The (or scattered disk) is a distant region of the Solar System that is sparsely populated by icy minor planets, a subset of the broader family of trans-Neptunian objects. The scattered disc objects (SDOs) have orbital eccentricities ranging as high as 0.8, inclinations as high as 40°, and perihelia greater than 30 astronomical units (4.5×109 km; 2.8×109 mi). These extreme orbits are believed to be the result of gravitational “scattering” by the gas giants, and the objects continue to be subject to perturbation by the planet Neptune. While the nearest distance to the Sun approached by scattered objects is about 30–35 AU, their orbits can extend well beyond 100 AU. This makes scattered objects “among the most distant and cold objects in the Solar System”. The innermost portion of the scattered disc overlaps with a torus-shaped region of orbiting objects known as the Kuiper belt, but its outer limits reach much farther away from the Sun and farther above and below the ecliptic than the belt proper.

Because of its unstable nature, astronomers now consider the scattered disc to be the place of origin for most periodic comets observed in the Solar System, with the centaurs, a population of icy bodies between Jupiter and Neptune, being the intermediate stage in an object’s migration from the disc to the inner Solar System. Eventually, perturbations from the giant planets send such objects towards the Sun, transforming them into periodic comets. Many Oort cloud objects are also believed to have originated in the scattered disc.

Centaurs are an unstable orbital class of minor planets that behave with characteristics of both asteroids and comets. They are named after the mythological race of beings, centaurs, which were a mixture of horse and human. Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a few million years. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km.

The first centaur-like object to be discovered was 944 Hidalgo in 1920. However, they were not recognized as a distinct population until the discovery of 2060 Chiron in 1977. The largest known centaur is 10199 Chariklo, discovered in 1997, which at 260 km in diameter is as big as a mid-sized main-belt asteroid.

No centaur has been photographed up close, although there is evidence that Saturn’s moon Phoebe, imaged by the Cassini probe in 2004, may be a captured centaur. In addition, the Hubble Space Telescope has gleaned some information about the surface features of 8405 Asbolus.

As of 2008, three centaurs have been found to display cometary comas: Chiron, 60558 Echeclus, and 166P/NEAT. Chiron and Echeclus are therefore classified as both asteroids and comets. Other centaurs such as 52872 are suspected of showing cometary activity. Any centaur that is perturbed close enough to the Sun is expected to become a comet.

Asteroids are mostly small Solar System bodies[e] composed mainly of refractory rocky and metallic minerals.

The main asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. It is thought to be remnants from the Solar System’s formation that failed to coalesce because of the gravitational interference of Jupiter.

Asteroids range in size from hundreds of kilometres across to microscopic. All asteroids save the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as Vesta and Hygiea may be reclassed as dwarf planets if they are shown to have achieved hydrostatic equilibrium.

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter. Despite this, the total mass of the main belt is unlikely to be more than a thousandth of that of the Earth. The main belt is very sparsely populated; spacecraft routinely pass through without incident. Asteroids with diameters between 10 and 10−4 m are called meteoroids.

Venus is covered with thick clouds that create a greenhouse effect that makes it very hot.
Venus has no moons.

• Diameter: 12,100 km. It is about 1040km smaller in diameter than Earth
• Temperature: Ranges from 900F+/- 50F (about 500°C +/- 32°C) at the surface
• Distance from Earth: At its closest, Venus is 41,840,000 km away* Atmosphere: Carbon dioxide (95%), nitrogen, sulfuric acid, and traces of other elements
• Surface: A rocky, dusty, waterless expanse of mountains, canyons, and plains, with a 200-mile river of hardened lava
• Rotation of its axis: 243 Earth days (1 Venusian Day)
• Rotation around the Sun: 225 Earth days
• Magnetic Field: No

Venus is the brightest object in the sky besides our Sun and the Moon. It is also known as the morning star because at sunrise it appears in the east and and evening star as it appears at sunset when it is in the west. It cannot be seen in the middle of the night.

A Venusian day is 243 Earth days and is longer than its year of 225 days. Oddly, Venus rotates from east to west (retrograde – opposite to that of earth). If you were on Venus, the Sun would rise in the west and set in the east.

Venus and Earth are close together in space and similar in size, which is the reason Venus is called Earth’s sister planet.

Mercury is the closest planet to the Sun and the smallest planet in the(0.055 Earth masses). Mercury has no natural satellites, and its only known geological features besides impact craters are lobed ridges or rupes, probably produced by a period of contraction early in its history. Mercury’s almost negligible atmosphere consists of atoms blasted off its surface by the solar wind. Its relatively large iron core and thin mantle have not yet been adequately explained. Hypotheses include that its outer layers were stripped off by a giant impact, and that it was prevented from fully accreting by the young Sun’s energy.

Mercury is bright when viewed from Earth, ranging from -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°: It can only be seen in morning and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface.

The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008. MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet.
Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K (-180 to 430 °C), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Recorded observations of Mercury date back to the Sumerians in the third millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes. Our name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes’ caduceus.

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The Sun is the Solar System’s star, and by far its chief component. Its large mass (332,900 Earth masses) produces temperatures and densities in its core great enough to sustain nuclear fusion, which releases enormous amounts of energy, mostly radiated into space as electromagnetic radiation, peaking in the 400–700 nm band we call visible light.

The Sun is classified as a type G2 yellow dwarf, but this name is misleading as, compared to the majority of stars in our galaxy, the Sun is rather large and bright. Stars are classified by the Hertzsprung–Russell diagram, a graph that plots the brightness of stars with their surface temperatures. Generally, hotter stars are brighter. Stars following this pattern are said to be on the main sequence, and the Sun lies right in the middle of it.

However, stars brighter and hotter than the Sun are rare, while substantially dimmer and cooler stars, known as red dwarfs, are common, making up 85 percent of the stars in the galaxy. It is believed that the Sun’s position on the main sequence puts it in the “prime of life” for a star, in that it has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is growing brighter; early in its history it was 70 percent as bright as it is today.

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The Sun is a population I star; it was born in the later stages of the universe’s evolution, and thus contains more elements heavier than hydrogen and helium (“metals” in astronomical parlance) than older population II stars. Elements heavier than hydrogen and helium were formed in the cores of ancient and exploding stars, so the first generation of stars had to die before the universe could be enriched with these atoms. The oldest stars contain few metals, while stars born later have more. This high metallicity is thought to have been crucial to the Sun’s developing a planetary system, because planets form from accretion of “metals”.
Facts about the Sun

• distance 149,597,900 km
• (92,975,699 mi., 8.3 light-minutes)
• spectral type G2V
• diameter 1,392,000 km (865,000 mi.)
• surface temperature 5,400ºC (9,800ºF)
• central temperature 14 million ºC (25 million ºF)
• mass (Earth = 1) 332,946
• density (water = 1) 1.409
• surface gravity (Earth =1 ) 27.90
• escape velocity 617.5 km/s (383.8 mi./s)
• rotation period 25.38 days
• apparent magnitude -26.8
• absolute magnitude 4.83