Objects forming the Solar System

1 Astronomical Unit (AU) = Distance from the Sun to the Earth.

Solar System Object Classification Example Bodies within Region

The Sun

The sun represents 99% of the mass of the Solar System.


The Inner Planets

With hard rocky surfaces.

The terrestrial planets. Mercury, Venus, Earth, Mars.

Near Earth Objects

These are asteroids that circle around the Sun near the Earth (obviously). They subdivide into 3 types -

  • Atens with mean orbital radii inside the Earth's orbit but which may cross the Earth's orbit at perihelion
  • Apollos with mean orbital radii outside the Earth's orbit but which may cross the Earth's orbit at aphelion
  • Amors which cross the orbit of Mars but do not reach Earth.


First discovered was Eros - the object about which most is known in the solar system (as at 2001). It ventures near to earth, it's eccentric orbit comes inside Mars, closest distance to earth is 22 million kilometers. Measures 33km x 13km. Spins once every 5.27 hours.Was visited by NEAR, the first asteroid orbiter which departed 1996 and was brought into orbit around Eros in Feb 2000.

The Asteroid Belt

The main belt lies between 2AUs and 5AUs

Asteroids lie at all orbits between Earth and Mars but there are gaps in the orbital space where no asteroids lie. These are called Kirkwood gaps and any asteroid orbiting in them would have an orbital period that would be a simple fraction such as 1/2 or 1/3 of the orbital period of Jupiter. Such periods would result in Jupiter soon pulling the asteroid into a new orbit. The exception is the simple fraction - 1. There are a few asteroids orbiting at about 5AUs at the same rate as Jupiter, either ahead or behind the giant planet and these are called 'Trojans'.

The Asteroids appear to be leftover material from the coagulation of the inner planets (in the same way that the comets from the EK belt and the Oort cloud can be regarded as leftover material from the creation of Neptune and Uranus).

Largest and first discovered is Ceres with a diameter of slightly less than 950km. Mean orbital radius of 2.77AU. Next are Pallas, Juno and Vesta and all four are classified as 'minor planets'. The only bodies larger than Ceres are the eight planets and a gaggle of TNOs (see below - more are being found all the time starting from around 2002).

The Gas Giants

Gas giants with rocky cores of unknown nature.

Jupiter & Saturn.

The Icy Planets

Probably formed from the icy cometary bodies, the remainder of which form the E-K belt. Neptune lies at 30 AU.

Uranus & Neptune.


Icy worlds that cross the orbits of the 4 outer planets. They are similar to asteroids but consist of ice like comets. The most common theory says that they originated in the Kuiper belt. At Feb, 2001, just 21 centaurs have been discovered.


Trans-Neptunian Objects

Objects beyond Neptune (which lies at 30AU) are known as Trans-Neptunian Objects and were predicted to exist by both Thomas Edgeworth in 1949 and Gerard Kuiper in 1951. The vast majority are icy bodies analagous to comets (dirty iceballs). Although most conform exactly to the prediction by Edgeworth and Kuiper, many have also been found near Neptune's orbit and much further out with extremely eccentric orbits leading to 4 or 5 sub-classifications as follows.

Table of TNOs in order of size

Name Diameter / Km - Est. (range) Distance from Sun (Class.)
2003 UB313 3000 (2390-3200) 38-98AU (Scattered Disk)
Pluto 2390 30-49AU (Plutino)
Sedna 1800 (1200 - 1800) 76-942AU (Inner Oort Cloud)
2005FY9 1500 (1150-1650) 39-53AU (E-K belt)
Santa (2003EL61) 1500 35-52AU (Plutino)
Orcus (2004DW) 1250 (840 - 1880) 31-48AU (Plutino)
Ixion (2001KX76) (1200 - 1400) 30-49AU (Plutino)
Quaoar (2002AW197) 1250 (1000 - 1350) 42-45AU (E-K belt)


30-49AU (Plutino moon)


The Plutinos

A third of TNOs lie in resonance with Neptune - their orbital period is an integer fraction of the planet. i.e. they pass quite close to the orbit line of Neptune but never at a time when Neptune is actually near so that Neptune's gravity is never able to exert significant influence upon them. The most common orbit observed lies at a mean distance of 39AU from the sun with a small eccentricity, a large inclination and a resonant period of two orbits for every three of Neptune.

Pluto is a TNO with an orbit typical of the most common orbits of TNOs in resonance with Neptune. It is therefore more accurately described as one of the largest of the TNO population and the largest Plutino, rather than a planet. Pluto is about 2/3 the diameter of our Moon. It has an eccentric orbit (another characteristic not shared by any solar planet) that goes from 30-49AU so that it actually comes within the orbit of Neptune sometimes.

Pluto's moon, Charon, appears to be a captured TNO.

The Edgeworth-Kuiper belt

The majority of TNOs have low eccentricity (near circular) orbits and are found beyond 40AU from the sun. These are called 'classical' TNOs and match exactly the description of the objects predicted by Edgeworth and Kuiper. According to Eugene Chiang and Mike Brown of Caltech, roughly 60,000 TNOs larger than 50Km across exist in the EKB and the total mass of the EKB is roughly 0.2 Earth masses, much larger than the mass of the main asteroid belt. It's maximum reach is 48AU and it is a doughnut shape. At Feb 2001, more than 300 Kuiper belt objects had been found.

A comet which returns regularly to the vicinity of the sun is called a short period comet. There are about 135 short period comets with a period of less than 200 years.

There is strong evidence that the E-K belt has a rather sharp edge at 50 AU.

The E-K belt is thought to be the reservoir from which the shorter period comets originate such as Comet Halley and Comet Encke.

The third largest non-planetary body after Pluto is 2001KX76, classified as a minor planet, which has a diameter of 1200-1400km.

Quaoar (2002AW197), the 12th largest solar system body (discounting moons) yet found and the largest K-B object. It measures around 1000-1400km across and orbits at a distance of 43 times the distance of the Sun to the Earth.

In 2004, a large planetoid was discovered, although @ April 2005, details are sketchy. It is known as 2004 DW and its diameter is 840 - 1,800km, most likely 1600km. It is 45AU from us in the E-K belt.

Scattered Disk Objects

The third group of TNOs have been observed to have highly eccentric orbits and only about 30 have been discovered. These are called 'scattered disk objects'.

Scattered Disk Objects

The most extreme scattered disk object is 2000CR-105 with an eccentricity of 0.94, an average distance from the sun of 675AU and an orbital period of 17,500 years. It seems likely that these objects were once in the EKB but were moved out of it by an encounter with another object.

The Inner Oort Cloud

Sedna, the 10th largest solar system body yet found (@ April 2005) never comes closer to the Sun than 76 AU and so it is part of a little understood and hypothetical "inner Oort cloud". A few other bodies have been found in this region leading to the proposals for such a region.


Sedna was discovered in March 2004 and 90 times further from the Sun than the Earth (3 times further away from the Sun than Pluto). Its orbit is elliptical with a period of 10,500 years and is around 1500 +/- 300km in diameter. At March 2005, it is believed that it should have a moon but this cannot be found, perhaps because it is too dark to be seen.

Called the 10th planet by some people, 2003 UB313 is about 3,000km across, a world of rock and ice and somewhat larger than Pluto. It flys at an orbit at an angle to the orbits of the other planets. It is currently 97 Earth-Sun distances away - but will actually come inside the orbit of Neptune at it's nearest point. It's size is not known exactly but the lower limit is definitely larger than Pluto with an upper limit of its size of 3,000 km. The discovery of 2003 UB313 comes just after the announcement of the finding of 2003 EL61, which appears to be a little smaller than Pluto.

The Oort Cloud

This is a globe of comets that surrounds our planetary system. A comet is similar in consistency to a dirty snowball.

There are believed to be about 10 trillion comets making up the spherical Oort cloud. Comets orbit in all planes, in all directions and paths that vary from extremely elliptic to circular like the planets. It is 6 light years across. The inner edge of the cloud starts at about 200 AUs. Beyond the outer edge of the cloud, the sun's gravity is too weak to hold a body in orbit.

If a comet in the cloud is displaced so that it travels near to the sun, it produces the characteristic comet tail as it's outer layers are blown off by the solar wind.


The Oort cloud is thought to be the reservoir from which the longer period comets originate such as Comet Hale-Bopp.


Probably more than 100,000 asteroids or minor planets larger than 1km float around the solar system.

The Titius-Bode law gives the distance of the planets, D in astronomical units using the formula

D=0.4 + 0.3 x X AU

Where X is as follows:

Mercury = 0, Venus = 2^0, Earth = 2^1, Ceres = 2^2, Mars = 2^3, Jupiter = 2^4, Saturn = 2^5 and Uranus = 2^6.