What Is the Oort Cloud? A Shell of Comets Nobody Has Seen

The reasoning was set out by the Dutch astronomer Jan Oort in 1950 from the orbits of long-period comets, and it has three steps. First, such comets arrive from every direction rather than from the plane in which the planets orbit, which means their source is a sphere rather than a disc. Second, a great many of them have orbits whose farthest point lies at roughly fifty thousand times the Earth-sun distance, clustering at a characteristic distance rather than being spread arbitrarily. Third, a comet passing close to the sun loses material and survives only a few hundred passes, so the supply must be continuously replenished from a reservoir far larger than the number currently visible. Ernst Opik had proposed a distant cometary reservoir in 1932, which is why the structure is occasionally named for both men.

Modelling suggests two regions with different shapes and behaviours:

• •An inner region, sometimes called the Hills cloud, roughly disc-shaped and running from about two thousand to twenty thousand astronomical units, probably containing most of the mass
• •An outer region, genuinely spherical, extending to somewhere between fifty thousand and a hundred thousand astronomical units, perhaps a quarter of the distance to the nearest star
• •A total mass estimated at a few times the Earth's, spread across an enormous volume, so the objects are separated by tens of millions of kilometres and any given cubic region is essentially empty
• •Composition of water, ammonia and methane ices with rock and dust, effectively unaltered material from the formation of the solar system
• •Gravitational binding to the sun that is extremely weak at those distances, which is the essential point, since a very small nudge can send an object inward or out of the system entirely

The objects did not form where they are, because the material at such distances is far too thinly spread to accumulate into anything. The accepted account is that they formed among the giant planets, in the region from roughly Jupiter to Neptune, and were then flung outward by close encounters with those planets as the young solar system rearranged itself. Most were ejected from the system completely; a fraction were thrown into distant orbits and then had those orbits rounded off by the gravity of passing stars and of the galactic disc, which lifted their closest approach away from the planets and left them stranded at the edge. That process also means the cloud is probably not purely ours: simulations suggest that a substantial fraction of its objects may have been captured from other stars in the cluster where the sun formed, and that a good number of ours are now in other systems.

Objects sit undisturbed for millions of years and then arrive because something perturbs them. Three mechanisms operate. The galactic tide, the differential pull of the Milky Way's mass on objects at different distances from the sun, gradually alters orbits and is the dominant influence. Passing stars occasionally come close enough to disturb a region of the cloud, and one, a red dwarf called Scholz's star, is calculated to have passed through the outer cloud around seventy thousand years ago, which is within the period when modern humans were alive to watch whatever followed. Giant molecular clouds produce similar effects on longer timescales. A nudge that changes an object's speed by a few metres per second is enough to drop its closest approach into the inner solar system, after which it either becomes a long-period comet, is captured into a shorter orbit by a planet, or is ejected. Comet Hale-Bopp in 1997 and comet Hyakutake in 1996 are recent arrivals from this population.

The objects are small, dark and extremely distant, and brightness falls off with the fourth power of distance for a body seen by reflected sunlight, which puts them far beyond direct detection with current instruments. Sedna, discovered in 2003, may be the closest thing to a confirmed member: it has a highly elongated orbit that never comes near Neptune and reaches around nine hundred astronomical units, which is too far for the known planets to have placed it there and suggests either an early stellar encounter or an inner cloud population. A handful of similar objects have since been found, and their orbital alignment prompted the hypothesis of an undiscovered large planet further out, which remains unconfirmed. No spacecraft will reach the cloud for a very long time: Voyager 1, the most distant human object, is around a hundred and sixty astronomical units out after nearly fifty years and would need roughly three hundred more to reach the inner boundary. For the foreseeable future the cloud remains a structure inferred entirely from the trajectories of the few objects it sends us.

The cloud is inferred from long-period comets arriving from every direction with farthest points clustered around fifty thousand times the Earth-sun distance, and from the fact that comets are destroyed quickly and must be replenished. It probably has a disc-like inner region and a spherical outer one reaching a substantial fraction of the way to the nearest star, holding perhaps a trillion icy bodies formed near the giant planets and flung outward. Galactic tides and passing stars nudge objects inward, and nothing in it has ever been directly observed.