Visions of the Cosmos

Planetary Science

The Solar System

Kuiper Belt

Denizens of the Frozen Zone

Pluto was discovered in 1930 by a young astronomer in 1930.  Clyde Tombaugh was only 24 years old when he made world news in 1930 by discovering the elusive ninth planet using a photographic telescope at Lowell Observatory in Arizona.  Until fairly recently very little was known about Pluto except that for most of its orbit it was the furthest planet from the Sun.   However, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit. Pluto crossed Neptune's orbit January 21, 1979, made its closest approach September 5, 1989, and remained within the orbit of Neptune until February 11, 1999. This will not occur again until September 2226.  Pluto has a very eccentric orbit and is also inclined at 17 degrees to the plane of the ecliptic which is far more than the other planets. The plane in which the Earth orbits the Sun is called the plane of the ecliptic.

Pluto's rotation period is 6.387 days and its year is approximately 247.9 Earth years

It is now known that Pluto is a very small planet and at the International Astronomical Union Conference in Prague in 24 August 2006 it was 'relegated ' to the status of a 'dwarf planet' along with several other small planets.  The dwarf planets are in fact far smaller, both in mass and size, to the Earth's Moon, the four large moons of Jupiter, and Titan and even Triton.   With the exception of the asteroid Ceres they are all similar to Pluto and orbit in the frozen zone beyond Neptune.  They are now known to be the largest bodies yet discovered to be orbiting in the Kuiper-Edgeworth Belt.

At the conference the astronomers present voted  that for a celestial body to qualify as a planet:

Apart from the asteroid Ceres all the bodies which may qualify as dwarf planets so far have been discovered in the Kuiper Belt They discussed below.

 Pluto and Charon

 Pluto has a moon called Charon

It was found that Pluto has a highly reflective south polar cap, a dimmer north polar cap, and both bright and dark features in the equatorial region. Pluto is much brighter than Charon.

Pluto has a moon called Charon.  The illustration shows images of Pluto and Charon taken with a ground based telescope and with the Hubble Space Telescope.  Pictures Courtesy NASA/ESA/HST


Pluto's diameter as 2,274 kilometers (1413 miles) and Charon's diameter as 1,172 kilometers (728 miles), just over half the size of Pluto. Their average separation is 19,640 km (12,200 miles). That's roughly eight Pluto diameters.

Pluto's has an average density between 1.8 and 2.1 grams per cubic centimeter. It is concluded that Pluto is 50% to 75% rock mixed with ices. Charon's density is 1.2 to 1.3 g/cm3, indicating it contains little rock. The differences in density tell us that Pluto and Charon formed independently, although Charon's numbers derived from HST data are still being challenged by ground based observations. 

Pluto's icy surface is 98% nitrogen (N2). Methane (CH4) and traces of carbon monoxide (CO) are also present. The solid methane indicates that Pluto is colder than 70 Kelvin. Pluto's temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 AU and as far away as 50 AU. There is a thin atmosphere that freezes and falls to the surface as the planet moves away from the Sun. NASA planned to launch a spacecraft, the Pluto Express, in 2001.  Although it was postponed a mission has now been sent of and is discussed under New Horizons below.  It will allow scientists to study the planet before its atmosphere freezes. The atmospheric pressure deduced for Pluto's surface is 1/100,000 that of Earth's surface pressure.

Not only is Charon tidally locked to Pluto but Pluto is also tidally locked to Charon

ERIS  U 203 UB313 Formally called Xena

Eris is the largest dwarf planet known, slightly larger than Pluto about 2,500 kilometers in diameter, was discovered in an ongoing survey at Observatory on Mount Palomar by astronomers Mike Brown (Caltech), Chad Trujillo (Gemini Observatory), and David Rabinowitz (Yale University).  Just to confuse matters this dwarf planet was called Xena at the time of its discovery - a name which remained for about 2 years until it was renamed Eris.  The new name was officially suggested the name on 6 September 2006, and it was accepted and announced on 13 September 2006. In Greek mythology, Eris was the goddess of  warfare and strife. It also has a moon named  Dysnomia, who in Greek mythology was Eris' daughter and the demon spirit of lawlessness. As Dysnomia is a bit of a mouthful, we tend to simply call the satellite Dy, for short.
The dwarf planet is the most distant object ever seen in orbit around the Sun, even more distant than Sedna the dwarf planet discovered almost 2 years ago. It is almost 10 billion miles from the sun and more than 3 times more distant than the next closest planet, Pluto and takes more than twice as long to orbit the sun as Pluto.  It has an elliptical orbit - its present distance from the Sun is about 97 astronomical units.

Quaor or 202 LM60

With the help of NASA's Hubble Space Telescope, astronomers have determined that 2002 LM60,  Quaoar (pronounced kwa-whar) is about half the size of Pluto. Like Pluto, Quaoar dwells in the Kuiper belt.

Only Hubble's sharp vision has been able to resolve the disk of this distant world, leading to the first-ever direct measurement of the true size of a Kuiper belt object. Quaoar's diameter is about 1,300 kilometers (800 miles ) 1300 kilometers. It is the farthest object in the Solar System ever to be resolved by a telescope. Quaoar is about 6.5 billion kilometers (4 billion miles)  from Earth, more than 1 billion miles farther than Pluto

Sedna or 203 VB12

Sedna was found at a distance 90 times greater than that from the Sun to the earth -- about 3 times further than Pluto.  Until the discovery of Eris it was the most distant object known in the Solar System.

Because of its frigid temperatures, it has been named  Sedna, after the Inuit goddess of the sea from whom all sea creatures were created.

Even though all we know for certain is that Sedna is smaller than 1800 km, we have evidence which suggests that the size might be pretty close to this number. We are virtually certain that the size is larger than the 1250 km size of Quaoar, though this object has shown many unexpected characteristics, so we can't completely rule out a smaller size.

The Kuiper Belt represents the icy material mainly water, ammonia, methane, carbon dioxide, hydrogen sulphide and nitrogen that was present in the outer regions of the protoplanetary disc.  It also contains some iron and rocky material , although far less than is found in the asteroids and the inner planets.   Most of the Kuiper Belt bodies are far smaller than Pluto and if they are disturbed in their orbits they can approach very close to the Sun - when this happens they acquire a tail (strictly speaking a double tail).  Apart from the dwarf planets the Kuiper Belt contains a very large number of small to extremely small objects.


Triton is the largest moon of Neptune and is discussed under Moons of Neptune.  It is larger than all the Kuiper Belt Bodies so far found.  However it has many of the characteristics of Pluto and is believed to have been captured by Neptune sometime in its early history.   There could be some justification in including it as a large Kuiper Belt object or dwarf planet.

The New Horizons Mission

On 19 January 2006 a Mission was launched from Cape Canaveral.  It was given the name New Horizons and is now on its way to Pluto.  IT is using a gravity assist as it passes Jupiter.  At the time of writing it is about to approach the giant planet.   The closest approach will occur Feb. 28, 2007. Moving about 47,000 miles per hour (about 21 kilometers per second), New Horizons would fly 3 to 4 times closer to Jupiter than the Cassini spacecraft, coming within 32 Jupiter radii of the large planet.

Caption Courtesy NASA

The spacecraft is about to swing past Jupiter and pick up even more speed on its voyage toward the unexplored regions of the planetary frontier.he fastest spacecraft ever launched, New Horizons will make its closest pass to Jupiter on Feb. 28, threading its path through an "aim point" 1.4 million miles (2.3 million kilometers) from the center of Jupiter. Jupiter's gravity will accelerate New Horizons away from the Sun by an additional 9,000 miles per hour half the speed of a space shuttle in orbit pushing it past 52,000 mph and hurling it toward a pass through the Pluto system in July 2015

After an eight-year cruise from Jupiter across the expanse of the solar system, New Horizons will conduct a five-month-long study of Pluto and its three moons in 2015, characterizing their global geology and geomorphology, mapping their surface compositions and temperatures, and examining Pluto's atmospheric composition and structure. Then, as part of a potential extended mission, New Horizons would conduct similar studies of one or more smaller worlds in the Kuiper Belt, the region of ancient, rocky and icy bodies far beyond Neptune's orbit.

The Oort Cloud.  

There is believed to be a very large number of cometary bodies well beyond the Kuiper Belt.  Unlike the rest of the Solar System the Oort Cloud is believed to form a shell around the System unkike the planets, asteroids and Kuiper Belt Bodies which lie in the plane of the disc.


Comets are believed to be a very old part of our Solar System. They are made of the leftover materials that didn't become part of the Sun, the planets, or the moons. If we knew more about comets, we would know more about how our Solar sSystem formed over four billion years ago!

Some comets live in a cloud of comets far outside the orbit of Pluto, the furthest planet from the Sun. This comet cloud is called the Oort Cloud. Comets in the Oort Cloud may take 30 million years to make one trip around the Sun!

Most of the comets we see in our part of the Solar System, though, come from just outside the orbit of Neptune in the region called the Kuiper Belt. These comets usually take 200 years or less to make one orbit of the Sun. They are called short-period comets. It's when one of them gets in close to the Sun that we can see it in our skies.

The nucleus, or solid part of a comet, is usually less than 10 kilometers (about 6 miles) across. The nucleus is like a dirty snowball. Although nobody knows for sure what they are like inside, comets seem to contain a lot of ice, some rocks and dust, and some gas. As they get closer to the Sun and start to heat up, some of their materials starts to boil off. This material forms a cloud around the nucleus. The cloud is called the coma and may be hundred of thousands of kilometers in diameter. Trailing out for long distances are the comet's tails.

There are three tails. The tails appear as the comet approaches the Sun. Sunlight pushes on things, but very gently. Because the comet dust particles are so small, they are pushed away from the Sun into a long tail. Another tail is made of electrically charged molecules of gas (called ions). It is usually seen as a separate tail at an angle to the main bright yellowish tail.   There is a third tail is made of sodium, which we usually don't see with our unaided eyes.

Missions to Comets

A number of missions have been sent to comets.  One of the most ambitious is the European Space Agency's Rosetta Mission which was launched in 2 March 2004 and is due to reach the comet 67 P/Churyumov- Gerasimenko. in 2015.

On 4 July 2004 the NASA Mission to Tempel 2 came to fruition when it impacted with the comet on a very symbolic day. NASA also launched aspacecraft to the comet Wild 2 to collect solid material in the form of dust and returned a sample from it to Earth in January 2006.  Scientists at the Naval Research Laboratory (NRL) have already analyzed samples from Comet Wild 2, the first solid sample return mission since Apollo. Over one hundred scientists at various institutions have participated in the preliminary analysis.


Solar System