When being asked the tough questions about why things are the way they are (usually by our own children!) one of the toughest questions to answer is, how did we get here on planet Earth? Well, if you were to answer this question scientifically, chances are it would unleash a whole sleuth of questions. And to be able to answer them, you would first have to be familiar with a little something known as Nebular Theory. This is the theory of how not only our own solar system, but all star systems were formed. Although still a theoretical model, it is the most widely accepted scientific explanation of how stars came to emerge from the cosmos.
This nebular hypothesis was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. Kant argued that gaseous clouds—nebulae, which slowly rotate, gradually collapse and flatten due to gravity and eventually form stars and planets. Laplace’s proposed a similar model in which a protosolar cloud (a nebular cloud) contracted and cooled, flattening and shedding rings of material in the process which later collapsed to form the planets. Over the course of the 20th century, this model came to be challenged by a number of theorists who proposed numerous models in an attempt to replace it. However, none of these attempts were successful and it was not until the 1970’s with Soviet astronomer Victor Safronov that the modern (and widely accepted) Solar Nebular Disk Model (SNDM) came into being.
According to this model, our star system was formed 4.568 billion years ago when a small part of a giant molecular cloud experienced a gravitational collapse. Most of the collapsing mass collected in the center forming the Sun while the rest flattened into a protoplanetary disk, out of which the planets, moons, asteroids, and other small Solar System bodies formed. Since that time, our system has evolved considerably due to collisions between objects, planetary migration and the capturing of extra-solar objects by our own system. While originally applied only to our own Solar System, the SNDM has since been used by theorists to explain star formation throughout the known universe.
While this remains the most widely accepted theory, alternative models still exist. Since the dawn of the space age in the 1950s and the discovery of extrasolar planets in the 1990s, all of these models have been both challenged and refined to account for new observations.
If you’d like more info on the Nebular Theory, check out the Theory of Planetary System Formation page, and here’s a link to NASA’s Solar System exploration page.
We’ve also recorded a series of episodes of Astronomy Cast about every planet in the Solar System. Start here, Episode 49: Mercury.