About 5 billion years ago, a vast, cold cloud of interstellar gas and dust, called a nebula, began to collapse. A star formed at the center, surrounded by a spinning disk of gas and dust. Small grains condensed, accreted into smaller rocky bodies, and ultimately grew into planets. Thus was born our solar system—the Sun and all the planets, moons, comets, asteroids, and meteoroids that revolve around it.

This solar nebula theory is well supported by modern astronomical observations of other nebulas, disks, and planets, and by radiometric dating. Samples of the Moon, Earth, Mars, and meteorites (the remnants of meteoroids that entered the atmosphere as meteors and landed on Earth) have a similar age of 4.56 billion years. The evidence implies that planets are a natural byproduct of star formation and that solar systems are common throughout the universe.

However, what initially caused the nebula to begin to collapse remains unknown. Some researchers suggest the death of a distant massive star in a violent, luminous explosion called a supernova sent a shock wave through space, compressing the cloud. Others argue that a supernova shock wave would scatter a gas and dust cloud, rather than collapse it. They propose instead that radiation emitted by a nearby massive star prior to its death could have nudged the nebula to collapse.

The debate centers on evidence from meteorites, which preserve a record of the chemical composition and conditions at the time of solar system formation. Researchers have found nickel-60 in the grains of meteorites. Nickel-60 is a radioactive decay product of iron-60, which forms inside massive stars and is dispersed by supernovas. Iron-60 decays rapidly into nickel-60, allowing scientists to use it like a clock. The presence of nickel-60 means that a supernova happened around the time that solid grains (later found in meteorites) began to condense out of the cloud.

This is tantalizing evidence that a supernova may have triggered the birth of the solar system—with the shock wave injecting iron and triggering the collapse at the same time. In contrast, proponents of the hypothesis that radiation from a nearby star collapsed the cloud believe the iron was injected later—by the supernova of that star—after the Sun and planets had already begun to form.

More research is needed to determine whether the birth of the solar system was triggered by a distant supernova or radiation from a nearby star, or possibly something else. An emerging hypothesis holds that no one star contributed the iron-60 and other short-lived radionuclides, but that they instead came from an ensemble of massive stars that formed in the nebula before the solar system.

In addition to iron-60, scientists are studying other short-lived radionuclides found in meteorites, such as aluminum-26, manganese-53, and iodine-129, which form in different ways, in stars of different masses, and have different half-lives. Thus, the proportion in which they appear can isolate which events occurred—in the right place, at the right time—to trigger the birth of the solar system and, ultimately, all life on Earth.

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