The Planets

What are they and how are they made?

For several hundred million years after the Big Bang, there were no planets, only stars. Planets couldn't form until the first stars ran out of fuel and exploded into supernovas, about 14 billion years ago. The supernovas spewed out thousands of tons of carbon, oxygen, iron and other elements into space. Planets are made out of the recycled atoms of old stars.

New stars formed wherever these atoms in space got a little thicker, and gravity began to pull them together. When the clouds of atoms got heavy and hot enough at their centers, that set off nuclear fusion and made a new star. But around the outside of these stars, you still had the thinner edges of the clouds floating around. The gravity of the star in the middle, through centrifugal force, pulled these thin clouds into orbit around the star.

Little by little, the whirling clouds around the star got thicker in some places and thinner in others. Where they were thicker, more atoms stuck together. The heaviest atoms, iron, ended up making the centers of planets, while the lighter atoms, like hydrogen, carbon, oxygen and helium, ended up on the surface. Because gravity pulled evenly in all directions, the planets were generally shaped like spheres. The first planets may have formed around 14 billion years ago, but not all planets formed then. The planets that go around our Sun, including Earth, probably formed only around 4.5 billion years ago, and new planets are still forming today around other stars.

Some planets formed closer to their star, and others formed further away. A planet that was close to a star was hotter, of course, but also usually smaller and harder, made mostly of iron, like our planets Mercury, Venus, Earth, and Mars. Planets that formed farther away from their star were colder, larger, and softer, made mostly of hydrogen, like our planets Jupiter, Saturn, Uranus, and Neptune. We call these "gas giants". There are planets around many other stars besides the Sun, and like our planets some of them have water on them, and organic molecules like methane, but we don't know yet whether there is anything alive on them.

Below is a list of the planets of our solar system.


The planet Mercury is one of the most difficult planets to study as it is always too close to the sun. Not even the Hubble Space Telescope can study the planet. Mercury, named after a Roman God, has a very elliptical orbit around the sun. At its closest point, its perihelion,  it is about 46 million km (28.6 million miles) from the sun and at its farthest, its aphelion, its about 70 million km (43.5 million miles) from the sun.Mercury is not easily seen from Earth due to its small angular separation from the Sun. Mercury moves around the sun faster than any other planet. Mercury travels about 48 km (30 miles) per second and it takes 88 Earth days to orbit the sun. The Earth goes around the sun once every 365 days (one year). Mercury is about 77.3 million km (48 million miles) from Earth at its closest approach. The planet rotates once on its axis every 59 Earth days but its interval between one sunrise and the next is about 176 Earth days. Mercury is the second densest major body in the solar system after Planet Earth and its density is slightly less than the Earths. Mercury's smaller mass makes its force of gravity only about a third as strong as that of the Earth. An object that weighs 100 pounds on the Earth would weigh only about 38 pounds on Mercury.


Venus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after the Roman goddess of love. Apart from the moon, it is the brightest natural object in the night sky and it is often called The Evening Star and The Morning Star because it reaches its maximum brightness before sunset and also before sunrise.

Venus is also reffered to as Earth's twin sister as it is similar in size, gravity and bulk composotion. However, during the last few years scientists have found that the kinship ends here. Venus is very different from the Earth. It has no oceans and is surrounded by a heavy atmosphere composed mainly of carbon dioxide with virtually no water vapor. Its clouds are composed of sulfuric acid droplets. At the surface, the atmospheric pressure is 92 times that of the Earth's at sea-level.


Our planet Earth was formed about 4.5 billion years ago. The Earth has a natural satellite called the Moon. Earth is the densest of all the rocky planets in our solar system and is made up of several layers. Below is a list of these layers and the depth of these layers in km: Crust: 0 - 4 Upper Mantle: 40 - 400, Transition Region: 400 - 650, Lower Mantle: 650 - 2700, D'' Layer: 2700 - 2890, Outer Core: 2890 - 5150, Inner Core: 5150 - 6378.

The outer layer of our planet is in constant motion due to a process called plate tectonics (continental shift). Continental plates float on the liquid magma and over a period of millions of years this movement has given us the shape of the continents. About 250 million years ago the Earth was made up of one large continent called Pangea. The supercontinent of Pangea subsequently fragmented and the pieces now account for Earth's current continents which are still in motion today.


Mars is the fourth planet from the Sun and is commonly referred to as the Red Planet. The rocks, soil and sky have a red or pink hue. The distinct red color was observed by stargazers throughout history. It was given its name by the Romans in honor of their god of war. Other civilizations have had similar names. The ancient Egyptians named the planet Her Descher meaning the red one. Before space exploration, Mars was considered the best candidate for harboring extraterrestrial life. Astronomers thought they saw straight lines crisscrossing its surface. This led to the popular belief that irrigation canals on the planet had been constructed by intelligent beings. In 1938, when Orson Welles broadcasted a radio drama based on the science fiction classic War of the Worlds by H.G. Wells, enough people believed in the tale of invading Martians to cause a near panic.

The first spacecraft to visit Mars was Mariner 4 in 1965.


Jupiter is the fifth planet from the sun and the largest in our solar system. It is a gas giant with mass slightly less than one-thousandth that of the Sun but is two and a half times more massive than all of the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn, Uranus and Neptune. Together, these four planets are sometimes referred to as the Jovian planets. When viewed from Earth, Jupiter will appear to be the third brightest object in the night sky next to the Moon and Venus. Jupiter is mainly composed of hydrogen with a quarter of its mass being helium. Like our planet Earth, Jupiter has a bulge at its equator and this is due to its rapid rotation. The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. The most dominant feature on Jupiter is its giant red spot. This giant red spot is actually a raging storm that is believed to have existed since the 17th century when it was first noticed through a telescope. Surrounding the plant is a powerful magnetosphere and at least 63 moons.


Planet Saturn

Saturn is the sixth planet from the sun and the second largest in the solar system after Jupiter and it is also a gas giant. Saturn is named after the Roman god Saturnus. The planet Saturn stands out from the rest of the objects in our solar system mainly because of its beautiful rings. The rings consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn. The ring particles are made almost entirely of water ice, with some contamination from dust and other chemicals. The brightness of Saturn is mainly due to the rings reflecting the sun's light onto the planet's surface but Saturn's rings are still not visible from Earth without visual aid. The rings were first seen by Galileo Galilei in 1610, the year he first looked at space through a telescope. During 1980 and 1981, Voyager 1 & 2 imaged the ring system extensively and the spaecraft's instruments observed gaps in the ring system. The rings have been given letter names in the order of their discovery. The main rings are, working outward from the planet, known as C, B, and A.


Uranus is the seventh planet from the Sun and the third largest in the solar system. Though Uranus is visible to the naked eye, it was never recognized as a planet to ancient observers due to its dimness and slow orbit. The planet was discovered on March 13th, 1781 by William Herschel and this expanded the boundries of the known solar system for the first time in modern history. Uranus is also a gas giant and like Saturn, it has a ring system. Uranus has several moons and the five largest are Miranda, Titania, Oberon, Umbriel, Ariel. Titania is the largest of all the Uranus moons with a radius only half that of our moon. Titania and Oberon were discovered by William Herschel in 1787. Ariel and Umbriel were discovered by William Lassell in 1851. John Herschel (William Herschel's son) gave the four then known moons their names in 1852. In 1948 Gerard Kuiper discovered the moon Miranda. Uranus is named after the ancient Greek God of the heavens and has been vistied by only one space probe, Voyager 2 on Jan 24 1986. One odd feature discovered by voyager 2 is that Uranus spins on its side resulting in its polar regions pointing towards the sun.


Neptune is the eight planet from the sun and is named after the Roman god of the sea. It is also a gas giant. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 Earth masses and not as dense. Neptune was the first planet found by mathematical prediction rather than by empirical observation(scanning the sky with a telescope). Unexpected changes in the orbit of Uranus led astronomers to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently found within a degree of its predicted position, and its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining 12 moons was located telescopically until the 20th century. Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. It was the last stop in 1989 for the Voyager 2 spacecraft on its grand tour of the solar system. Neptune has eight known moons: Triton, Thalassa, Naiad, Despina, Galatea, Larissa, Proteus and Nereid. Neptune has been particularly challenging to study from the ground because its disk is small and badly blurred by the Earth's atmosphere at that distance.

Pluto....The Dwarf Planet:

There are oficially only eight planets in our solar system. According to scientists, there are hundreds of dwarf planets yet to be discovered and we already know of 44.

Pluto was discovered in 1930 by a fortunate accident. Calculations which later turned out to be in error had predicted a planet beyond Neptune, based on the motions of Uranus and Neptune. Pluto is usually farther from the Sun than any of the eight planets; 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 the most eccentric orbit of all the planets in the solar system. Its orbit takes it to 49.5 AU (7.4 billion kilometers) at its farthest point from the Sun. And its orbit takes it as close as 29 AU (4.34 billion kilomters) to the Sun. It takes 249 years for Pluto to complete its orbit. This means that a single Pluto year is 249 earth years long.