Discovery of the Oort Cloud

Saying discovery is premature.  No one has seen an object within the Oort cloud yet so we cannot be sure it exists.  The beauty of science…prediction, calculation, observation, experimentation, and continue until evidence supports or refutes the prediction, the hypothesis. The asteroid and Kuiper belts have been confirmed – numerous objects have been observed in both.

Why is it called a cloud? Maybe it’s a misnomer.  The Oort cloud’s hypothetical shape is a bubble, a bubble shaped collection of objects (comets) surrounding the vast outer regions of our solar system.

Astronomers in the early 20th century were unsure about the nature of what they called long-period comets.  Some thought they were one-time visitors with parabolic trajectories.  Others, notably A.O. Leuschner, postulated they were members of the solar system with extremely long elliptical orbits.  In 1932 Estonian astronomer Ernst Öpik suggested the existence of a “cloud” of comets at the outer reaches of our solar system.  Using this model of long-period comet origins, Jan Oort in 1950 was able to resolve an apparent cometary dynamics paradox.  That is, due to their inherently unstable orbits comets either hit the Sun, or a planet, get ejected, or eventually boil away. Oort was convinced that there must be a reservoir of comets at the extreme edges of the solar system to explain pristine long-period comets.

Shouldn’t it be called the Öpik-Oort cloud?  It should, and sometimes is.

The challenge then was to gather evidence in support of this hypothetical cloud.  The challenge continues today. The Oort cloud is so far out our current technology cannot spot individual resident objects.  Using the astronomical unit (au = the distance from the Sun to the Earth, 93 million miles) we can appreciate its distance.  Pluto, the nearest Kuiper belt object is about 30au from the Sun.  The Kuiper belt extends outward to 50au.  The Oort cloud’s inner edge could be farther than 2000au extending to over 100,000au at its outer reaches.  Voyager 1 has passed through the Kuiper belt but will not reach the Oort cloud for another 300 years.  It will be a cold, dead spacecraft by then.

As part of the original disk of protoplanetary material it is thought the Oort cloud formed because of gravitational perturbations, and collisions.  Another hypothesis is the Sun captured the comets from its birth cluster siblings.  In other words, we don’t know.

To complicate things, there might be an inner cloud.   In 1981 Jack G. Hills proposed an inner, torus (doughnut) shaped ring possibly more densely populated with comets.  Most of what is “known” about the Oort cloud comes from comet study and computer modeling. While the cloud is stable a comet occasionally gets kicked out and into orbit around the Sun, becoming a long-period comet.  An Oort cloud comet.

What’s in the Sky?

Halloween; after sunset; east: October’s second full Moon – a Blue Moon, Hunter’s Moon, and minimoon!

November 1; 2am:  Daylight Saving Time ends