How can planets form around a star

In this article we explain the process of star formation for regular Sun-like stars. Stars form from an accumulation of gas and dust, which collapses due to gravity and starts to form stars. The process of star formation takes around a million years from the time the initial gas cloud starts to collapse until the star is created and shines like the Sun.

The leftover material from the star's birth is used to create planets and other objects that orbit the central star. Observing star formation is difficult, because the dust is not transparent to visible light. It is, however, possible to observe these dark stellar nurseries using radio waves, because radio waves travel freely down to us and our radio telescopes.

Stars, like our own Sun, have not always been around. Stars are born and die over millions or even billions of years. Stars form when regions of dust and gas in the galaxy collapse due to gravity. Without this dust and gas, stars would not form. A galaxy contains not only billions of stars, but also large amounts of gas and dust. These regions of gas and dust in the galaxy lie in the space between the stars. If the galaxy were a street, the houses would be stars and the regions of gas and dust would be the gardens in between the houses.

The space between the stars in a galaxy is called the interstellar medium , because it is the medium, or substance, that makes up the space between stellar objects.

The regions of gas and dust are called molecular clouds , because of their content. Molecular clouds are made of a mix of atoms, molecules, and dust. Atoms are the small building blocks of all the stuff around us.

Molecules consist of two or more atoms joined together. The molecules present in molecular clouds are typically molecular hydrogen, H 2 , but can also be more complex molecules, such as methanol, which consists of six atoms, or water, which consists of three atoms. Dust grains are even larger clumps of matter and they can be up to a few millimeters in size, which is huge compared with atoms or molecules.

Molecular clouds in the interstellar medium are large. In fact, a single molecular cloud can be hundreds of thousands of times heavier than the Sun. Their volumes also vary: a molecular cloud can be the same size as, or many times bigger than, our entire solar system. These enormous molecular clouds undergo turbulent motion. This means that the gas and dust within the clouds do not stay in the same place as time passes. These substances move around in all directions, like children running around in a school yard.

This turbulent motion of the gas and dust distributes the atoms and molecules unevenly, so that some regions of the molecular cloud will have more matter in them than other regions Figure 1A. Within these clumps, the attraction of each part for all the other parts caused the clumps to shrink steadily, squeezing their material into ever-smaller volumes.

As the clumps continued to contract, the resulting increase in density caused a corresponding rise in temperature at the clumps' center. Eventually, as this central temperature rose above 10 million degrees, atomic nuclei began to fuse.

The onset of nuclear fusion, which marks the birth of a new star, occurred nearly 5 billion years ago in the case of our Sun. Some break apart, but others hold on.

These are the building blocks of planets, sometimes called "planetesimals. Where the disk is colder, far enough from the star that water can freeze, tiny fragments of ice hitch a ride with dust. Dirty snowballs can amass into giant planetary cores. These colder regions also allow gas molecules to slow down enough to be drawn onto a planet.

This is how Jupiter, Saturn, Uranus and Neptune, the gas giants of our solar system, are thought to have formed. Jupiter and Saturn are thought to have formed first and quickly within the first 10 million years of the solar system. In the warmer parts of the disk, closer to the star, rocky planets begin to form.

Planets that are rocky like Mercury, Venus, Earth and Mars may take tens of millions of years to form after the birth of the star.