Sun Facts:

The Sun is the star at the center of our solar system, and is by far the largest member with a thousand times more mass than all the planets, moons, asteroids and comets put together. The Sun itself is a very typical G-type, main sequence, Population I star. The average distance between the Sun and the Earth is 1 AU or 149,598,000 km. The diameter of the Sun is about 1,392,000 km, or about 109 times the diameter of the Earth. The temperature at the surface of the Sun is about 5800 K, or 5,527 °C and the temperature at the center is about 1.55 x 10⁷ K. By mass, about 74% of the Sun is hydrogen, 25% is helium and 1% is other elements. It is orbiting the center of the Milky Way Galaxy every 220 million years at a speed of 220 km/s. Light from the Sun takes 8.32 minutes to reach the Earth.

The Structure of the Sun:

From the center of the Sun to about 0.25 of the solar radius is the core. The core of the Sun is extrememly hot and dense and is fusing hydrogen to helium at a rate of 600 million metric tons per second. The density at the center of the Sun is 160,000 kg/m³ and by 0.3 of the solar radius, the pressure has dropped to 13,000 kg/m³. For comparison, the density of air on Earth at sea-level is approximately 1.2 kg/m³. 94% of the mass of the Sun is located withing the inner 0.5 of a solar radius. Outside of the core, the density and temperature are too low for thermonuclear reactions to take place. Out to about 0.7 of the solar radius, energy flows by radiative diffusion. The radiative zone is transparent enough that photons can travel moderate distances before being absorbed or scattered. Beyond 0.7 of a solar radius, the temperature is low enough that hydrogen nucleii can join with electrons to form atoms, and these atoms are very effective at absorbing photons. From here to nearly the surface of the sun is the convective zone where hot atoms and molecules rise to the surface where they cool and sink, causing convection currents.

The Atmosphere of the Sun:

The Sun has an atmosphere composed of three main layers. The lowest one is the photosphere and essentially all of the visible light we see from the Sun comes from this thin layer of gas. The Sun's density at the photosphere is very low and we can see about 400 km into the photosphere. When atronomers observe the photosphere with telescopes they see a blotchy pattern of light and dark regions. This effect is called granulation. Each granule is about 1000km across and is caused by the convection of gas. Hot gas rises in the middle of a granule, and this area appears brighter, then the gas cools and sinks on the edges of the granule and these areas are darker. Granules only last several minutes. On a larger scale are supergranules which are superimposed on the granules but are about 35,000 km across and last about 24 hours. The surface of the photosphere is about 5800 K and has a density of about 10^-4 kg/m³. This is about 0.01% the density of air at sea level. 

A view of an irregular-shaped sunspot and granules on the Sun's surface, seen on August 22, 2003. (Swedish 1-m Solar Telescope (SST) operated by the Royal Swedish Academy of Sciences, Oddbjorn Engvold, Jun Elin Wiik, Luc Rouppe van der Voort, Oslo)

The next layer out is the chromosphere, which is one ten-thousandth as dense as the photosphere. The chromosphere is about 1600 km thick and is only visible when the Sun is eclipsed by the moon or when astronomers use instruments that block out the disk of the Sun. The temperature of the chromosphere increases with altitude, so the temperature at the top of the photosphere is approximately 4400 K, at the top of the chromoshpere it is nearly 25,000 K! This seems counter-intuative, but can be explained by the magnetic fields generated by the Sun, which interact with the charged particles in the chromosphere and cause them to accelerate, which makes them hotter. Astronomers do not yet fully understand the mechanisms that cause this.

The Sun's Magnetism:

The Sun has a very complex magnetic field, which astronomers are actively studying. The magnetic field causes many of the features on the sun including sunspots, prominences, flares and coronal mass ejections.

Sunspots are irregularly shaped dark regions in the photosphere of the Sun such as the one in the image above. They have an inner dark area called the umbra, and a brighter edge region called the penumbra. Although these terms are the same as the ones used to describe eclipses, sunspots are not shadows. The are regions of cooler temperature on the photosphere. The umbra of a sunspot typically has a temperature of around 4300 K while the penumbra is typically 5000 K. This is quite a bit cooler than the photosphere's average temperature of 5800 K. Sunspot groups usually last about two months, and Galileo was the first to use a telescope and filter to observe the movement of sunspots to determine that the Sun rotates about once every four weeks. The number and position of sunspots varies with time over an 11 year cycle. At the beginning of each cycle, sunspots typically form near latitudes of 30 north or south. As the cycle continues, the spots usually form closer to the equator. At the beginning of each cycle there are very few sunspots, called a sunspot minimum. These occured in 1976, 1986, 1996 and 2008. A sunspot maximum occured in 1979, 1989 and 2000. Sunspots are caused by concentrations in the Sun's magnetic field, which deflect the charged gas of the photosphere, creating areas that are cooler than the rest of the surface. In recorded history there have been periods of relatively low sunspot activity for longer than the typical cycle. from about 1645 to 1715 virtually no sunspots were seen, and Europe experienced a very cold period referred to as the Little Ice Age. In the eleventh and twelfth centuries there was a period of increased sunspot activity and the climate on Earth was temporarily warmer.

Prominences are giant arching colums of gas in the corona that often form just before a sunspot appears below them in the photosphere. They are a result of the interactions of the gas in the corona with the magnetic fields of the Sun. Particularly energetic promineces may break free from the magnetic fields of the Sun and burst into space.

Flares are violent eruptions on the Sun that occur in complex sunspot groups. Within a few minutes, the temperature in the region may rise as much as 5,000,000 K and large quantities of particles and radiation are blasted into space. The energy of a solar flare can be as much as the energy that would be released by 100 trillion nuclear bombs!

Coronal mass ejections are larger scale versions of solar flares caused by major disturbances in the Sun's magnetic field. In a coronal mass ejection, over a billion tons of high temperature gas is ejected from the corona at very high speed. They occur every few months and if an ejection is pointed towards the Earth, it can disrupt satellites, interfere with communication equipment on Earth, and pose health hazards to astronauts in orbit.

Image Credit: SOHO Consortium, ESA, NASA

Many images of the solar system do not do justice to how small the planets are relative to the Sun, or how distant they are from the Sun and each other. The solar system is really mostly empty space. The Toilet Roll Solar System activity helps give an idea of the relative distances between the planets. This Powers of Ten Demonstration shows relative sizes and distances by zooming out. Some other fun things to try include finding your age and weight on other planets.

The image below by Dave Jarvis shows relative sizes of the planets in our solar system compared to the Sun and to other stars.

The planets in our Solar System are believed to have formed from the same spinning disc of dust that formed the Sun. This disc, called the solar nebula, was composed mainly of hydrogen and helium, but also had other elements in smaller proportions. The nebula had a certain amount of angular momentum orbiting the forming Sun. Particles in the spinning disc began to clump together as gravity attracted them to each other. Over a few million years many of these chunks had merged together and there were about 10⁹ objects called planetesimals, with diameters of about 1000 m. Over time the planetesimals continued to collide and join together, attracted by gravity. These larger objects, about the size and mass of our Moon, are called protoplanets. The accumulation of material to form planets in this way is called accretion.

The temperature of the early solar system explains why the inner planets are rocky and the outer ones are gaseous. As the gases coalesced to form a protosun, the temperature in the solar system rose. In the inner solar system temperatures were as high as 2000 K, while in the outer solar system it was as cool as 50 K. In the inner solar system, only substances with very high melting points would have remained solid. All the rest would have vaoprized. So the inner solar system objects are made of iron, silicon, magnesium, sulfer, aluminum, calcium and nickel. Many of these were present in compounds with oxygen. There were relatively few elements of any other kind in a solid state to form the inner planets. The inner planets are much smaller than the outer planets and because of this have relatively low gravity and were not able to attract large amounts of gas to their atmospheres. In the outer regions of the solar system where it was cooler, other elements like water and methane did not vaporize and were able to form the giant planets. These planets were more massive than the inner planets and were able to attract large amounts of hydrogen and helium, which is why they are composed mainly of hydrogen and helium, the most abundant elements in the solar system, and in the universe.

Mercury is the closest planet to the Sun with an average distance of 0.387 AU or 5.79 x 10⁷ km. It takes 88 Earth days to orbit the Sun, and rotates very slowly at a rate of 1 rotation every 58.7 Earth days. Its mass is 3.3 x 10²³ kg or 0.055 Earth masses. It has no moons, little to no atmosphere and generally a very hot surface. The surface temperature ranges from 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F) with the hottest temperatures where the Sun's rays hit perpendicular to the surface, and the coolest temperatures in craters at the poles.

Mercury can be seen with the naked eye for a few hours before or after sunset, depending on which side of the Sun it is on relative to Earth. It never travels more than 28° from the Sun from our point of view. It has a large iron core and a magnetic field about 1% as strong as Earth's. Its surface is covered with craters where there is even water ice! Mercury's orbit has a 3:2 spin–orbit resonance, and rotates three times for every two revolutions around the Sun. Mercury's orbit is eccentric and this resonance makes it stable.

Mercury was visited from 1974 to 1975 by Mariner 10, and again in 2008 and 2009 by MESSENGER. Mariner 10 mapped about 45% of Mercury's surface, and MESSENGER mapped another 30% during its two flybys. It is scheduled to go into orbit around Mercury on March 18, 2011.

 Venus is the second closest planet to the Sun with an average distance of 0.723 AU or 1.082 x 10⁸ km. It takes 224.7 Earth days to orbit the Sun, and rotates retrograde (opposite the direction it orbits) very slowly at a rate of 1 rotation every 243 Earth days. Its mass is 4.868 x 10²⁴ kg or 0.815 Earth masses and it is nearly the same size as Earth with a radius of 0.9488 the Earth's radius. It has no moons, and a very thick atmosphere composed mainly of carbon dioxide. Its surface is extrememly hot with and average temperature of 733 K (460 °C or 860 °F).

Venus can be seen with the naked eye for a few hours before or after sunset, depending on which side of the Sun it is on relative to Earth. It never travels more than 47° from the Sun from our point of view. Scientists are not yet sure whether it has a liquid core, but it does not have a magnetic field. It has many large volcanoes and its atmosphere has evidence of recent volcanic activity. Because of its thick atmosphere which causes a huge greenhouse effect, Venus is the hottest planet in the solar system. The atmosphere also has clouds of concentrated sulfuric acid. Venus seems not to have plate tectonics like Earth does because its crust is too thin and its interior too hot for large plates to form from the crust. Instead it may have flake tectonics where the thin surface crust flakes and crumples.

Many spacecraft, including the Soviet Venera space probes 3-14, and Mariners 2,5 and 10, have flown by or landed on Venus. Most of the spacecraft that landed on Venus lasted only a very short time because of the high pressure and extreme heat.

Earth is the third closest planet to the Sun with an average distance of 1.0 AU or 1.496 x 10⁸ km. It takes 365.256 Earth days to orbit the Sun, and rotates at a rate of 1 rotation every 23.9345 hours. Its mass is 35.974 x 10²⁴ kg. It has one moon, and an atmosphere composed of 78% nitrogen, 21% oxygen, 0.035% carbon dioxide, and 1% water vapor. The surface temperature ranges from 183 to 333 K (−90 °C to 60 °C, −130 °F to 140 °F).

The Earth is the only planet in the solar system with life on it as far as we know. About 70% of the Earth's surface is covered with water. 97% of Earth's water is salty and is contained in oceans, seas, and some lakes and rivers. 80% of the Earth's fresh water is frozen at the north and south poles. 99.5% of the remaining fresh water is unavailable for human use because it is either too far underground, trapped in soil, polluted, etc., so only about 0.003% of Earth's water is potable. 

Like Venus and Mars, Earth has volcanos, but in addition Earth has tectonic activity. The surface layer of Earth is called the crust. On the continents, this crust is about 35 km thick, while the crust under the oceans is about 6 km thick. The crust and the upper layer of the mantle (together called the lithosphere) ride on top of the viscous athenosphere, which is very hot and has convection currents flowing throughout causing the plates on top to move. When the plates colide, this leads to earthquakes and mountains forming. When the plates pull apart, molten rock known as lava, seeps upward in the space created and cools. Scientists believe that originally there was one large continent, which has since split into the seven we see today. 

Earth is the first planet from the Sun to have a moon. Despite being Earth's closest neighbor, the Moon is very different from the Earth. It has no atmosphere and the surface is billions of years old. The dark areas in the image below are called maria. They are regions that were struck by large asteroids in the past, which caused molten rock from beneath the crust of the Moon to seep out. The Moon has hardly any magnetic field, although rocks studied on the Moon show evidence that in the past, the Moon probably had a small, but stronger magnetic field than it has now.

The Moon's Exterior

The Moon is much smaller than the Earth with a diameter of about 3476 km or 2160 miles. It is about 384,000 km or 238,900 miles from Earth.

The surface of the Moon is covered with craters because the Moon doesn't have an atmosphere to protect it from incoming asteroids and debris, and it has no plate tectonics that would shift the surface and cause craters to disappear over time.

The rocks brought back to Earth from the Moon are all between about 3 and 4 billion years old. They are all igneous rock, and most likely formed when the Moon was molten, and unlike rocks on Earth, contain absolutely no water.

The Moon's Interior

The Moon has a crust, upper and lower mantles, and and iron-rich core in the center. The average thickness of the crust is about 60 km on the side facing the Earth and about 100km on the back side of the Moon. 

The Moon's History

The most widely accepted theory for the creation of the Moon is called the Collisional Ejection Theory or Giant impact hypothesis. This theory is that when Earth was forming, a large, Mars-sized object slammed into it, causing a large mass to be ejected. This mass became our Moon.

Phases of the Moon

The Moon goes through phases every month, where different amounts of the Moon's surface appear bright. This is because as the Moon orbits the Earth, one side of the Moon is always facing the Sun. As the Moon's orbits, we see the Moon in different positions, and in these different positions we see varying amounts of the surface lit by the Sun.

Credit: This diagram based on a similar diagram

The lunar phase depends on the Moon's position in orbit around the Earth. This diagram looks down on Earth from north. Earth's rotation and the Moon's orbit are both counter-clockwise here with sunlight is coming in from the right. The times marked on Earth show the local time at various location around the Earth. This diagram shows, for example, that the full moon will always rise at sunset and that the waning crescent moon is high overhead around 9:00 am local time.

This animation shows the phases of the Moon in detail.

We always see the same face of the Moon because it is in a synchrnous orbit with Earth. Each time it orbits the Earth it spins once on it's axis. As a result it keeps the same face toward Earth.

Eclipses

Lunar Eclipses:

As the Moon orbits the Earth, it occasionally moves into Earth's shadow. This happens only a handful of times each year because the Moon's orbit around the Earth is inclinde at a 5° angle to the plane of the Earth's orbit around the Sun. For the Moon to be in position to move into the Earth's shadow, it must be in the full moon position. On average two or three times a year the Moon happens to be at a point not far above or below the plane of the Earth's orbit during a full moon and a lunar eclipse can be observed from Earth. Most of the time, when the Moon is in full moon position, it is either above or below the plane of Earth's orbit and does not come into Earth's shadow.

The Earth casts two shadows called the umbra and penumbra. The umbra is a completely dark area where light from the Sun is totally blocked. The penumbra is an area where much, but not all light coming from the Sun is blocked. 

There are three main types of eclises. A lunar eclips is visible to all parts of the Earth where it is night time during the eclipse. Sometimes the Moon travels only through the penumbra. This is called a penumbral eclipse, and the Moon does not appear much dimmer than usual. These eclipses are not very noticable. Sometimes the Moon travels into the penumbra and partly into the umbra. Such an eclipse is called a partial eclipse. And sometimes the Moon travels through the umbra and appears to go almost completely dark. This is called a total eclipse. During a total eclipse, the moon will appear slightly red because some sunlight travels through the Earth's atmosphere and reaches the Moon.

Solar Eclipses:

A solar eclipse happens when the Moon blocks sunlight from reaching part of the Earth. A solar eclipse can only happen when the Moon is in the new moon position, and can only be observed for a few moment by observers in a small strip of Earth's surface called the path of totality. Outside the path of totality is a larger region where a partial solar eclipse can be observed.

Please note: It is very important to use a special filter approved for solar viewing when observing any type of solar eclipse. Permanent eye damage orblindness could result otherwise.

The Exploration of the Moon

Between July 20th, 1969 and December, 1972, there were 6 manned lunar landings. Lunar missions began in 1959 with unmanned lunar landers and orbiters. Since 1972, many unmanned landers and orbiters have been sent to the Moon.

Mars is the 4th planet from the Sun with an average distance of 1.524 AU or 2.279 x 10⁸ km. It takes 686.98 Earth days to orbit the Sun, and rotates at a rate of 1 rotation every 24 hours 37 minutes and 22 seconds. Its mass is 6.418 x 10²³ kg or 0.107 Earth masses. It has two moons, and a thin atmosphere composed of 95% carbon dioxide, 2.7% nitrogen, 0.03% water vapor and 2% other gases. The surface temperature ranges from 133 to 293 K (−140 °C to 20 °C, −220 °F to 70 °F) with an average temperature of 250 K (-23 °C or -10 °F)

Mars is visible to the naked eye and is one of the brightest objects in the night sky. It is a bright red color because its surface contains large amounts of iron oxide. Mars' two moons, Phobos and Deimos are small and irregularly shaped, and are probably captured asteroids. The largest volcano in the solar system is located on Mars: Olympus Mons, and the deepest canyon in the solar system the is Valles Marineris.

Many spacecraft have been sent to Mars, including most recently the two Mars Exploration Rovers, Spirit and Opportunity.

Asteroids are rocky objects which orbit the Sun in our Solar System, but are too small to be considered planets. They are in fact, commonly known as Minor Planets due to their size. The majority of the asteroids in our Solar System orbit the Sun in what we call the "Asteroid Belt." This is located between Mars and Jupiter as can be seen in the diagram below from the NASA Lunar and Planetary Institute. Some other asteroids are found in Jupiter's orbit at Lagrangian points points in Jupiter's orbit where they have a stable orbit. These asteroids are called the Trojan asteroids.

However, there are some asteroids that have left this region after being influenced by the gravitational forces exerted by the planets in the Solar System, and are on paths which bring them near to the Earth. If they approach the Earth at a distance of less than 1.3 AU (1 AU is the distance between the Earth and the Sun), then they are considered to be a Near Earth Object, or NEO. Although the chances of such asteroids hitting the Earth are very slim, our planet has been struck in the past. An example of an asteroid impact creating a crater on Earth is shown in the image of the Barringer Meteor Crater in Arizona.

Asteroids come in different shapes and sizes, ranging from about 1000km across, to a few cm. They are thought to be the leftovers  of the formation of the Solar system which didn’t quite make it into a planet, but remained in orbit around our Sun. Most of our knowledge of asteroids comes from our study of meteorites found on Earth. An asteroid which is on a collision path with the Earth is called a meteroid. If the meteoroid then enters the Earth’s atmosphere, as it burns up due to the speed of its entry, it will leave a streak of light in the sky, and it becomes known as a meteor (or a shooting star!). What is left of the meteor, when it hits the Earth, is known as a meteorite.

One of the largest asteroids, and also the brightest, is Vesta. Discovered in 1807, this asteroid is 530km in diameter. The smallest asteroid known to date is approx. 3-6m in diameter. It passed very close-by to the Earth in 2003 - in fact, it was the closest approach by an asteroid that didn’t hit the Earth that has ever been documented, at a distance of 88,000km. This is less than a quarter of the distance to the Moon, and therefore, much less than 1.3AU, earning it NEO status. The largest asteroid in the asteroid belt used to be Ceres until it was given "dwarf planet" status in 2006.

- edited from text provided by Dr. Sarah Roberts

Jupiter is the 5th planet from the Sun with an average distance of 5.203 AU or 7.783 x 10⁸ km. It takes 11.86 Earth years to orbit the Sun, and rotates very quickly at a rate of 1 rotation every 9 hours 50 minutes and 28 seconds. It is the largest planet in the Solar System with a mass of 1.899 x 10²⁷ kg or 317.8 Earth masses. It has 63 confirmed moons, and the four largest are Ganymede, Callisto, Io, and Europa (also known as the Galilean moons), would be considered dwarf planets if they orbited the Sun rather than Jupiter. The average temperature of Jupiter at the cloudtops is about 165 K (−108 °C or −162 °F).

Jupiter can be seen with the naked eye and is one of the brightest objects in the night sky. It is 2.5 times more massive than all the other planets in the Solar System combined. Jupiter has a very thin ring system, discovered by Voyager 1 in 1979. The Great Red Spot is a gigantic storm that has been in existence since at least 1831, possibly even as early as 1665. The storm is large enough to contain 2-3 Earths. Io is the most volcanic body in the Solar System. Europa is completely covered in ice with a hypothesized water ocean beneath it, and is one of the other very few places in the Solar System that has the possibility for life to exist. Ganymede is the largest moon in the Solar System. Io has very active volcanos and lightning has been observed on it as well.

Jupiter has been visited by several spacecraft including Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Cassini and New Horizons. They discovered a strong magnetic field generated by Jupiter, which is 14 times stronger than Earth's magnetic field.

Dwarf planets are objects in the solar system which orbit the Sun. They are large enough that their gravity has caused them to form a spherical shape. Unlike planets, they have not cleared their orbits, meaning that there could be several or many dwarf planets orbiting the Sun at the same distance and in the same plane. Satellites of planets are not considered dwarf planets.

The definition of a dwarf planet was agreed upon in 2006 by the International Astronomical Union (IAU) after being debated for years. Pluto had been considered a planet for several generations, but in 1992, other objects were discovered in the same region as Pluto, and then in 2003, Eris was discovered, and found to be larger than Pluto. Astronomers realized that more and more objects were being discovered in the region known as the Kuiper belt, and that if Pluto was going to be considered a planet, many (possibly hundreds of) other objects were going to be added to the number of planets in the solar system. Instead, the IAU decided to create a new category with Pluto as the first member. At the moment, there are 5 known objects in our solar system that meet the criteria to be called a dwarf planet. In order from the Sun they are: Ceres, Pluto, Haumea, Makemake, and Eris.

When the definition of dwarf planets was agreed upon, the largest member of the asteroid belt, Ceres, also qualified as a dwarf planet. 

Comets

Comets are small, irregularly shaped bodies in the solar system composed mainly of ice and dust that typically measure a few kilometers across. They travel around the sun in very elliptical orbits that bring them very close to the Sun, and then send them out past Neptune. There are two categories of comet, based on the amount of time they take to orbit the Sun. Short-period comets take less than 200 years, and long-period comets take over 200 years, with some taking 100,000 to 1 million years to orbit the Sun. The short-period comets are found near the ecliptic, which means they are orbiting the Sun in same plane as the planets. The short-period comets are thought to originate in the Kuiper Belt, an area outside Neptune's orbit (from about 30 to 50 AU) that has many icy comet-like objects. The long-period comets tend to have orbits that are randomly oriented, and not necessarily anywhere near the ecliptic. They are thought to originate in the Oort cloud. The Oort cloud has never been observed, but is believed to have at least 10¹² icy objects located between 3000 AU and 100,000 AU in a spherical distribution around the Sun.

As comets travel close to the Sun, the Sun's heat begins to vaporize the ices and causes them to form a fuzzy, luminous area of vaporized gas around the nucleus of the comet known as a coma. Outside the coma is a layer of hydrogen gas called a hydrogen halo which extends up to 10¹⁰ meters in diameter. The solar wind then blows these gases and dust particles away from the direction of the Sun causing two tails to form. These tails always point away from the Sun as the comet travels around it. One tail is called the ion tail and is made up of gases which have been broken apart into charged molecules and ions by the radiation from the Sun. Since the most common ion, CO+ scatters the blue light better than red light, to observers, this ion tail often appears blue. The other tail is called a dust tail and normally appears white. The dust in this tail is less strongly affected by the solar wind since the particles of dust are much larger than the ions in the ion tail. That is why the dust tail is usually curved rather than straight, and does not point directly away from the Sun, because it is also influenced by the motion of the comet. The tails of the comet can be extremely large and my extend a distance of up to 1 AU (the distance between the Earth and the Sun)! Both tails can be seen in the image of Comet Hale-Bopp to the right, taken by Malcom Ellis in England.

Kuiper Belt

The Kuiper belt is a region between about 30 and 50 AU from the Sun in the plane of the ecliptic. It is believed to be where most of the trans-Neptunian objects are, including Pluto and several other recently discovered dwarf planets. It is also thought to be the origin of many of the solar system's short-period comets. There are several types of Kuiper belt objects, or KBOs. Classical KBOs orbit between 30 and 50 AU from the Sun with most between 42 and 48 AU. They tend to have orbital inclinations of less than 30°. Another type of KBO are called scattered KBOs which have much higher orbital eccentricities. They probably moved into these irregular orbits as a result of gravitational interactions with gas giants, especially Neptune. They are thought to be one source of short period comets. The last type of KBO are called resonant KBOs because these objects are in resonance orbits with Neptune. Many are in 3:2 orbital resonance with Neptune, while others are in 4:3, 5:3 or 2:1. These orbital resonances are relatively stable orbits, and prevent the objects being pushed out of orbit by Neptune's gravity. KBOs in a 3:2 resonance are called plutinos, named after Pluto.

Oort Cloud

The Oort cloud has never been observed but is thought to be a spherical distribution of icy objects like comets orbiting our Sun at distances between 3000 and 100,000 AU. It is also believed to be the origin of many of the long-period comets in the solar system. The objects in the Oort cloud probably formed closer to the Sun, around the present day orbits of Uranus and Neptune, and were then pushed out to their current positions by gravitational interactions with the planets. Astronomers theorize that there are approximately 10¹² to 10¹³ members of the Oort cloud with a total mass of about 100 Earth masses. Objects within the Kuiper belt are affected by the gravitation of the planets. Further out, is a region of the Oort cloud from 50-2000 AU where objects are not affected by the planets. From 2,000-15,000 AU, objects in the cloud are affected by galactic tidal forces, and in the outer Oort cloud, from 15,000-100,000 AU, objects are affected by the gravity of other stars. Outside the Oort cloud, the Sun's gravitation is not strong enough to keep objects in orbit.