Beyond the orbit of Neptune and the Keiper belt, lies a vast region of our solar system known as the Oort Cloud filled with icy bodies weakly bound to the sun in very long orbits. The Oort cloud is defined as a region of space that is spherical shell around the sun starting at 2,000 AU away from the sun, the beginning of the inner Oort cloud, and ending at about 50,000 AU away, the outer Oort cloud. The absolute maximum distance the Oort cloud extends to is usually defined by the Sun’s tidal truncation radius, which is 100,000 to 200,000 AU. This is the radius away from the sun at which the gravitational pull of the sun is about equal to the galactic ‘tide,’ which is essentially the gravitational pull of other stars and the galaxy as a whole.
image from wikipedia
The farther out in the Oort cloud an object is, the weaker the pull of sun is, and thus the pull of gravity by the galactic tide plays a much larger role in changing the motion of the object. Galactic tide is similar to ocean tides in that as we orbit the galaxy, gravity pulls more strongly on one side of the solar system than the other, causing the sun’s field of gravity to warp from a spherical shape to more of an ellipsoid periodically. The inner planets like Earth never notice this effect because we are so close to the sun, the galactic tide would never shake us from the strong pull of the sun, but objects very far out in the Oort cloud are.
The cloud was named after Jan Oort, who proposed that comets originated from this cloud of icy bodies due to a slight displacement from its orbit that pulled it into a long elliptical orbit close to the sun. At the time, it was speculative to what would cause the displacement creating the comet’s orbit, but with computer simulations, many think that as much as 90% of comet’s orbits are made from changes in the periodic galactic tide versus random events such as stars or giant molecular clouds just happening to get closer to the solar system.
It was first proposed by Newton and showed by Edmond Halley (of which the famous comet is named after) that comets must orbit the sun in long elliptical orbits lasting hundreds to thousands of years. The only reason we can see comets is because as they approach the perihelion or their orbit, at 3-4 AU, the flux (energy/area) of the sun’s energy hitting the comet increases enough to melt the ice on the comet, creating a gas cloud on the surface called a coma. A dust tail is then formed due to radiation pressure from the sun and solar winds pushing dust off the cloud. This tail of matter coming off the comet is then illuminated by the sun’s UV light exciting the gas causing emission of visible wavelengths. Thus, making comet’s very noticable in the night sky for this time. Once the comet passes the perihelion, it then begins to freeze again, going dark in the night sky. This effect is what enabled Halley and others to learn about these far objects without large, advanced telescopes.
image from wikipedia
As scientists improve telescopes, especially in the infrared range, we are finding more and more Oort objects. Since all these objects are invisible to us the visible range, infrared telescopes are useful for detecting the larger objects that could have core heat signature in the infared range. Sedna and VP 2113 are two inner Oort cloud objects that have been discovered in the past 10 years. We also now believe that much of the outer Oort cloud formed with the rest of the solar system, while the very outer edges may act like an ‘eddy’ in the tides of galaxy collecting objects that exist in the interstellar medium between stars. What remains a challenge however is detecting the objects in the outer Oort cloud, as it takes precision equipment as well as luck. There remain many questions on the distribution and size of these objects, as well as how they formed, and their evolution over time.
Sources used:
http://solarsystem.nasa.gov/planets/profile.cfm?Object=KBOs&Display=OverviewLong
http://www.nasa.gov/content/nasa-supported-research-helps-redefine-solar-systems-edge/#.Uzwnlq1dWr8
http://en.wikipedia.org/wiki/Oort_cloud
http://en.wikipedia.org/wiki/Comet
http://www.space.com/53-comets-formation-discovery-and-exploration.html
