I. The Oort Cloud


- The Oort Cloud is a region of the solar system far beyond the orbit of the dwarf planet Pluto in which up to a few billion to two trillion icy objects made of ammonia, water, and methane move in nearly circular orbits unless one is pulled into a highly eccentric elliptical orbit by a passing star. The "cloud" ranges from around 2,000 AU (Astronomical Units) from the Sun and is around one light-year thick, which supposedly surrounds our whole planetary system. This "cloud" is believed to have been the origin of many of the Solar System's long-period comets, such as Hale-Bopp, as well as many of the "Centaurs" of the Solar System. However, the outer extension of the Oort cloud is said to be in the area of space where the sun's gravitational pull is weaker than the gravitational pull of other stars.


II. Hypothesis

Ernst Öpik

A) Early Life

Ernst Julius Opik was born on October 23, 1893 in Kunda, Estonia. After finishing secondary school, Opik attended the University of Moscow to study Minor Bodies such as comets, asteroids, and meteors. Opik estimated the densities of multiple stars in his article that was published by Astrophysical Journal.

B) Works in Astronomy

  • Opik published a paper in 1922 in which he estimated the distance of the Andromeda Galaxy to be about 450 kpc (kiloparsec ) only using a novel and simple astrophysical methods. His guess was relatively close to the most recent estimates at 778 kpc compared to Hubble's estimate of 275 kpc. Opik's method is still in use today.
  • Also in 1922, Opik correctly predicted the frequency of Mars' craters before being detected by space probes.
  • In 1932, he speculated that the comets in the Solar System came from a cloud beyond the orbit of the then-ninth planet, Pluto.
  • Opik invented a rocking camera to study meteors.

Jan Oort

A) Early Life

Jan Hendrick Oort was born on April 28, 1900 in Franeker, Friesland in the Netherlands. He was a Dutch astronomer who studied in the field of radio astronomy. After many achievements in the world of Astronomy, Oort became the professor at the Observatory of the University of Leiden. Oort was the first to say that comets came from one common origin. This was proven "wrong", but in the right "ballpark". From Oort's idea, Astronomers found that similar types of comets come from the same area of the outer solar system. The "Oort Cloud" of comets was named after him.

B) Works in Astronomy

  • Suggested the idea that comets came from a common place outside our Solar System (now called the Oort Cloud)
  • Oort raised money to fund a radio telescope in Dwingeloo (East Netherlands) in order to discover the middle of the galaxy.
  • First person to discover evidence of Dark Matter
  • Calculated exactly where our Solar System is in the Milky Way Galaxy (19,200 light years from the center)

III. Structure

Because the Oort Cloud is a hypothetical space object, one cannot know for sure the "structure" of the Oort Cloud. However, as part of speculation, astronomer have put together a possible 'sketch' of the Oort Cloud.

  • Immense in Size ( In between 2,000 - 5,000 AU to 50,000 AU )
  • Spherical in Shape ( 20,000 - 50,000 AU )
  • Believed to have a dense 'core' of comets with an outer edge containing a majority of the Oort Cloud's comets.

IV. Origin of Comets

The Oort Cloud is thought to be the 'home', so to speak, of Long-Period Comets. This cloud is said to have been formed from the sun's proto-planetary disk. Comets that come into the Solar System and stay near the Solar System (i.e. Short-Period Comets) are kept here by planetary gravity. The Oort Cloud is so far from the Sun, comets on the outer edge can easily fall out of the Oort Cloud due to other star's gravity. Until the comets start moving towards the Milky Way, they stay put in the Oort Cloud as large ice balls. The prevailing theory on how those ice balls made their way into the Oort Cloud in the first place is that they are what's called "planetesimals" - objects that failed to fully form as planets that condensed in the disk of gas surrounding the newly formed Sun four and a half billion years ago. These planetesimals were then scattered by the gravitational force from the Gas Giants, namely Jupiter.

V. Oort Cloud and Kuiper Belt Relationship

The Kuiper Belt and the Oort Cloud are regions of space. The known icy worlds and comets in both regions are much smaller than Earth's moon. The Kuiper Belt and the Oort Cloud surround our sun, a star. The Kuiper Belt is a doughnut-shaped ring, extending just beyond the orbit of Neptune from about 30 to 55 AU. The Oort Cloud is a spherical shell, occupying space at a distance between five thousand and 100 thousand AU. Long-period comets (which take more than 200 years to orbit the sun) come from the Oort Cloud. Short-period comets (which take less than 200 years to orbit the Sun) originate in the Kuiper Belt. There may be are hundreds of thousands of icy bodies larger than 100 km (62 miles) and an estimated trillion or more comets within the Kuiper Belt. The Oort Cloud may contain more than a trillion icy bodies. Both the Kuiper Belt and the Oort Cloud are named for the astronomers who predicted their existence during the 1950s: Gerard Kuiper and Jan Oort.

VI. Summary

The Oort Cloud, though hypothetical, is believed to be one of the largest objects in our 'near' vacinity. It is basically the largest, and only trail of about 2 trillion comets orbiting the sun in such a long distance of our solar system. If the Oort Cloud exists, it would explain the origins of Comets and give astronomers a better understanding of the workings of the outer realms of our solar system, and, perhaps, our galaxy. Each comet that is included in the Oort cloud is separated from the next comet by over one billion kilometers.