WILLIAM & DEBORAH HILLYARD

Solar System -

Trans-Neptunian Objects

The relationship between the Kuiper Belt and the Oort Cloud with respect to Pluto's orbit. 
                    Credit: NASA/JPL

Oort Cloud Objects (OCOs)

Well beyond the scattered disk objects, we find the Oort Cloud.  While there is strong evidence for the Oort Cloud, it remains hypothetical as it is too far away to observe.  It is believed to comprise two distinct regions; an inner cloud, in the form of a torus or doughnut more or less in the plane of the ecliptic, and a spherical outer cloud.  The inner boundary is probably around half a trillion km (c. 310 billion miles) from the Sun, with the outer edge, representing the gravitational boundary of the Solar System, at anything up to two or three light-years, and possibly even more. 

Objects in the cloud comprise water, ammonia, methane and, possibly, nitrogen ices.  While short-period comets are believed, predominantly, to originate in the scattered disc and possibly in the Kuiper Belt, long-period comets are thought to originate in the Oort cloud, some of which take hundreds of thousands or even millions of years to orbit the Sun.  However, the Halley-family comets, named for Halley's Comet, perhaps the most famous comet, are short-period comets that almost certainly originated in the Oort cloud.  They started as long-period comets, but the gravitational affect of the gas giant planets sent them into closer orbits. 

The outer Oort cloud is very  loosely gravitationally bound to the Sun, so objects are easily affected by the gravitational pull of nearby stars and even the Milky Way galaxy as a whole, usually destabilizing them so that they fall towards the inner Solar System.  The inner regions of the Oort Cloud are much more heavily populated, and objects migrate from the inner to the outer regions maintaining the outer cloud population as objects are lost to inter-stellar space. 

Planetary astronomers suggest that the material that formed the Oort Cloud was part of the original disc that formed around the Sun from which the planets formed.  The objects originated closer in, around the gas and ice giant planets, and were ejected outwards into very long elliptical orbits by their gravitational effect early on in the formation of the Solar System.   Over very long time scales, gravitational affects from nearby stars and the galaxy as a whole caused objects further out to assume more circular orbits, which is why the outer cloud tends to be spherical.  Objects further in, being gravitationally bound to the Sun, are less affected.  The total mass of the Oort Cloud is estimated at about 30 to 40 times the mass of the Earth, with about 85% of the mass in the inner cloud. 

2006 SQ372 is a small object; perhaps 50 to 100 km (30 to 60 miles) across.  At its closest approach to the Sun it is about 24 AU away, while at its furthest it is over 2,000 AU away; that is around 300 billion km (186 billion miles) away.  It may be an inner Oort Cloud object. 

2008 KV42, also known as Dracula or Drac, has a very odd orbit around the sun being, at 104 degrees, almost perpendicular to the ecliptic.  It also orbits in retrograde. This orbit suggests that it may have been pulled into the Solar System from the Oort cloud.  As stated above, long period comets are thought to originate from the Oort cloud and this discovery may finally show how they transition from the Oort cloud to become objects like Halley's comet. 
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In the image to the right the scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance.  One AU is the distance from the Sun to the Earth, which is about 150 million kilometers (93 million miles).  Neptune, the most distant planet from the sun, is about 30 AU out.  The a Centauri system, comprising three stars, is the closest star system to the Earth at about 4.37 light-years. 
In this image, the scale bar is in astronomical units, with each set distance beyond 1 AU representing 10 times the previous distance.  One AU is the distance from the Sun to the Earth, which is about 150 million kilometers (93 million miles).  Neptune, the most distant planet from the sun, is about 30 AU out.  The a Centauri system, comprising three stars, is the closest star system to the Earth at about 4.37 light-years.