Named after the Dutch astronomer Jan Oort, but sometimes also known as the Öpik–Oort cloud, this hypothetical cloud of small, icy planetesimals is thought to surround the solar system out to a distance of more than 3 light years. Refer to the image above for some details of the structure of the Oort cloud, and also note that by way of comparison, the Kuiper belt that exists beyond the orbit of Neptune is about one thousand times closer to the Sun than the inner wall of the Oort cloud is to the Sun. Below are 10 more interesting facts about the Oort cloud you may not have known.
The Oort cloud defines the solar system’s boundary
In practical terms, the outer edge of Oort cloud defines the boundary of the solar system, and the limit of the Sun’s Hill sphere. In simple terms, the limit of Sun’s Hill sphere (named after American astronomer George William Hill, who defined this limit) can be seen as the point where the Sun’s gravity no longer dominates in the face of the gravitational effects of a more massive body, which in this case would be either the Milky Way galaxy, or the gravitational effects of a star passing close to the outer limit of the Sun’s Hill sphere.
Long-period comets may originate in the Oort cloud
Although the question around the origin of the Oort cloud is far from settled, astronomers have used the observed orbits of long period comets such as Halley’s Comet as the basis for the notion that all long period comets, as well as “centaurs” and Jupiter-family comets have their origin in the Oort cloud. However, even though most short-period comets are thought to originate in the scattered disc (NOT a part of the Oort cloud) it is entirely possible that the ultimate origin of at least some short-period comets may have been in the outer parts of the Oort cloud.
The Oort cloud is really, really big
Although the Oort cloud has not been directly observed, it is thought to resemble a spherical ball with a wall thickness that starts at about 2,000 – 5,000 AU (0.03 – 0.08 light years) from the Sun, and stretches to about 100,000 – 200,000 AU (1.58 – 3.16 light years) from the Sun. This is really big, considering that Proxima Centauri, closest star closest to the Sun, is only 4.22 light years away.
The Oort cloud is only about 5 times as massive as Earth
Based on complex computer models, it is estimated that the Oort cloud contains at least several trillion objects that are bigger than 1 km (0.62 mile) in diameter, and a further several billion objects that are around 20 km (12 miles) in diameter, with the typical distance between objects being on the order of a few tens of millions of km. Although the total mass of the Oort cloud is not known, calculations based on the mass of Halley’s Comet (a suspected Oort-cloud comet) yield a total mass of the objects in the Oort cloud of about 3×1025 kilograms, which is roughly 5 times the mass of Earth. Note though that the mass of the inner Oort cloud (a torus-shaped part of the principal structure) has to date not been calculated.
The Oort cloud contains material from other stars
While conventional wisdom holds that the Oort cloud represents the remains of the original protoplanetary disc out of which the solar system had formed about 4.6 billion years ago, new research has shown that the proto solar system once existed as part of a star cluster that consisted of between 200 and 400 stars. These findings imply that the Oort cloud could therefore not have formed close to the Sun as has been suggested, and could also therefore not have been “puffed up” to its present location and size by the action of the giant gas planets as they migrated away from the Sun.
Moreover, improved modelling techniques suggest that since the structure of the Oort cloud is largely compatible with the notion that other stars may have contributed to its formation, close stellar encounters with the cloud would have been far more frequent then than it is now. In fact, in 2010 Harold F. Levison et al used highly complex computer models to show that up to 90% or more of the material in the present-day Oort cloud originated in the protoplanetary discs of other stars in the cluster that once accommodated the Sun.
The Oort cloud is flexible
Since the outer reaches of the Oort cloud falls in an area where the gravity of the Sun is in direct competition with the gravitational effects of the Milky Way, the Oort cloud is both stretched in one direction, and compressed in another direction by the tidal forces of the galactic gravitational field. This” kneading” effect is thought to be the principal mechanism that perturbs some objects out of their otherwise relatively stable orbits, to become long period comets when the Sun’s gravity overtakes that of the Milky Way galaxy, which happens at distance of between 100,000, and 200,000 AU from the Sun. In astronomical parlance, this point is known as the “tidal truncation radius” beyond which the Milky Way’s gravity is stronger than that of the Sun.
Stars sometimes pass through the Oort cloud
Apart from the gravitational effects of the galactic tide, another mechanism that perturbs the Oort cloud sufficiently to send comets into the inner solar system is the passing of nearby stars through the cloud. For example, the dim binary star designated WISE J072003.20-084651.2 (Scholz’s star) passed through the outer reaches of the Oort cloud about 70,000 years ago, although its high speed and low mass limited the effects of its passing. One other star, Gliese 710, has the potential to dislodge large numbers of comets from the Oort cloud during the next 10 million years or so.
Oort-cloud comets can disappear
Soon after Jan Oort developed a model that predicted how many long-period comets from the Oort cloud will return to the solar system, he noted that far fewer comets actually do so than his model had predicted. To date, no known purely dynamical process can account for this, and although the number of returning comets that return to the outer solar system far exceeds the number that return to the inner solar system, the issue remains unresolved.
Possible explanations for the observed under count of comets include destruction of comets by collisions with outer planets and other bodies, fragmentation caused by tidal stresses, or the depletion of all volatile material in the cometary nucleus, which would render such comets invisible.
The Oort cloud contains both comets and asteroids
If what is known about the composition of known comets is taken to be representative of all comets, most objects in the Oort cloud will consist of various ices such as frozen water, methane, ethane, carbon monoxide, and hydrogen cyanide. However, the discovery of an object dubbed 1996 PW that has an orbit that is similar to long-period comets, and which has a composition and appearance similar to D-type asteroids, suggests that between 1% and 2% of the Oort cloud population consists of asteroids.
Nobody has seen the Oort cloud-yet
Everything that is known about the Oort cloud is based on inference, deductive reasoning, theoretical computer models, and some intelligent guesses about the origin of long-period comets, since nobody has actually observed the cloud yet. The space probe that is currently closest to the Oort cloud is Voyager 1, and although it is the fastest of all the current space probes, it will only reach the inner reaches of the Oort cloud in another 300 or so years, and will need about 30,000 years to pass through the cloud’s wall.
Sadly though, the nuclear reactors that supply Voyager 1 with power is expected to cease working in about 2025, and none of the other space probes now in service is expected to be functional by the time they reach the Oort cloud.