozen as hard as steel. There are also polar ice caps of frozen methane, which partially melt when Pluto is nearest the Sun, releasing the gas that forms Pluto's atmosphere.

Appearance in Earth's sky:

Pluto is invisible without a moderate-sized telescope.

Telescopic appearance:

Even with the largest telescopes on Earth, Pluto appears as a faint star-like point. No details can be seen at all. The Hubble Space Telescope has been able to resolve some rough details on Pluto's surface, and also detect its moon, Charon.