About 4.6 billion years ago, at the very beginning of our solar system, a vast disk of dust, gas and ice circled the young Sun. Over thousands of years, this original solar nebula was pulled together by gravity to form the celestial bodies as we know them today. Some of the small rocky and icy bodies in this disk would come together to build the planets, but billions of these ancient objects remained after the rocky planets and gas giants began to take shape. These small worlds, the remnants of that time long ago, are the asteroids, comets, dwarf planets, and meteoroids of today. They still orbit the Sun: remnants of that early, fast and furious time of solar system formation. And today, according to the International Astronomical Union (IAU), our solar system contains one star, eight planets, more than five dwarf planets, countless asteroids, comets, and tens of thousands of fragments of rock, ice and other "small bodies" in the solar system.
Keys to the Past
The small worlds of our solar system may conceal answers to some of the most fundamental questions about our own origins. How did our sun's family of planets, moons and small bodies originally form? How did our solar system evolve into the diverse collection of worlds we see today? How did life begin on Earth, and could the same processes have caused life to form on other celestial bodies?
At the very beginning of our solar system, before there was an Earth, Jupiter or Pluto, a massive swirling cloud of dust and gas circled the young sun. The dust particles in this disk collided with each other and formed into larger bits of rock. This process continued until they reached the size of boulders. Eventually this process of accretion formed the planets of our solar system.
Billions of small space rocks never fully evolved. Amazingly, many of these mysterious worlds have been altered very little in the 4.6 billion years since they first formed. Their relatively pristine state makes comets, asteroids and dwarf planets windows to the past- revealing what conditions were like billions of years ago in the early solar system. They can reveal secrets about our own origins, chronicling the processes and events that led to the formation of Earth and life, as we know it.
Comets and asteroids may have delivered some of the water and other ingredients that allowed the complex chemistry of life to begin on Earth. For example, the amino acid glycine was discovered in the comet dust returned to Earth by the Stardust mission. Glycine is used by living organisms to make proteins. The discovery supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts.