The Kuiper Belt and The Oort Cloud
Facts
- Careful orbital calculations done in 1950 by
Jan Oort indicate that a huge spherical "cloud"
(now called the Oort Cloud)
of perhaps a trillion (1e12) or more comets orbit the Sun far beyond the orbit of
Pluto from about 30,000 AU
to a light-year or more. This is the source of the
long-period comets.
- The Oort Cloud may account for a significant fraction of the mass of the solar
system, perhaps as much or even more than Jupiter.
(This is highly speculative, however; we don't know how many comets there are
out there nor how big they are.)
- The Kuiper Belt
is a disk-shaped region past the orbit of
Neptune roughly 30 to 100
AU from the Sun containing many small icy bodies.
It is now considered to be the source of the short-period comets.
- Occasionally the orbit of a Kuiper Belt object will be disturbed by the
interactions of the giant planets in such a way as to cause to cross the orbit
of Neptune. It will then very likely have a close encounter with Neptune
sending it out of the solar
system or into an orbit crossing those of the other giant planets or even
into the inner solar system.
- There are presently six known objects orbiting between Jupiter and Neptune
(including 2060 Chiron (aka 95 P/Chiron) and 5145 Pholus; see the MPC's
list).
The IAU has designated
this class of objects as Centaurs.
These orbits are not stable. These objects are almost certainly "refugees" from
the Kuiper Belt. Their future fate is not known.
- Curiously, it seems that the Oort Cloud objects were formed closer to the
Sun than than the Kuiper Belt objects. Small objects formed near the giant
planets would have been ejected from the solar system by gravitational
encounters. Those that didn't escape entirely formed the distant Oort Cloud.
Small objects formed
farther out had no such interactions and remained as Kuiper Belt.
- Several Kuiper Belt objects have been discovered recently including 1992 QB1
and 1993 SC (picture 1).
They appear to be small icy bodies similar to Pluto and
Triton (but smaller).
As of early 1996 there are 32 known trans-Neptunian objects
(not counting Pluto and
Charon); see the MPC's
list.
Nine of these have distances between 31 and 36 AU, the other eight between
40 and 45 AU. None have so far been found in the gap in between; this may be
an effect of Neptune's gravitational attraction.
Many orbit in 3:2 resonance with Neptune (as does Pluto).
Color measurements of some
of the brightest have shown that they are unusually red.
- It is estimated that there are at least 35,000 Kuiper Belt objects greater
than 100 km in diameter, which is several hundred times the number (and mass)
of similar sized objects in the main asteroid
belt.
- A team of astronomers led by Anita Cochran report that
the Hubble Space Telescope
has detected extremely faint Kuiper Belt objects
(picture 2).
The objects are very small and faint perhaps only 20 km or so across.
There may be as many as 100
million such comets in low-inclination orbits and shining brighter
than the HST's magnitude-28 limit.
(A follow-up HST observation failed to confirm this observation, however.)
- Spectra and photometric data have been obtained for 5145 Pholus. Its albedo is
very low (less than 0.1). Its spectra indicates the presence of organic
compounds, which are often very dark (e.g. the nucleus of
Comet Halley).
- Chiron is by far the largest known object of this type.
It is about 170 km in
diameter, 20 times larger than Halley. If it ever is
perturbed into an orbit that approaches the Sun it will be a truly spectacular
comet.
- Some believe that Triton, Pluto and its moon
Charon are merely the largest examples of Kuiper
Belt objects.
- But these are more than distant curiosities. They are almost certainly pristine
remnants of the
nebula
from with the entire solar system was formed.
Their composition and distribution places important contraints of models of the
early evolution of the solar system.
Pictures
- (above) Object 1993 SC
155k gif
small Kuiper Belt object (HST)
109k jpg;
225k gif
(caption)
- Object 1995-QY9
72k jpg
(caption)
Object 1995 WY2
(caption and animation)
More about Kuiper Belt objects
Open Issues
- The very existence of the Oort Cloud is
somewhat questionable. Our only evidence is very indirect.
- HST's recent images seem to confirm the existence of the Kuiper Belt.
But how many of them are there? What are they made of?
- The proposed Pluto Express mission may be
also be extended
to include a fly-by of a Kuiper Belt object.
... Sun
... Small Bodies
... SL-9
... Kuiper/Oort
... Asteroids
... 
Bill Arnett; last updated:
1996 June 26
