Comets are among the oldest objects in the Solar System, almost unchanged since they were formed billions of years ago. The primordial Solar System was a large, flattened cloud that spun slowdy, its center slowly building to become the Sun, its outskirts condensing to become the planets.
Wandering through this cloud were the comets. As Jupiter and the other giant planets grew, their gravity slung many of these comets into a large sphere, now called the Oort Cloud and located well beyond Pluto's orbit. Others survive in the Kuiper Belt, just beyond Pluto and more closely confined to the plane of the Solar System.
As far as a light-year away from the Sun, the Oort Cloud contains billions of comets, their ices forever frozen unless disturbed by the gravitational tug of a passing star or by the Solar System's passage through the galaxy. A comet may then leave the cloud, either ejected from the Solar System altogether or following a new orbit that will head it toward the Sun. After 4 1/2 billion years in deep space, the comet begins to boil off its gases, or sublimate, as it nears the Sun. A comet newly discovered may be in just this state, warming up for the first time and releasing ancient gases.