Solar nebula theory (SNT)
This is the post of how planets are formed, and begins with a huge cloud of gas and dust starting to collapse under its own gravity – perhaps prompted by blastwaves from a nearby supernova explosion. At the centre of the cloud, the density and temperature becomes high enough to trigger nuclear fusion, and a star begins shining.
Meanwhile, the debris around it begins to coalesce into even larger chunks. These ‘planetesimals’ then become the cores for full-blown planets, the debris around them helping to make their initially highly elliptical orbits more circular. The end result is one or more planets orbiting the central star.
The capture theory
An alternative to the solar nebula theory is the capture theory, in which the Sun acquired the gas and dust from a star in the process of forming nearby. Most stars begin life in so-called stellar clusters, where close encounters are common. In the capture theory, one such encounter between the newly created Sun and a nearby ‘proto-star’ led to the capture of some of the proto-star’s material by the Sun’s gravitational field. Once in orbit around the Sun, the disk of debris coalesced to form four ‘gas giants’ – Jupiter, Saturn, Uranus and Neptune – and the four rocky inner planets – Mercury, Venus, Earth and Mars.
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