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The Outer Solar System Overview

Oort Cloud

The inner solar system consists of the sun and the eight planets. The outer solar system comprises everything beyond the planet Neptune but within the gravitational pull of the sun. The outer solar system starts at about 30 AU (astronomical units - thirty times the distance of the earth from the sun). The outer solar system ends about 200,00 AU from the sun, where the sun’s gravitational field fully ends, the solar wind stops, and becomes part of interplanetary space.

The outer solar system consists of the Kuiper (Ky' per) Belt and the Oort Cloud. The Kuiper belt starts at 30 AU and ends about 50 AU. There is a gap called the Kuiper cliff about 50 AU to 50,000 AU , where there is almost nothing in regular orbit. The Oort Cloud starts about 50,000 AU and extends up to about 200,000 AU. The Oort Cloud is only loosely bound to the Solar System, and therefore is easily affected by the gravitational pull of passing stars.

The Kuiper belt is home to millions of floating asteroids consisting of rock and ice. It is the starting point of many short-period comets. Short-period objects in the Kuiper belt have orbital periods up to 200 years. The Oort Cloud is the origin point of many long-period comets with orbits up to a million years or more.

While the dwarf planets (like Pluto) receive most of the attention, most of the bodies in the outer solar system are the size of comets. Trillions of ice particles and millions of comets smaller than a kilometer in width coexist with an estimated 75,000 frozen bodies over a hundred kilometers in width.  Top

The Chemistry Of Outer Space

Outer Map

Most of the objects in the outer solar system are a combination of rocks and ice. Compounds necessary for life like methane and hydrocarbons are found there. A number of objects are covered with tholin, a hydrocarbon molecule that forms after long periods of exposure to ultraviolet light.

There is probably no liquid water in the outer solar system. Objects this far from the sun are hardly warmed by it, with temperatures that average about 50 Kelvin. Not far above absolute zero where even hydrogen freezes.

Some outer space objects may or may not have sufficient radioactive elements to have a warm core, which would permit a liquid ocean of water to exist beneath the frozen surface.

They may also have cyro-volcanoes, where jets of frozen water or methane erupt due to the gravitational tug of war between the Sun, Neptune and other bodies.  Top

The Kuiper Belt

The Kuiper Belt

The Kuiper Belt is populated with hundreds of thousands of icy bodies larger than 100 km (62 miles) across and an estimated trillion or more comets. Kuiper Belt Objects (KBOs) are remnants from the formation of the solar system about 4.6 billion years ago.

The dwarf planet Pluto is the best known of the larger objects in the Kuiper Belt. Comets from the Kuiper Belt take less than 200 years to orbit the sun and travel approximately in the plane in which most of the planets orbit the sun.

The Kuiper Belt is named after Gerard Kuiper who in 1951 proposed that a belt of icy bodies probably lay beyond Neptune. It was the only way, he figured, to solve a baffling mystery about comets. Comets beyond Neptune never stuck together, remaining instead primitive and individual. They occasionally fall toward the sun and become short-period comets. These short-period comets encounter the sun so often that they quickly evaporate - vanishing in about a few hundred thousand years.

The discovery of Eris in 2005, orbiting the sun and similar in size to Pluto (which was then designated the ninth planet), forced astronomers to consider whether Eris should be classified as the tenth planet. Instead in 2006, the International Astronomical Union created a new class of objects called "dwarf planets" and placed Pluto in this category.  Top

Pluto - A Dwarf Planet


Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune. Not knowing of the error, Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona did a very careful sky survey which turned up Pluto anyway. Pluto orbits the sun in the Kuiper belt.

Pluto has an atmosphere consisting of mainly nitrogen extending to 1,600 km above the surface. Methane is another constituent of the atmosphere and it is likely caused by sunlight breaking down methane gas particles into ethylene and acetylene. The atmosphere and gases were determined by the space satellite New Horizons. NASA's New Horizons launched in January 2006 and conducted a six month long reconnaissance flyby study of Pluto and its moons in the summer of 2015 - 9 years later!

Pluto has a heart shaped region called Sputnik Planum. See the pink area to the center right in the photo to the left. The region is composed of nitrogen, carbon monoxide and methane ices.  These ices flow like glaciers in the minus 234 C environment.  The glaciers are thought to flow just as glaciers here on earth do, although water ice on Pluto is very hard and virtually immovable.

Some interesting facts about Pluto follow. Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Also, Pluto rotates in the opposite direction from most of the other planets. Like Uranus, the plane of Pluto's equator is at almost right angles to the plane of its orbit. The surface temperature on Pluto varies between about -235 and -210 C (38 to 63 K). Finally, Pluto has five moons:  Charon, Hydra, Nix, Kerberos, Styx.  Top

Other Dwarf Planets

Dwarf Planets

Eris was discovered in 2003, is slightly larger than Pluto and is the largest dwarf planet. Some reports initially referred to it as the tenth planet. However as mentioned above, in 2006 the International Astronomical Union (IAU) created a new class of objects called "dwarf planets" and placed Pluto, Eris and Ceres in this category.

A "planet" is now defined by the IAU as a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a nearly round shape, and (c) has cleared the neighborhood around its orbit (no other asteroids or comets in its pathway).

A "dwarf planet" is a celestial body that (a) is in orbit around the sun, (b) has sufficient mass for its self-gravity to overcome body forces so that it has a nearly round shape, (c) has "not cleared" the neighborhood around its orbit, and (d) is not a satellite.

Closest to home, Ceres is the largest and most unique asteroid in the main Asteroid Belt between Jupiter and Mars, but the smallest of the dwarf planets. Ceres was discovered way back in 1801and was originally called a planet, then an asteroid. Composed of rock and ice, Ceres was the first dwarf planet to be visited by a spacecraft. NASA's Dawn mission visited it during March of 2015. There are also two additional outer space objects currently defined as dwarf planets: Haumea discovered in 2003, and Makemake (ma ki' ma ki') discovered in 2005. Astronomers expect that there may be as many as 50 dwarf planets in the solar system.  Top

Planet Nine

The 9th Planet

In January of 2016, Caltech researchers revealed that they found evidence of a giant planet tracing a bizarre, highly elongated orbit in the very outer Solar System. The object, which the researchers have nicknamed Planet Nine, has a mass about 10 times that of earth and orbits about 20 times farther from the sun on average than does Neptune (which orbits the sun at an average distance of 2.8 billion miles). In fact, it would take this new planet between 10,000 and 20,000 years to make just one full orbit around the sun.

Planet Nine, at 5,000 times the mass of Pluto, is sufficiently large that there should be no debate about whether it is a true planet. Unlike the class of smaller objects, like the above dwarf planets, Planet Nine gravitationally dominates its neighborhood of the Solar System. In fact, it dominates a region much larger than any of the other known planets. A predicted consequence of Planet Nine is that a second set of confined objects should also exist. Six of these objects are forced into positions relative to Planet Nine and into orbits that are perpendicular to the plane of the Solar System. Five known objects fit this prediction precisely (See the 5 purple orbits above excluding Sedna.)

Scientists have long believed that the early Solar System began with four planetary cores that went on to grab all of the gas around them, forming the four gas planets: Jupiter, Saturn, Uranus, and Neptune. Over time, collisions and ejections shaped them and moved them out to their present locations. But there is no reason that there could not have been five cores rather than four. Planet Nine might be that fifth core, and if it got too close to Jupiter or Saturn, it could have been ejected into its very distant eccentric orbit.

This ninth planet, that seems like such an oddball to us, would actually make our solar system much more similar to other planetary systems that astronomers have found around other stars. First, most of the planets around other sun like stars have no single orbital range. That is, some orbit extremely close to their host stars, while others follow exceptionally distant orbits. Second, the most common planets around other stars range between 1 and 10 earth-masses. Does this sound familiar?

The Oort Cloud

Oort Cloud

A giant shell of icy bodies known as the Oort Cloud encircles the whole solar system. The Oort Cloud is a spherical cloud of comets with its inner edge about 50,000 astronomical units from the sun. Its outer edge is about 100,000 astronomical units which is about 1.75 light years away.

This icy shell is named after Dutch astronomer Jan Oort who theorized its existence. In 1950 Jan Oort suggested that some of the comets entering the solar system must come from a cloud of icy bodies that lie as far as 100,000 times earth's distance from the sun (93 trillion miles). This extreme distance makes it challenging for scientists to identify objects within the Oort Cloud.

When the Oort Cloud comets interact with passing stars, molecular clouds, and/or gravity from the solar system, they may find themselves spiraling inward toward the sun, or cast completely out of the solar system into distant space. If they travel towards the sun, the comets have orbital periods that range up to thousands of years and are said to have "long-periods".

Comet Hale-Bopp was a long-period comet that traveled in from the Oort Cloud. Visible for nearly a year and a half (July 1995 to April 1997), it passed within 122 million miles of the earth. Halley's Comet, with a period of 75 years and last seen in 1986, is also believed to have originally come from the Oort Cloud, although it is now in the Kuiper Belt. Both of these Oort Cloud objects had their orbits drastically changed as a result of passing through the inner solar system.

Astronomers have identified five other objects that they believe are part of the Oort Cloud. The largest of the five, Sedna, thought to be three-quarters of the size of Pluto, lies 8 billion miles from earth and orbits the sun approximately every 10,500 years. The other four objects: Hyakutake was seen in 1996, CR105 in 2000, SQ372 in 2006, and KV42 in 2008. Their sizes range from 30 to 155 miles in width.  Top

Comets, Asteroids and Meteoroids

Comet Hale-Bopp

A comet is an icy solar body that originates in the Oort Cloud, orbits around the sun, heats up and begins to outgas, displaying a visible atmosphere called a coma. Some comets also have a tail. See the comet Hale-Bopp to the left. These effects are due to radiation from the sun onto the nucleus of the comet.

Comet nuclei range from a few hundred meters to tens of kilometers across. They are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the earth without a telescope. Comets have been observed and recorded since ancient times by many cultures.

As of November 2014 there were 5,253 known comets, a number that is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the number of comet-like bodies in the in the Oort Cloud is estimated to be one trillion. Roughly one comet per year is visible to the naked eye, though many of these are very faint and unspectacular.

Asteroid Belt

Asteroids are rocky bodies that may contain minerals and are at least one meter in size. However, the term asteroid has never been formally defined. Very large rocky bodies have also been called planetoids.

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the solar system that never grew large enough to become planets. A large majority of asteroids orbit in the Asteroid Belt between the orbits of Mars and Jupiter. See the white area in the image to the left.

The main difference between an asteroid and a comet is that a comet has a coma and a tail due to their icy bodies passing through radiation from the sun. An asteroid is mainly rocky material.

Why does the Asteroid Belt exist? The main theory that astronomers suggest is that 4.6 billion years ago, when our solar system was forming, a planet tried to form between Mars and Jupiter. However, Jupiter’s gravitational force was too strong and the material was unable to coalesce and form a planet.

Even if a planet had formed, it would not have been anything to write home about. It is estimated that if you put all the asteroids in the Asteroid Belt together into one body, they would form an object less than half the size of our moon.


See the asteroid Mathilde at the left. Mathilde is a main Asteroid Belt object about 50 km in diameter. It has a relatively elliptical orbit that requires more than four years to circle the sun. This asteroid has an unusually slow rate of rotation requiring 17.4 days to complete a 360° revolution about its axis. It is a C-type asteroid, which means the surface has a high proportion of Carbon giving it a dark surface that reflects only 4% of the light that falls on it.

With all these asteroids orbiting around the sun and not too far from the earth, it is reasonable that from time to time the path of our planet and that of an asteroid will cross. In fact, scientists believe that about 65 million years ago, an asteroid about six miles across had a major collision with the earth and caused the extinction of the dinosaurs.

More recently, in 1989 an asteroid that was a quarter-mile in diameter came within 400,000 miles of earth. The asteroid weighed 50 million tons and was traveling at 46,000 miles per hour. Astronomers estimate that the asteroid and the earth passed through the same point in space only six hours apart!


A meteoroid is a very small rocky, metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from a grain of sand to one meter wide. Objects smaller than a grain of sand are classified as space dust. Most meteoroids are fragments from comets or asteroids, but a few are collision impact debris ejected from bodies such as the Moon or Mars. Almost all meteoroids contain extra-terrestrial nickel and iron. About 15,000 tons of meteoroids enter earth's atmosphere each year.

When a meteoroid, (or comet or asteroid) enters the earth's atmosphere at a speed in excess of 45,000 miles per hour, aerodynamic heating of the object produces a streak of light, both from the glowing object and the trail of glowing particles that it leaves in its wake. This phenomenon is called a meteor. Most meteoroids that become meteors are about the size of a grain of sand. A meteorite is the portion of a meteoroid that survives its passage through the atmosphere and hits the ground without being destroyed.

A series of many meteors appearing seconds or minutes apart, and from the same fixed point in the sky, is called a meteor shower. Millions of meteors incur the earth's atmosphere daily. Meteors become visible between about 45 to 75 miles above the earth. They usually disintegrate at altitudes of 30 to 60  miles high. Most meteors glow for about a second.

Meteors have roughly a fifty percent chance of a daylight collision with the earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognized.