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Far Solar System: An Overview

Image: sci.esa.int, Schematic diagram of inner solar system (left), planets and Kuiper belt (middle) and Oort Cloud.

Dr. Vinaya Kumar Jha

The history of successful interplanetary space mission is begun by United State in December 1962 after sending the Mariner 2 to the planet Venus which collected the data from around 35,000 km of the Venus. In a last few decades, due to advancement in technology, robotic spacecraft is being used in the space exploration.

The first robotic spacecraft is sent under joint venture by the NASA, Indian Space Research Organization, Soviet Union, European Space Agency and the Japan Aerospace Exploration Agency which has produced number of discoveries and revolutionized our view about Solar system.

In 1977, the Voyager mission was launched which was one of the fruitful missions as it provided many examples of the process of discovery.  Voyager revealed that the moon of the Jupiter named Io was volcanically active and confirmed the theories of tidal heating, and another moon, Europa is nearly crater-free icy surface which suggests recent resurfacing and the possibility of liquid water.

Voyager also noticed about the Saturn’s largest moon Titan that its atmosphere is thick and rich in hydrocarbons, and another moon, Enceladus (the sixth largest moon of Saturn) to have amazingly geologically young surface. Until now, much more information has been obtained about major planets of the solar system but a little has been known about a trans-Neptune region, the Kuiper belt and Oort cloud. The Kuiper belt is a disc-shaped region of icy bodies which includes dwarf planets such as Pluto and comets beyond the orbit of Neptune, extending from the orbit of Neptune (at ~ 30 AU) to approximately 50 AU from the Sun.

The astronomical unit (symbol A.U.) is the unit of distance in space science and it is approximately 150 million km. This belt is similar to the asteroid belt, but is far larger ~ 20 times as wide and 20 to 200 times as massive. Beyond this Kuiper belt, there is another belt called Oort cloud. This article comprises an overview on the Kuiper belt and Oort cloud.

A sednoid is a trans-Neptunian object with a perihelion greater than 50 AU and a semi-major axis greater than 150 AU.[1] Only two objects are known from this population, 90377 Sedna and 2012 VP113, both of which have perihelia greater than 75 AU, [2] but it is suspected that there are many more. These objects lie outside an apparently nearly empty gap in the Solar System starting at about 50 AU, and have no significant interaction with the planets.

They are usually grouped with the detached objects. Some astronomers, such as Scott Sheppard,[2] considered the sednoids to be inner Oort cloud objects (OCOs), though the inner Oort cloud (or Hills cloud) was originally predicted to lie beyond 2,000 AU, several times as far as the aphelia of the two known sednoids.

 Kuiper Belt

Kuiper belt (pronounced Kai-per) is disc-shaped very cold zone of the solar system which extends from 30 to 100 A.U. and is never inside orbits of jovian planets. It contains materials totalling less than a tenth the mass of the Earth.  Based on different models developed, scientists conclude that the entire Kuiper belt formed closer to the Sun and was transported outward during the final stages of planet formation.

The Kuiper Belt consists of Icy rocks, and it a major source of short-period Comets in the Solar system.  Extending beyond the planet Neptune, Pluto was discovered to be one of the largest objects in the belt which is now in the family of dwarf planet.

The Kuiper Belt is one of the two main reservoirs of comets in the Solar System along with the Oort cloud. Jupiter Family comets are believed to have formed here and to have migrated inwards only later, to their current orbits with aphelia near the orbits of Jupiter and the other giant planets. Mass of all comets probably are comparable to the mass of terrestrial planets. The schematic diagrams of inner solar system, outer solar system and Oort cloud have been shown in figure above.

Oort Cloud

The mysterious Oort cloud is a collection of Comets which is thought to extend far beyond the planets of the Solar System.  It is better called ‘Cloud’ as the Oort comets are distributed spherically about the Sun, possibly extending as far as halfway to the next star.

The orbits of OCOs are not regular or planar like the Asteroid and Kuiper belts, and supposed that they are often perturbed through gravitational interactions during their long journeys on highly eccentric orbits. The belts and the Oort cloud show just how many rocks there are in the Solar System.  The Planets certainly may be the largest objects, but they are certainly not alone. This region is filled with billions of comets.

Already mentioned above, one of the major sources of comets are the Oort cloud along with the Kuiper belt. The gravity of Jupiter, a Gas Giant planet affects objects in the Kuiper Belt and Oort cloud, sending them into our Solar System towards the Sun. In Figure above, the right panel shows the Oort cloud, the other main reservoir of comets located well beyond the outer Solar System. They have highly eccentric orbits and periods of around 200 years or more. Comets from the Oort cloud are believed to have originated in the vicinity of the giant planets and to have been ejected later, via gravitational interactions, to their current orbits.

The Oort cloud is thought to have a roughly spherical distribution. Although Nemesis is assumed to be the most distant object in the solar system, probably the most amazing hypothetical trans-Neptunian object. This is a predicted companion star to the Sun, with an elliptical orbit between 20,000 and 90,000 AU from the Sun and a period of 30 million years. The IRAS satellite surveyed the entire sky at and could not find any evidence about the existence of Nemesis.

In November 2015, space scientists found the icy body named V774101which is supposed to be the farthest body in the solar system. This claim is based on its reflectivity. [3,4] This icy body is supposed to be 500 to 1,000 km in diameter which is roughly half the size of Pluto. It is around 10 billion miles from Earth, or three times farther away than Pluto.



  1. Sheppard, Scott S. “Known Extreme Outer Solar System Objects”. Department of Terrestrial Magnetism, Carnegie Institution for Science, April 17, 2014.
  2. Chadwick A. Trujilloand Scott S. Sheppard.“ ASedna– like body with a perihelion of 80 astronomical units”, Nature 507, 471- 474, March 14, 2014.
  3. Kelly Beautty. “ V774104: Solar System’s Most Distant Object”, SKY & TELESCOPE, November 15, 2015.
  4. Deborah Byrd. “New most distant object in solar system”, Science wire/Space, December 1, 2015.

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