Nobody has seen the Oort cloud, yet its comets keep arriving
No telescope can image the Oort cloud, a shell of icy bodies that may stretch more than three light-years from the Sun. Jan Oort deduced it in 1950 from a simple puzzle: comets that swing close to the Sun should have burned out long ago, so something must keep sending fresh ones.
Every pass near the Sun boils ices off a comet until it breaks up or seals itself under a dark crust. Orbits that dip into the planetary zone are also unstable over millions of years, ending in collisions or ejection. Oort reasoned that comets seen today cannot have been circling this close for the 4.5 billion years since the planets formed; they must have spent almost all that time in cold storage far away. He noticed that long-period comets' farthest points clustered around 20,000 astronomical units, one unit being the distance from Earth to the Sun, hinting at a spherical reservoir.
The modern picture has two parts. An inner, doughnut-shaped Hills cloud, proposed by Jack Hills in 1981, runs from about 2,000 to 20,000 units and probably holds far more objects. Beyond it lies a spherical outer cloud reaching perhaps 50,000 units, and possibly 200,000. Even its closest edge is more than a thousand times farther out than the Kuiper belt. The outer region marks the edge of the Sun's gravitational reach, so passing stars and the pull of the Milky Way easily nudge its members, occasionally sending one inward as a new comet.
The numbers are huge but the spacing is lonelier still. There may be trillions of bodies over a kilometre wide, yet neighbours sit tens of millions of kilometres apart. Most are probably water, methane, carbon monoxide and other ices, with perhaps a small rocky fraction.
They were not born there. The favoured story is that they formed among the planets, then were flung outward by the young giants such as Jupiter, while stars and galactic tides rounded their orbits into a sphere. Some may even have been captured from the disks of neighbouring stars in the cluster where the Sun was born.
Source: Oort cloud
