Our Sun probably had thousands of sibling stars it has since lost
About 4.6 billion years ago a fragment of a giant gas cloud collapsed, perhaps nudged by an exploding star. The Sun likely grew up in a crowded cluster of between one thousand and ten thousand stars, and a close pass by one of them may explain why distant icy bodies like Sedna follow such strange orbits.
The leading explanation, the nebular hypothesis, was sketched by three 18th-century thinkers: Swedenborg, Kant and Laplace. It fell from favour because it seemed unable to explain why the Sun holds so little angular momentum compared with the planets, but from the early 1980s telescopes found young stars wrapped in cool discs of gas and dust, just as predicted, and the idea returned. Even the phrase Solar System is relatively recent, first recorded in 1704.
In this picture a cloud about 65 light-years wide broke into fragments, one of which, the presolar nebula, became our system. Roughly 98 percent of its mass was hydrogen and helium from the Big Bang; the rest had been forged in earlier stars. Meteorites carry decay products of iron-60, an isotope made only in short-lived exploding stars, which suggests a supernova went off nearby and may have triggered the collapse. The oldest meteorite inclusions, 4,568.2 million years old, offer one definition of the system's age.
As the nebula shrank it spun faster and heated up. Over about 100,000 years it flattened into a disc some 200 astronomical units across around a hot protostar, and within 50 million years hydrogen fusion ignited in the core, putting the Sun on the main sequence where it remains. Jupiter shares the Sun's composition and has been proposed as a failed second star, though it lacked the mass for fusion. Later, moons formed from discs, were captured, or, like Earth's Moon, may have come from giant collisions, while planetary migration rearranged orbits.
Kuiper belt evidence points to a birth cluster that began dispersing between 135 and 535 million years after formation. The Sun may also have drifted, forming perhaps 3 kiloparsecs closer to the galactic centre. Its future is mapped too: in roughly five billion years it will swell into a red giant, shed its outer layers as a planetary nebula and settle as a white dwarf. Over tens of billions of years, passing stars will probably strip away every original body orbiting it.
Source: Formation and evolution of the Solar System
