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The new solar system.

Since 2006, the details of bodies making up our solar system have been revised. This was largely as a result of new discoveries of a number of planet-like objects beyond the orbit of Pluto. The International Astronomical Union redefined what constituted a planet and established two new classifications--dwarf planets and plutoids. As a result, the way in which we view the solar system has changed and science teachers need to be aware of such changes.


Towards the end of the eighteenth century, only six planets were known--Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, British astronomer William Herschel accidentally discovered the seventh planet --Uranus. In 1846, Urbain Leverrier in France, and John Adams in England used Newton's gravitational laws to independently predict that variations in the orbit of Uranus were due to the influence of an eighth planet. Soon after, the Berlin Observatory found the predicted planet and named it Neptune. In the early twentieth century, Percival Lowell and William Pickering predicted that another planet should exist beyond Neptune. In 1930, Clyde Tombaugh found such a body (which was named Pluto) close to where Lowell and Pickering had predicted it to be. Since 1930, Pluto has been regarded as the ninth planet of our solar system. However, in the last decade there have been many new discoveries made of planet-like bodies that orbit the Sun beyond the orbit of Pluto, and our solar system is suddenly becoming quite crowded.


In 2006, a meeting of the International Astronomical Union (IAU) decided on a definition of a planet that excluded Pluto, relegating it to being a 'dwarf planet', along with many of the other newly-discovered bodies. Then in 2008, the IAU introduced another classification, that of a plutoid (IAU, 2008).

In addition, the outer reaches of our solar system have been divided into three regions: the Kuiper Belt, the Scattered Disc and the Oort Cloud. As a result, we now have what many call 'the new solar system' (Wilkinson, 2009).


Traditionally, a planet has been regarded as a spherical body that orbits a star and is visible because it reflects sunlight. The spherical shape is only possible when the object has enough mass for gravity to pull it into a spherical shape.

In August 2006 the IAU decided on the following definition of a planet: To be a planet a body must:

1. be in orbit around the Sun,

2. have sufficient mass for self-gravity to overcome rigid body forces, so that it assumes a hydrostatic equilibrium (nearly round) shape, and

3. have cleared the neighbourhood around its orbit.

The IAU also introduced a new classification--that of a 'dwarf planet'. A dwarf planet is one that is in orbit around the Sun, has sufficient mass for self-gravity, has NOT cleared the neighbourhood around its orbit, and is not a satellite. All other objects orbiting the Sun are collectively known as 'small solar system bodies'. Currently, there are a number of bodies regarded as dwarf planets and more are expected to be added to the list over the next few years as more information about them becomes available. Dwarf planets are not considered to be true planets mainly because they have not cleared their orbital path of other material they exist in populated regions such as the Asteroid Belt and the Kuiper Belt. In 2008, the IAU also introduced another classification, that of a plutoid. A plutoid is basically a dwarf planet that orbits the Sun beyond the orbit of Neptune. Plutoids also require a certain minimum brightness--they can be regarded as a subclass of dwarf planets.

As of mid-2009, there are eight major planets in the solar system--Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune, and five dwarf planets Ceres, Pluto, Haumea, Makemake and Eris. The dwarf planets Pluto, Haumea, Makemake and Eris are also plutoids. Ceres is not a plutoid as it orbits in the inner solar system rather than beyond Neptune.

Many of the planets in the solar system have natural satellites or moons orbiting them. The planets Jupiter and Saturn have the most moons orbiting them. True moons are large enough for gravity to have pulled their mass into a spherical shape, while smaller moons do not have enough mass and gravity and are irregular in shape. To be a moon, a body must be naturally orbiting a planet and be smaller than that planet. Many smaller moons have been captured by planets during the formation of the solar system and their irregular shape suggests they would be better classed as 'moonlets' (Wilkinson, 2009).

As a result of recent space probes such as Galileo Cassini and Voyager, more moons have been discovered around the outer planets. Jupiter, for example, now has 63 moons, and Saturn has 62. To date, the IAU has not decided on a formal definition of what constitutes a moon.

The four largest (gaseous) planets, Jupiter, Saturn Uranus, and Neptune, also have planetary rings of varying size and complexity. These rings are composed primarily of dust or particulate matter. The origin of such rings is not known, but they may be leftover debris from moons which have been torn apart by tidal forces. The E Ring of Saturn is known to contain material ejected from Enceladus, one of its moons. Saturn is the only planet whose rings can be observed through telescopes from Earth.

Most textbooks in schools have out-of-date information tables on the planets. Table 1 should be of value to teachers requiring an update.


Two striking changes to the solar system after the 2006 IAU decision were the demotion of Pluto from major planet status, and the inclusion of Ceres as a dwarf planet.

Ceres is the largest member of the Asteroid Belt--a region of rocky bodies orbiting between the planets Mars and Jupiter. Ceres is large enough to have formed a spherical shape (diameter 930 km) but it has not cleared the neighbourhood around its orbit. It is a member of a larger population of bodies--the Asteroids. Interestingly, Ceres was initially classed as a planet when it was first discovered in 1801 (but was quickly reclassified as an asteroid). The second-largest asteroid, Vesta, has not been classified as a dwarf planet because its shape is not spherical enough to have reached hydrostatic equilibrium (it is actually irregular).

When Pluto was discovered in 1930, it was classified as a planet. The 2006 IAU decision resulted in Pluto being demoted to dwarf planet and then plutoid status. Pluto orbits in a region of rocky and icy bodies known as the Kuiper Belt and although it is spherical it has not cleared the neighbourhood around its orbit. Pluto also has other peculiarities. For example, Pluto's orbit is inclined at 17 degrees to the orbital plane of the other planets and its rotational axis is tipped over to one side. Its eccentric path occasionally carries it closer to the Sun than Neptune. Pluto is also very unlike its inner neighbours--the gas giants Uranus and Neptune. Pluto has more in common with Triton, the largest moon of Neptune than it does with any of the other planets (Wilkinson, 2007). Like Triton, Pluto is thought to consist of 75% rock and 25% water ice and its thin atmosphere contains nitrogen and methane. Pluto takes 248 Earth years to orbit the Sun, and its distance from the Sun varies from 30 AU to 50 AU. Pluto has three moons, Charon (discovered in 1978) and Nix and Hydra (discovered in 2005 and named in 2006). The largest moon, Charon, is about half the size of Pluto. Recent studies by the Hubble Space Telescope show that Charon is bluer than Pluto and has a different surface composition. Charon is so large in comparison to Pluto that many scientists considered the two to be a double planet. The two bodies actually rotate around a common centre of gravity that is located in the space between them.


A Trans-Neptunian Object (TNO) is any object in the Solar system that orbits the Sun at a greater distance on average than the planet Neptune. A range of objects fall into this category, many of which have only recently been discovered. At last count, astronomers had catalogued over 1000 TNOs.

The first astronomer to suggest the existence of a Trans-Neptunian population was Frederick C. Leonard in 1930. In 1943, Kenneth Edgeworth postulated that in a region beyond Neptune, the material from the primordial solar nebula would have been too widely spaced to condense into planets. From this he concluded that the outer region of the solar system should contain a very large number of smaller bodies that could, from time to time, venture into the inner Solar system. In the last few decades, astronomers have identified three regions that exist beyond Neptune: the Kuiper Belt, the Scattered Disc, and the Oort Cloud.


The Kuiper Belt is a vast region of the solar system beyond Neptune's orbit. It is best described as a flat doughnut-shaped disc that extends from 30 AU to 50 AU around the Sun. It is similar to the Asteroid Belt, although it is much larger and more massive. Like the Asteroid Belt, it contains mainly small bodies, but while asteroids are composed mainly of rock and metal, the objects in the Kuiper Belt are made mostly of rock and ices (such as methane, ammonia and water).

The belt is named after Dutch-born American astronomer Gerard Kuiper, who first predicted its existence in 1951. The objects in this region are called Kuiper Belt Objects or KBOs. The orbits of many of these objects are highly elliptical, destabilised by Neptune's gravity. KBOs move slowly and it takes a long time to determine their orbital characteristics.

The first KBO was discovered by David Jewitt and Jane Luu in Hawaii in August 1992 and is called 1992 QB1. This object was found 42 AU from the Sun. Six months later, these two astronomers discovered a second object, 1993 FW, in the same region. It is suspected that there may be as many as 35,000 objects in the Kuiper Belt with diameters of 100 km or greater, and an even larger number of smaller objects. The region is also thought to be the home of short period comets (those with periods less than 200 years).

The former planet Pluto and its companion Charon are two of the larger KBOs. Several other large KBOs have been discovered recently, including Quaoar (2002 LM60) and Orcus (2004 DW).

Once discovered, KBOs are given temporary designations that indicate the year and time of the month (using a letter code) when the discovery was made. When details are confirmed and orbital characteristics are determined, official names are given (see Table 2).

Of the KBOs listed in Table 2, only Pluto, Haumea and Makemake are classed as plutoids. However, Haumea (originally 2003 EL61 ) is one of the strangest known objects in the Kuiper Belt, and some scientists (including the author) have questioned its status as a plutoid. To be a plutoid, the object should be spherical, but Haumea has a shape like an Aussie Rules football. It spins end over end every four hours like a football that has been kicked. It appears to be made almost entirely of rock, with a glaze of ice over the surface. It is orbited by two tiny moons and is followed by a swarm of other small icy bodies. Why is it a plutoid? Scientists think that originally this object may have been spherical (reached hydrostatic equilibrium), but that it collided with another body at high speed. This collision broke it into pieces and left what we see today (Brown, 2007).

It is likely that more KBOs will be classified as plutoids in the future when more is known about them.


The Scattered Disc is a sparsely-populated region beyond the Kuiper Belt, extending to 100 AU and beyond. Objects in this region have highly eccentric orbits and are often wildly inclined to the orbital plane of the major planets. Two of the first scattered disc objects (SDOs) to be recognised are 1995 TL8 (at 53 AU from the Sun) and 1996 TL66 (at 83 AU). Other objects include 1999 TD10, 2002 XU93 and 2004 XR190 (at 58 AU). Many of the SDOs are doomed in the long term because, sooner or later, their orbits will carry them close to the giant planets to undergo more scattering. They may last for a few million years, or even 100 million years in their current orbits, but eventually Neptune will flip them towards Uranus, Saturn or Jupiter. These planets will in turn fling them outward, far beyond the Kuiper Belt, out of the solar system entirely or closer to the Sun (where they will become comets).

One of the major scattered disc objects is Eris (2003 UB313, previously known as Xena). The elliptical orbit of this body takes it to within 37 AU of the Sun and as far as 100 AU. The discovery of this object in 2003 prompted astronomers to decide on a new definition of a planet. If Eris had been classified as a planet, there could have been as many as fifteen planets in the solar system. In the end, the IAU decided on a definition that excluded Eris and Pluto as major planets, instead classifying them as dwarf planets and then as plutoids.

Eris has a diameter of 2400 km, which makes it as large as Pluto. It is the largest object found in orbit around the Sun since the discovery of Neptune and its moon Triton in 1846. Eris takes more than twice as long to orbit the Sun as Pluto (560 years). In 2005, a near infrared spectrograph on the Gemini Telescope in Hawaii showed the surface of Eris to be mainly methane ice. Methane ice suggests a primitive surface unheated by the Sun since the solar system formed. If Eris ever had been close to the Sun, the methane ice would have been boiled off. The interior of the dwarf planet is probably a mix of rock and ice, like Pluto's. The elliptical orbit of Eris is tilted at an angle of 45 degrees to the orbital plane of the major planets. Eris currently sits about three times Pluto's distance from the Sun, following an orbit that is about twice as eccentric and twice as steeply inclined to the plane of the solar system. Eris has also been found to have a moon, which has been named Dysnomia.


The Oort Cloud is an immense spherical cloud surrounding the solar system between 1,000 and 100,000 AU (30 trillion km) from the Sun. The cloud is named after Dutch astronomer Jan H. Oort, who first suggested its existence in 1950. The region contains billions of small icy objects probably left over from the formation of the solar system. Sometimes, the orbit of one of these objects is disturbed by other bodies, causing it to come streaking into the inner solar system as a long period comet. In contrast, short period comets take less than 200 years to orbit the Sun and originate from the Kuiper Belt.

One of the major Oort Cloud objects is Sedna (2003 VB12), which was discovered in November 2003 by a team led by Mike Brown at Palomar Observatory near San Diego, California (USA). Sedna has a highly elliptical orbit that is inclined at about 12[degrees] to the ecliptic. Its distance from the Sun varies between 76 AU and 975 AU, so it is best described as an inner Oort Cloud object. The object will reach its closest approach to the Sun (perihelion) about the year 2076 and will be furthest from the Sun (aphelion) in the year 8207. The shape of its orbit suggests it may have been captured by the Sun from another star passing by our solar system, or its orbit could be affected by another larger object further away in the Oort Cloud.


Sedna is the second reddish-coloured object in the solar system after Mars. Its size is estimated to be between 1200 and 1800 km (about three-quarters the size of Pluto) and it takes 12,000 years to orbit the Sun. Recent estimates put its rotational period at about 10 hours and its surface temperature at a very chilly -250[degrees]C. Sedna appears to have very little methane ice or water ice on its surface.


There is no doubt that more discoveries will be made in the Kuiper Belt and the Oort Cloud. Any new objects discovered will necessarily be faint, cold and far away from Earth. It is also likely that the IAU will change its definition of what constitutes a planet in the future. However, there are a number of other questions left for discussion. It can be argued that a 'dwarf planet' is a form of planet (just as a dwarf star is still a star), regardless of what the intent of the IAU might have been. So what happens when a student asks how many planets are in our solar system? The answer is probably eight major planets, five dwarf planets, and a large number of small solar system bodies. Other questions are: Can you have a dwarf planet beyond Neptune that is not a plutoid? Is a plutoid really a sub-class of dwarf planets or is it a separate class? Can a TNO that is not spherical be a plutoid?

Teachers need to keep up to date with changes to the way in which we classify objects in the solar system. Websites such as Wikipedia already have updates for plutoids and dwarf planets, but these often change as new information is added. Teachers also need to be aware that many websites contain out-of-date information, so only use the most recent sites.

Teachers should not be concerned about such changes--this is the way science evolves--new information leads to changes in our understanding. Astronomy used to be a fairly static science but in recent years it has changed on an almost daily basis.

Further information about, and photographs of, the objects discussed in this article can be found in the author's book, Probing the New Solar System (CSIRO Publishing).


Brown, M. (2007). 2003 EL61--the strangest known object in the Kuiper Belt. Retrieved May 5, 2007 from

IAU (2008). Plutoid chosen as name for Solar system objects like Pluto. International Astronomical News Release IAU0804. Retrieved from

Wilkinson, J. (2007). A Changing Solar system. Lab Talk, V51 (4), 32-34.

Wilkinson, J. (2009). Probing the New Solar system. CSIRO Publishing, Melbourne, Australia.

John Wilkinson has been a science teacher and university lecturer for over 30 years and is the author of many science text books. He has research interests in astronomy and science education. His latest book is Probing the New Solar System (CSIRO Publishing).
Table 1) Major planets and dwarf planets in the Solar system
(as of 2009).

                           DISTANCE               NUMBER       RING
                           FROM SUN   DIAMETER   OF MOONS     SYSTEM
NAME           CLASS         (AU)       (KM)

Mercury    major planet      0.38       4880         0         none
Venus      major planet      0.72      12,104        0         none
Earth      major planet      1.00      12,756        1         none
Mars       major planet      1.52      6,794         2         none
Ceres      dwarf planet      2.76       933          0         none
Jupiter    major planet      5.20     142,984       63         faint
Saturn     major planet      9.54     120,536       62       prominent
Uranus     major planet     19.20      51,118       27         faint
Neptune    major planet     30.10      49,532       13         faint
Pluto      dwarf/plutoid    30-50      2,320         3         none
Haumea     dwarf/plutoid    35-52       1500         2         none
Makemake   dwarf/plutoid    38-53       1600         0         none
Eris       dwarf/plutoid    38-100      3000         1         none

Note: Distances are given in astronomical units (AU), where
one AU is the distance between the Earth and the Sun.
Distances to the plutoids vary because of their highly
elliptical orbits.

Table 2) Main Kuiper Belt Objects (as of mid-2009).

              DIAMETER      AV. ORBITAL    PERIOD
NAME             (KM)       RADIUS (AU)   (YEARS)

Pluto            2320          39.5         248
Ixion            800           30-49        250
Varuna           900            43          283
Quaoar           1250           42          288
2002AW197        750           41-53        325
Haumea           1500          35-52        285
Orcus            950           30-48        247
Makemake         1600          38-53        309

               ORBITAL        NUMBER

Pluto           0.249            3
Ixion           0.242            0
Varuna          0.051            0
Quaoar          0.034            1
2002AW197       0.132            0
Haumea          0.189            2
Orcus           0.225            1
Makemake        0.159            0

Note: Orbital eccentricity is a measure of how elliptical an
orbit is (a perfect circle is 0 and a straight line. 1).
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Title Annotation:Hands On
Author:Wilkinson, John
Publication:Teaching Science
Geographic Code:1USA
Date:Dec 1, 2009
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