While all planets in the solar system have asteroids and moons of different sizes orbiting them, Earth seems to make an exception. It has the biggest single moon, relative to the planet's size. There are different theories regarding how it formed, but only one stands out as the most popular.

It looks like the Moon has formed as a result of a planetary collision, right after the Sun formed about 4.5 billion years ago. The original Earth intersected orbit with a so-called planet destroyer, a celestial body roughly the size of the planet Mars and suffered a violent collision that blasted both bodies apart, spreading large quantities of smoldering rock and dust in space. The leftovers eventually cooled and collapsed back to form the current planet Earth, but some of the debris remained in orbit around our planet and formed the big Moon we all see today.

Studies made by NASA's Spitzer Space Telescope by observing large clods of stellar dust suggest that moons similar to our own may form in about 5 to 10 percent of the planetary systems. Astronomers hoped that they would find large numbers of moons forming in the dust clouds around other stars, but were disappointed to find out that this wasn't the case.

The circumstances regarding the formation of the Moon are unique in our solar system, most of the planets having asteroids orbiting them, captured by the planet's gravitational field, or small moons relative to the size of the planet, which formed around them. For example Mars has two small asteroids orbiting it, while large gas planets such as Jupiter or Saturn have moon of sizes comparable to that of the planet Mercury.

Astronomer Nadya Gorlova from the University of Florida in Gainesville and her colleagues studied the dust signs of what appears to be evidence of similar planetary collisions, present around 400 stars, with ages of about 30 million years old, close to the age when the Earth and the Moon formed. Out of all the studied stars, only one seemed to present evidence that displayed the distinct dust signature. By calculating the frequency of planetary systems forming around other stars and the time needed for the dust clouds to collapse to form planets, scientists were able to determine how often bodies are formed through the same process as our moon.

However due to the large distances to these stars and limited resolution of the observations, astronomers argue that the frequency of such events resulting in a moon-forming process could be even smaller than calculated. Scientists studying infant stars that have large clouds of gas around them say that the process resulting in the birth of new planets and moons may take between 10 to 50 million years, after the star was formed. The only star that presented evidence of collision-generated dust clouds indicated that these collisions would occur about 30 million years after the birth of the star.

Observations of stars that have spinning dust clouds around them have been carried out for more than 20 years, but there has been no evidence of that the debris was collision-derived or material for planet forming, because the stars were a lot older than our Sun, or about the same age meaning that they have finished the planet forming process and collisions resulting in Earth-moon-like systems had already taken place.

Though moons similar to ours may be rare, the immensity of space might hold billions of rocky planets which have big moons orbiting them.