The Oort cloud, alternatively termed the Öpik-Oort Cloud, is a
postulated
spherical cloud of
comets situated
about 50,000
to 100,000
AU from the Sun.
This is approximately 2000 times the distance from the Sun to
Pluto or roughly one
light year,
almost a quarter of the distance from the Sun to
Proxima Centauri, the star nearest the Sun.
The Oort cloud would have its inner disk at the
ecliptic
from the
Kuiper belt. Although no confirmed direct observations have been made of
such a cloud, astronomers believe it to be the source of most or all comets
entering the inner
solar
system (some short-period comets may come from the
Kuiper
belt), based on direct observations of the
orbits of comets.
There is also a theory of a denser, inner part of the Oort cloud coined the
Hills
cloud;[1]
it would have a well-defined outer boundary at 20-30 000 AU, a less well defined
inner boundary at 50 to 3000 AU, and would be about 10 to 100 times denser than
the remainder.[2]
In 1932 Ernst Öpik
, an Estonian
astronomer, proposed[3]
that comets originate in an orbiting cloud situated at the outermost edge of the
solar system. In 1950
the idea was revived and proposed[4]
by
Dutch astronomer
Jan Hendrick
Oort to explain an apparent contradiction: comets are destroyed by several
passes through the inner solar system, yet if the comets we observe had really
existed for billions of years (since the generally accepted origin of the solar
system), all would have been destroyed by now. According to the hypothesis, the
Oort cloud contains millions of comet nuclei, which are stable because the sun's
radiation is very weak at their distance. The cloud provides a continual supply
of new comets, replacing those that are destroyed. It is believed that if the
Oort cloud exists and supplies comets, in order for it to supply the necessary
volume of comets, the total mass of comets in the Oort cloud must be many times
that of Earth. Estimates range between 5 and 100 Earth masses.
Origin
The Oort cloud is thought to be a remnant of the original solar nebula that
collapsed to
form the Sun and planets approximately 4.6 billion years ago, and is loosely
bound to the solar system.
The most widely-accepted hypothesis of its formation is that the Oort cloud's
objects initially formed much closer to the
Sun as part of the
same process that formed the
planets and
asteroids,
but that gravitational interaction with young gas giants such as Jupiter ejected
them into extremely long
elliptical or
parabolic orbits. This process also served to scatter the objects out of the
ecliptic plane, explaining the cloud's spherical distribution. While on the
distant outer regions of these orbits, gravitational interaction with nearby
stars further modified their orbits to make them more circular.
A recent alternative hypothesis for the origin of the Oort cloud is that the
comets were already present in the original solar nebula, even before the
protosun and the
protoplanetary disk was formed. Regarding the current distribution of the
comets, this alternative hypothesis makes the same predictions as does the
protoplanetary disk formation of comets.
It is thought that other
stars are likely to
possess Oort clouds of their own, and that the outer edges of two nearby stars'
Oort clouds may sometimes overlap, causing perturbations in the comets' orbits
and thereby increasing the number of comets that enter the inner solar system.
Star Perturbations and Nemesis Theory
The known star with the greatest possibility of perturbing the Oort cloud in
the next 10 million years is
Gliese 710.
However, physicist
Richard A. Muller and others have postulated that the
Sun has a heretofore
undetected companion star in an elliptical orbit beyond the Oort cloud. This
star, known as
Nemesis, is theorized to pass through a portion of the Oort cloud
approximately every 26 million years, bombarding the inner
solar
system with comets. Although the theory has many proponents, no direct proof
of the existence of Nemesis has been found.
Oort Cloud Objects (OCO)
So far, only three potential Oort cloud objects have been discovered,
90377
Sedna,
2000 OO67 and
2000 CR105.
90377
Sedna, with an orbit that ranges from roughly 76 to 928 AU, is much closer
than originally expected and may belong to an "inner" Oort cloud. If Sedna
indeed belongs to the Oort cloud, this may mean that the Oort cloud is both
denser and closer to the Sun than previously thought. This has been proposed as
possible evidence that the Sun initially formed as part of a dense cluster of
stars; with closer neighbors during Oort cloud formation,
objects ejected by gas giants would have their orbits circularized closer to the
Sun than was predicted for situations with more distant neighbors.
90377
Sedna is considered to be simply a
Trans-Neptunian object; its orbit does not carry it completely out to the
assumed position of the Oort Cloud and is too far out for it to be truly
considered as a
Kuiper
Belt object.
Some astronomers include the objects
2000 CR105
and
2000 OO67 as part of the Oort cloud. The object
2000 CR105
has a perihelion of 45 AU, an aphelion of 415 AU and an orbital period of 3,241
years while the object
2000 OO67 has a perihelion of 21 AU, an aphelion of 1,000 AU and
an orbital period of 12,705 years.
