Before there were planets
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.
Gravity pulls dust clouds together
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.
What are planets made of?
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.
Why are planets round?
Because gravity pulled evenly in all directions, the planets were generally shaped like spheres.
How old are the planets?
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”.
Do all the stars have planets?
No, not all stars have planets. But there are planets around many other stars besides the Sun. 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.
Who discovered the planets?
You can see five of the planets in our solar system just by looking up into the sky on a clear night when there’s no moon. So Stone Age people knew about those five planets. The other planets were discovered by European astronomers once they had telescopes, about 1600 AD.
How did the planets get their names?
In ancient Mesopotamia, astronomers named those planets after their gods: Marduk, Ishtar, Nergal, Ninurta, and Nabu. The Greeks and Romans translated those Mesopotamian planet names. So they gave the planets the names of their own gods. Marduk became Zeus, and then Jupiter. Ishtar became Aphrodite, and then Venus. Nergal became Ares, and then Mars. Ninurta became Kronos, then Saturn. Nabu became Hermes, and then Mercury. European astronomers named the last two planets- Neptune and Uranus – after other Roman gods.