What are planets made of?

D. Indumathi, The Institute of Mathematical Sciences, Chennai

You all know that there are many planets that circle our Sun and that our own home, the Earth, is only one of them. You may also have heard about Pluto which was once considered a planet and is now called a dwarf planet.
For thousands years human beings from many countries around the world have looked up into the sky and wondered about the objects they saw in it. Even earlier, animals and plants learned to instinctively plan their day or breathing/eating/hunting habits depending on the position of that fiery eye in the sky: our Sun. The earliest sundials are recorded in 3500 B.C. while planets were charted from about 2000 years ago. A ceiling in the tomb of Senenmut around 1500 B.C. in Egypt shows an astronomical chart (see picture).
The Babylonians who lived in Mesopotamia knew about Mercury, Venus, Mars, Jupiter and Saturn, and were perhaps the first civilisation to learn so much about them. The Sun and the Moon were also considered to be planets. The Greeks including the famous astronomer Ptolemy, also knew the same list as also the Chinese. They were, in increasing order from Earth (in Ptolemy's order): the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. The Indians including Aryabhata also soon joined the list of famous astronomers about 1500 years ago. But it was only with the invention of telescopes by the Europeans that the remaining planets were discovered.
By the early 17th century, Galileo Galilei had built many telescopes that could be used to study the sky. He made many observations of Venus and also studied Saturn and its rings. He also noticed Neptune and tracked its path for some time but it was not realised that it was a planet until 200 years later, in 1846! His observations led him to realise that the Earth was not at the centre of the Universe but that it was just another planet going around the Sun. This theory was finally given shape to by Johannes Kepler. Galileo also observed sunspots, the lunar craters and the Milky Way galaxy and realised that there were literally thousands of stars in the sky.

Nebulae
A cloud of dust and gases in space is called a nebula. Nebula means "cloud" in Latin and early astronomers saw many of these clouds way up in the sky, clearly not part of Earth's atmosphere. The gases are mostly hydrogen and helium, among the simplest that can form. These gas clouds form in several ways. As a star like our Sun reaches the end of its life it grows bigger and hotter; it is called a Red Giant. Soon it has no more fuel to burn: it ejects its outer layers and transforms into a small compact white dwarf star. The outer layers form a planetary nebula. In fact, this will be the fate of our Sun in a few billion years.
Sometimes the star is very massive, about 20 times more than our Sun, it ends its life in a spectacular way: the star literally collapses in a supernova explosion. The gas surrounding the exploding star also forms a nebula. Of course, in the huge Universe, there are many places where dust and gases can simply accumulate and stay held together by gravity. These form diffuse nebulae and can be seen by reflected light of nearby stars. The Orion nebula is the nearest and most well-known of these.
Nebulae are often star-forming regions, such as in the Eagle Nebula (shown on the back inside cover). This image is one of NASA's most famous images, and was named the "Pillars of Creation". In these regions the formations of gas, dust, and other materials "clump" together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

How stars form
By the 19th century, all the planets were known and their orbits (paths around the Sun) were also known. However, even today we are not sure how planets form. The theory is that a nebula collapses and forms a protostar at its centre or core. This core is surrounded by a rotating disk called protoplanetary disk. As the name suggests, protoplanetary means "that which forms a planet". Dust particles accumulate on the disk and make it more massive. Some parts of the disk have more mass and so higher gravitational attraction; this causes more particles to be pulled in at this point so that the material in this part of the disk begins to clump together to form a protoplanet.
In the meanwhile, the protostar is also growing in mass and since it is tightly held together by its own gravity it gets squeezed and so begins to get hot and ignites. It starts to shine and becomes a star. Its heat causes the dust material in the outer disk to disperse. However, the places in the disk where the matter is clumped does not disperse. Some of the clumps come together and form larger clumps and these finally become planets or satellites of planets. Just as the protodisk circled the protostar, so do the planets continue to circle the star. So we believe that the planets are formed along with their stars in the regions where nebulae collapse. Entire galaxies are formed this way. So nebulae are the factories where stars and planets are made.

How planets take shape
When the clumps of matter collide to form planets, the matter gets heated up so the planet partially melts. The denser, heavier matter settles down and since planets (due to gravity) are mostly spherical, "down" means the centre or core!! That is why the Earth's core is made of iron, which is one of the densest metals known. Surrounding the core is a more fluid (liquid or gas) mantle. In fact, with the discovery of exo-planets (planets in other galaxies circling other stars) these theories can be tested. We know for instance that metal-rich stars are likely to have more planets than metal-poor ones.

Our Solar System
Excluding Pluto, we have eight planets. There are several dwarf planets and two asteroid belts, one between the orbits of Mars and Jupiter and one outside the orbit of Neptune.
The picture shows the planets and some dwarf planets, in actual scale so you can compare their sizes. The top row has Uranus (left) and Neptune (right). The next row has Earth, Sirius B and Venus from left to right. The smaller planets and dwarf planets below, in decreasing size are Mars and Mercury in a row with the Moon, Pluto, and Haumea below.
Mercury, Venus, Mars and Earth are called the Terrestrial Planets since they are largely composed of rock. Earth is the largest terrestrial planet.
Jupiter, Saturn, Uranus and Nepture are called Gas Giants or Jovians, named after Jupiter, which is the largest and most massive one, 318 times heavier than Earth. In fact, all the gas planets are more massive than Earth.
Uranus and Neptune are sometimes also called Ice Giants, since they have both ice and rock in them.

Core, mantle and Crust
In the terrestrial planets such as Earth, the top of the fluid mantle solidifies into a hard crust, just as many liquids do when they are left in the open! In gas giants the mantle simply dissolves into the upper cloud layers so the surface of the planet is not clearly separated from its atmosphere. All planets (except Mercury) have atmospheres which is air held in place by the gravitational attraction of the planet.
The terrestrial planets possess cores of magnetic elements such as iron and nickel, and mantles of silicates (sand). Jupiter and Saturn are believed to possess cores of rock and metal surrounded by mantles of metallic hydrogen. Uranus and Neptune, which are smaller, possess rocky cores surrounded by mantles of water, ammonia, methane and other ices. The fluid action within these planets' cores creates a geo-dynamo that generates a magnetic field. You must have learned about Earth's magnetic field as well.
The gas giants have atmospheres of hydrogen and helium (as in the Sun). But these gases are very light and so the smaller terrestrial planets are not able to hold them down and they have escaped. This is good for us because hydrogen burns easily and it would be a catastrophe for life-forms on Earth! Also, we have oxygen in Earth's atmosphere from the plant life and this is not there elsewhere.