4.5 billion years ago, a gigantic cloud of dust and gas drifted through space. This dark and cold molecular cloud consisted mainly of hydrogen (74%) and helium (24%), the most abundant elements in the universe. However, it also contained minute traces of iron, oxygen, and silicon—the remnants of earlier stars that had exploded as supernovae.
Possibly triggered by a shock wave from the explosion of a neighboring star, smaller dense regions within the molecular cloud began to accumulate more matter. Under the influence of their own gravity, these regions began to collapse in on themselves. As a result, pressure and temperature rose steadily until nuclear fusion finally began, and the newly formed Sun released enormous amounts of energy.
The remnants of the matter left over from the Sun’s formation surrounded the newly formed star as a flat, slowly rotating disk of dust and gas. Within this disk, collisions and compression gave rise to the celestial bodies of the planetary system as we know it today: the eight planets and their moons, as well as countless smaller bodies, such as asteroids and comets.
Near the extremely hot Sun, light elements such as hydrogen and helium were blown away by its radiation. Only heavy elements such as iron, nickel, and silicon survived there. From these, the four rocky planets—Mercury, Venus, Earth, and Mars—formed.
The outer planets, which are much farther from the Sun, were able to attract enormous amounts of gas. Thus, the “gas giants” Jupiter and Saturn formed in the outer solar system, and even farther from the Sun, the “ice giants” Uranus and Neptune. They are accompanied by numerous moons.
The two groups—the rocky planets and the gas giants—are clearly distinct from one another. Between the orbits of Mars and Jupiter, which are very far apart (525 million km), the asteroid belt contains about one million (2026) small bodies with diameters of up to 1,000 km. These include the 963-kilometer-wide dwarf planet Ceres and the majority of the solar system’s known asteroids to date.
Beyond Neptune’s orbit, extending to a distance of several billion kilometers, lies the belt named after the Dutch-American astronomer Gerard Peter Kuiper. In this flat, disk-shaped region, more than 70,000 celestial bodies with diameters exceeding 100 kilometers, as well as countless smaller objects, orbit. Many comets and comet fragments are believed to originate from this region.
The Sun is the center of the solar system, which consists of eight planets, numerous moons, and many millions of smaller celestial bodies. With a mass of 333,000 Earth masses, it is a thousand times more massive than all the other bodies in the solar system combined.
It is a star in the Milky Way: a hot ball of gas with a diameter of about 1.4 million kilometers. In reality, this distance is 109 times the diameter of Earth, but in our model it is only 31 centimeters.
Inside the Sun, under extreme pressures and temperatures (15 million degrees), 400 million metric tons of hydrogen nuclei fuse into helium nuclei every second (nuclear fusion). This process releases a vast amount of energy, causing the Sun to shine with a surface temperature of 5,500 °C.
With a diameter of just under 1.4 million kilometers, the Sun contains nearly the entire mass of the solar system (99.9%). Its immense gravitational pull keeps all other bodies in elliptical orbits (Kepler’s laws). Depending on the shape of these ellipses, the orbiting bodies (e.g., planets) do not—as would be the case with ideal circular orbits—always remain at the same distance from the Sun, but instead approach the Sun at “perihelion” during each orbit and then move away again to “aphelion”. Their orbital velocity also changes in the process; it is greater at “perihelion” than at “aphelion.”