The Solar System and Planets Guide and FactsheetA guided tour of the solar system from the Sun to the Oort Cloud stopping off at the major attractions along the way like the asteroid belt.
Our solar system is defined as the celestial objects that are trapped, and bound, to the Sun's gravitational field. This consists of the eight planets and their moons, dwarf planets and the newly discovered Xena, the Kuiper Belt and the Oort Cloud defining the border. The solar system is also home to a large number of asteroids, meteoroids, comets and interplanetary dust.
Please note that the graphics shown are not to scale.
One Astronomical Unit (AU) is equal to the distance between the Earth and the Sun and is approx 149,597,871 km.
The Solar System
Our solar system consists of an average star we call the Sun, the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. It includes the satellites of the planets; numerous comets, asteroids, and meteoroids; and the interplanetary medium.
The Sun
The Sun is the star at the centre of our solar system. The Earth and other matter (including other planets, asteroids, meteoroids, comets and dust) orbit the Sun, which by itself accounts for more than 99% of the solar system's mass. Energy from the Sun - in the form of sunlight, supports almost all life on Earth via photosynthesis and via heating from insulation - drives the Earth's climate and weather.
- Mass: 1.989×1030 kg (332,946 × Earth)
- Diameter: 1.392×109 m (109 × Earths)
- Temperature (surface): 5778 Kelvin
- Temperature (corona): 5×106 Kelvin
- Luminosity: 3.846×1026 W
Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered, such as why its outer atmosphere has a temperature of over a million Kelvin while its visible surface (the photosphere) has a temperature of just 6,000 K. Current topics of scientific enquiry include the sun's regular cycle of sunspot activity, the physics and origin of solar flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin of the solar wind.
- Never, ever look at the Sun directly through any optical instrument - instant blindness could be the result. Even looking at the Sun with the eye can be harmful.
- DO NOT look at the Sun with magnifying glasses, cameras, binoculars or telescopes, or any optical instrument in fact, without the use of a properly designed, approved and tested filter or specialised instrument.
- DO NOT use photographic film, smoked or tinted glass, plastic or metal film.
- Use only materials and instruments designed for the specific purpose of viewing the Sun.
- If in doubt, seek expert advice first!
Mercury
Mercury is the innermost planet in our solar system, orbiting the Sun once every 88 days. It ranges in brightness from about 2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°, meaning it is only seen in twilight. The planet remains comparatively little known: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40% - 45% of the planet's surface.
- Distance from the Sun: 0.39 AU
- Mass: 0.055 times Earth
- Diameter: 4880 km (0.38 times Earth)
- Surface temperature: 440 Kelvin
- Orbit period: 88 days
Venus
Its size is very similar to Earth, but the conditions on the surface of Venus are completely different. Nearly the whole atmosphere consists of carbon dioxide. Therefore the high temperatures (supposedly caused by the greenhouse effect) make the rocks on the surface glow in red, and the pressure is 95 times higher than this on Earth. Venus is covered by a thick layer of clouds consisting of sulphuric acid.
Venus's surface has been mapped in detail only in the last 20 years. It shows evidence of extensive volcanism, and some of its volcanoes may still be active today. In contrast to the constant crustal movement seen on Earth, Venus is thought to undergo periodic episodes of plate tectonics, in which the crust is subducted rapidly within a few million years, every few hundred million years.
Venus is also known as the Morning or Evening Star and is the third brightest object in the sky (after Sun and Moon).
- Distance from the Sun: 0.72 AU
- Mass: 0.81 times Earth
- Diameter: 12,104 km (0.95 times Earth)
- Surface temperature: 737 Kelvin
- Orbit period: 224 days
Earth
Our home and still, for a long while, the only known planet with life. There are many tens of millions of different species on the planet living in almost every conceivable type of climate. Earth is the only planet not named after a god.
- Distance from the Sun: 1 AU
- Mass: 5.9736×1024 kg
- Diameter: 12,745 km
- Surface temperature: 287 Kelvin
- Orbit period: 365.256 days
Mars
The Red Planet takes its name from the iron oxide (rust) which gives it its colour. It has a thin atmosphere and icy polar caps which could contain water ice.
Mars can be seen from Earth by eye with a brightness of up to -2.9 magnitude, only surpassed by Venus, the Moon and the Sun.
Until the first flyby of Mars by Mariner 4 in 1965, it was thought that Mars had channels of liquid water. We now know that these channels do not exist. Still, out of any planet in our solar system after the Earth, Mars is the most likely to harbour liquid water, and perhaps life. It is the only planet that has seasons, it has a rotational period nearly the same as our own. It has the highest mountain in the solar system, Olympus Mons, the largest canyon in the solar system, Valles Marineris, and polar ice caps. There are parts of Mars that are more photographed than parts of Earth.
Mars is currently host to 4 orbiting spacecraft and the two Mars Exploration Rovers, many more than any planet has ever had before.
Scientists have long speculated about the possibility of life on Mars due to that planet's proximity and similarity to Earth. It remains an open question of whether life exists on Mars now or existed there in the past.
- Distance from the Sun: 1.52 AU
- Mass: 0.11 times Earth
- Diameter: 6,794 km (0.53 times Earth)
- Surface temperature: 210 Kelvin
- Orbit period: 686 days
Asteroid Belt
The asteroid belt is a region of the solar system falling roughly between the planets Mars and Jupiter where the greatest concentration of asteroid orbits can be found.
Despite popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be highly improbable to reach an asteroid without aiming carefully. Nonetheless, tens of thousands of asteroids are currently known, and estimates of the total number range in the millions. About 220 of them are larger than 100 km.
The biggest asteroid belt member is Ceres, which is about 1000 km across. The total mass of the Asteroid belt is estimated to be 3.0-3.6×1021 kilograms, which is 4% of the Earth's Moon. And of that total mass, one-third is accounted for by Ceres alone.
Jupiter
Jupiter is the fifth planet from the Sun and by far the largest within the solar system. Jupiter and the other gas giants - Saturn, Uranus, and Neptune - are sometimes referred to as "Jovian planets." Jupiter has been known since ancient times and is visible in the night sky. The Romans named the planet after the Roman god Jupiter (also called Jove).
Jupiter is usually the fourth brightest object in the sky (after the Sun, the Moon and Venus); however, at times Mars appears brighter than Jupiter.
Jupiter is composed of a relatively small rocky core, surrounded by metallic hydrogen, surrounded by liquid hydrogen, which is surrounded by gaseous hydrogen. There is no clear boundary or surface between these different phases of hydrogen; the conditions blend smoothly from gas to liquid as one descends.
Jupiter is the second most visited planet in the solar system, after Mars, with 7 spacecraft providing images and results about the giant.
- Pioneer 10 flew past Jupiter in December of 1973, followed by Pioneer 11 exactly one year later. Pioneer 10 made the first-ever close-up images of Jupiter, studied its atmosphere, and detected its magnetic field.
- Voyager 1 flew by in March 1979 followed by Voyager 2 in July of the same year. The Voyagers vastly improved their understanding of the Galilean moons and discovered Jupiter's rings. They also took the first close-up images of the planet's atmosphere.
- In February 1992, Ulysses solar probe performed a flyby of Jupiter at a distance of 450,000 km (6.3 Jovian radii). The flyby was required to attain a polar orbit around the Sun. The probe conducted studies on Jupiter's magnetosphere.
- So far the only spacecraft to orbit Jupiter is the Galileo orbiter, which went into orbit around Jupiter on December 7, 1995. It orbited the planet for over seven years and conducted multiple flybys of all of the Galilean moons and Amalthea. The spacecraft also witnessed the impact of Comet Shoemaker-Levy 9 into Jupiter as it approached the planet in 1994, giving a unique vantage point for this spectacular event.
- In 2000, the Cassini probe, en route to Saturn, flew by Jupiter and provided some of the highest-resolution images ever made of the planet.
- Distance from the Sun: 5.87 AU
- Mass: 318 times Earth
- Diameter: 142,984 km (10.84 times Earth)
- Surface temperature: 129 Kelvin
- Orbit period: 11.86 years = 4332.6 days
Saturn
Saturn is the sixth planet from the Sun. It is a gas giant, also known as a Jovian planet, after the planet Jupiter, the second-largest planet in the solar system after Jupiter. Saturn has a prominent system of rings, consisting mostly of ice particles with a smaller amount of rocky debris and dust. It was named after the Roman god Saturn.
Saturn is probably best known for its planetary rings, which make it one of the most visually remarkable objects in the solar system.
The rings can be viewed using a quite modest modern telescope or with a good pair of binoculars. They extend from 6,630 km to 120,700 km above Saturn's equator and are composed of silica rock, iron oxide, and ice particles ranging in size from specks of dust to the size of a small automobile.
In November 1980, the Voyager 1 probe visited the Saturn system. It sent back the first high-resolution images of the planet, rings, and satellites.
Almost a year later, in August 1981, Voyager 2 continued the study of the Saturn system. More close-up images of Saturn's moons were acquired, as well as evidence of changes in the atmosphere and the rings.
- Distance from the Sun: 9.35 AU
- Mass: 95.16 times Earth
- Diameter: 120,536 km (8.96 times Earth)
- Surface temperature: 97 Kelvin
- Orbit period: 29.5 years = 10759 days
Uranus
Uranus is a gas giant, the third largest by diameter and the fourth largest by mass. It is named after Uranus, the Greek god of the sky and progenitor of the other gods.
Uranus was the first planet to be discovered that was not known in ancient times; although it had been observed on many previous occasions, it was always mistakenly identified as a star.
NASA's Voyager 2 is the only spacecraft to have visited the planet and no other visits are currently planned. Launched in 1977, Voyager 2 made its closest approach to Uranus on January 24 1986, before continuing its journey to Neptune.
- Distance from the Sun: 20 AU
- Mass: 14.54 times Earth
- Diameter: 51,118 km (3.97 times Earth)
- Surface temperature: 76 Kelvin
- Orbit period: 84 years = 30685 days
Neptune
Neptune is the outermost gas giant in our solar system. For most of its 165-year orbit it is the eighth planet from the Sun and occasionally the ninth due to Pluto's eccentricity. It is the fourth largest planet in the solar system by diameter and the third largest by mass; Neptune is more massive than its near-twin Uranus as its stronger gravitational field has compressed it to a higher density. The planet is named after the Roman god of the sea.
Discovered on September 23 1846, Neptune is notable for being the only planet discovered based on mathematical prediction rather than regular observations. Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence. It has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25 1989.
- Distance from the Sun: 30 AU
- Mass: 17.15 times Earth
- Diameter: 49, 532 km (3.85 times Earth)
- Surface temperature: 72 Kelvin
- Orbit period: 165 years = 60,189 days
Kuiper Belt
The Kuiper belt is an area of the solar system extending from the orbit of Neptune (at 30 AU) to 50 AU from the Sun. The objects within the Kuiper Belt together with the members of the scattered disk extending beyond, are collectively referred to as trans-Neptunian.
The interaction with Neptune (2:1 orbital resonance) is thought to be responsible for the apparent edge at 48 AU (a sudden drop in the number of objects) but the current models have yet to explain this peculiar distribution in detail.
Over 800 Kuiper belt objects (KBOs) have been discovered in the belt, almost all of them since 1992. Among the largest are Pluto and Charon, but since the year 2000 other large objects that approached their size were identified. 50000-Quaoar, discovered in 2002, which is a KBO, is half the size of Pluto and is larger than the largest asteroid, 1-Ceres 2005-FY9 and 2003-EL61, both announced on 29 July 2005, are larger still. Other objects, such as 28978-Ixion (discovered in 2001) and 20000-Varuna (discovered in 2000) while smaller than Quaoar, are nonetheless quite sizable. Sedna, a small red planetoid with a diameter roughly halfway between Pluto and Quaoar, was first sighted on November 14th 2003. The exact classification of these objects is unclear since they are probably fairly different from the asteroids of the asteroid belt. The largest recent discovery is 2003-UB313, codenamed Xena. It has led scientists to question the definition of the term Planet, as it is larger than Pluto and has already been called a tenth planet by some.
Update:
As of Thursday 24 August 2006, Pluto is no longer the ninth planet in the solar system. Its status was demoted by the IAU. You can read more about plutos demotion to a dwarf planet by the IAU .
Pluto
Pluto is the ninth and the smallest of the traditional planets of the Solar system is a dwarf planet at the outer reaches of the solar system. Its status as a planet was disputed in recent years with the discovery of Xena, which is slightly larger.
Pluto is a very small and icy chunk of rock. It has a reddish surface made of water, nitrogen and methane ice. Pluto has a very eccentric orbit, which in part takes as close to the Sun as Neptune.
The New Horizons spacecraft, which lifted off from Cape Canaveral, Florida on January 19, 2006, is expected to become the first spacecraft to fly by Pluto on July 14, 2015.
- Distance from the Sun: 29 - 49 AU
- Mass: 0.0021 times Earth
- Diameter: 2,274 km (0.179 times Earth)
- Surface temperature: 50 Kelvin
- Orbit period: 248 years = 90,465 days
Eris
On July 30th 2005 the discovery of a tenth planet was announced by its discoverer Michael E. Brown. Its official scientific name is 2003 UB313 and was later officially named Eris. Its informal name used by its discoverers was Xena. After its discovery, the International Astronomical Union (IAU) published an updated definition of the term "planet" in early September 2006, which classified Eris as a dwarf planet, and since Eris is larger than Pluto, pluto was demoted to dwarf planet.
The planet is very cold and has an almost white surface. Probably it is covered with frozen Methane. Its axis is lent 44 degrees compared to the other planets. This is one reason why it wasn't discovered earlier. The planet is located in the Kuiper belt and is slightly bigger than Pluto.
Eris has a moon with which has is at least one-tenth of Eris's size.
- Distance from the Sun: 97 AU
- Diameter: 2390 km (0.188 times Earth)
- Orbit period: 557 years = 203,440 days
Oort cloud - The Edge of the Solar System
Alternatively termed the Opik-Oort Cloud, is a postulated spherical cloud of comets situated about 50,000 to 100,000 AU from the Sun. This is approximately 1000 times the distance from the Sun to Pluto or roughly one light year, almost a quarter of the distance from the Sun to Proxima Centauri, the star nearest the Sun.
The Oort cloud would have its inner disk at the ecliptic from the Kuiper belt. Although no direct observations have been made of such a cloud, it is believed to be the source of most or all comets entering the inner solar system (some short-period comets may come from the Kuiper belt), based on observations of the orbits of comets.
