Outer Solar System

What is the Outer Solar System?
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 well of the Sun. The outer solar system starts at about 30 astronomical units, thirty times the distance of the Earth from the Sun. The outer solar system ends around one light year from the Sun, where the Sun’s gravitational field ends and the solar wind stops and becomes part of the general hydrogen gas that is part of open space.

Outer Solar System

Layers of the Outer Solar System
The outer solar system consists of the Kuiper Belt and the Oort Cloud. The Kuiper belt starts at 30 AU and ends around 50 AU. There is a gap called the Kuiper cliff around 48 AU, where there is nothing in regular orbit. The Oort Cloud starts at about 50 AU and extends up to 100 AU, though some astronomers think it ends closer to 0.8 AU.

The Kuiper belt is home to millions of floating asteroids consisting of rock and ice. It is the likely starting point of many short period comets. The Oort Cloud is probably the origin point of many long term comets.

Objects in the Kuiper belt have orbital periods of 150 years to 500 years. Objects in the Oort Cloud have orbital periods ranging from centuries to thousands of years.

Kuiper Belt

Kuiper Belt

Chemistry of the Outer Solar System
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 trans-Neptunian 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 though to average 50 Kelvin, not far above absolute zero, where even hydrogen freezes. Bodies like Sedna 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.

Bodies in the Outer Solar System
While dwarf planets like Sedna and Eris 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 across coexist with an estimate 50,000 to 100,000 frozen bodies over a hundred kilometers across.
Some bodies in the outer solar system are large enough to have their own moons or moon-lets. The moonlet Weywot orbits dwarf planet Quaoar. Charon and Pluto orbit each other, often considered a pair of dwarf planets, while two smaller moonlets orbit Pluto.
Many dwarf planets in the Kuiper belt are in orbital resonance with Neptune.
A formation called the scattered disc ranges from the Kuiper belt up to a hundred astronomical units away. Most of this mass is dust. It also includes small bodies that are not large enough to be dwarf planets.

Oort Cloud

Oort Cloud

Study of the Outer Solar System
Study of the outer solar system has taken off since the 1960s. Several major space probes are now in the outer solar system, though not all of them are sending data back to Earth. Voyager 2 is now in the outer solar system, having passed Neptune in 1989, but it stopped sending back radio astronomy data in 2008. Voyager 1 formally exited the outer solar system and entered deep space in 2012 when it passed the heliopause, where the solar wind’s pressure falls and equals that of deep space.
The outer solar system is continually surveyed by a number of Earth bound and orbiting telescopes. Even known objects like Pluto are sources of discovery, such as the discovery of its smaller moons Nix and Hydra in 2005. Larger moon Charon had been discovered in 1978.
New Horizons was launched in 2006 with the mission to explore the Kuiper belt. New Horizons’ target is Pluto and Charon, but it may visit other Kuiper belt objects.
Ongoing digitization of millions of astronomical photos will probably allow for the discovery of new objects in the outer solar system. Sedna has an orbit of eleven millennia. Objects this far out are only found by comparing images taken years apart to confirm that they have, indeed, moved.

heliopause

heliopause

Literal Gaps in the Outer Solar System
There are ongoing scientific models of Trans-Neptunian Objects in an effort to understand their orbits. Some bodies like Sedna are in inexplicably elongated orbits and have orbital periods we cannot explain. Astronomers turn to mathematical models in an effort to explain them. Theories for these eccentric or long orbits range from yet undiscovered Mars sized bodies in the Oort Cloud to the influences of a star near the Sun early in its history. This theoretical planet orbiting in the Oort cloud has been named Tyche.
The Kuiper belt ends at 48 astronomical units, with a significant empty gap before the Oort cloud begins. No objects over a hundred kilometers across have been found past 48 AU until the Oort cloud begins. This area may have been swept clean by the gravitational pull of Neptune. Neptune’s effect would range from sucking in planetoids into itself, throwing them into the inner solar system as comets, capturing them in orbital resonance or throwing them into deep space. Another theory is the existence of a large Earth sized body, though there is no proof to date.