MARK GARLICK / SCIENCE PHOTO LIBRARY MARK GARLICK / SCIENCE PHOTO LIBRARY
Planetary system formation. Illustration of a primordial nebula (solar nebula, in the case of the Sun) surrounding a newborn star (centre). The Sun was formed in this way as gas coalesced until it was dense enough and hot enough to begin the nuclear reactions that sustain a star. The solar nebula, originally a sphere of gas, dust and rock, became a disc that eventually, under gravity, coalesced into planets. Some planets are already seen forming in the disc (red glows), carving out distinct gaps as they attract material in their orbits. It is thought the planets formed around 4.6 billion years ago. Astronomers have now found numerous similar objects in the Milky Way.
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