Just how big is the Solar System? Extremely big, if you
count the Oort Cloud as being part of it!
The Oort Cloud takes its name from Dutch astronomer Jan Oort
(1900-92) who in 1932 postulated the idea that a vast array of small objects
must lie right on the edge of the Solar System, and that this was where the
long-period comets originated that visited the inner Solar System so
infrequently that they were recorded once in human history and never again. (Many
short-period comets may also have started in the Oort Cloud but been captured
by the gravitational influence of the outer giant planets and now have smaller
orbits).
Although it is impossible to view the cloud directly from
Earth, and no space probes could possibly reach it and transmit data about it
back to Earth, most astronomers now accept that it exists and are in reasonable
agreement about its composition.
It is believed that the Oort Cloud contains trillions of
objects, mostly lumps of rock and ice no more than a kilometer wide, that were
part of the Sun’s original disc of planetary material that never coalesced to
form planets but were scattered to the far reaches of the Solar System under
the gravitational influence of the giant planets (e.g. Saturn and Jupiter) when
they formed.
The Oort Cloud is aptly termed a cloud because it does not
form a disc of objects that orbit the Sun in roughly the same plane as the
planets but is a spherical halo that extends in all directions. However, there
is also thought to be an “inner Oort Cloud” that is in the form of a disc.
There is plenty of debate over the extent of the Oort Cloud.
A convenient shorthand for explaining distances within the Solar System is the
Astronomical Unit (AU), which is the average distance from Earth to the Sun.
One estimate of the distance of the Oort Cloud has it beginning at 2,000 AUs
and extending to 50,000 AUs. However, some astronomers reckon that the outer
edge could be anything from 100,000 AUs to 200,000 AUs.
To put these distances into some sort of context, the
outermost planet, Neptune, is 37 AUs from the Sun. The Kuiper Belt, that
contains a mixture of lumps of rock and dwarf planets including Pluto, extends
to 50 AUs.
If the furthest estimate for the extent of the Oort Cloud is
accepted, and expressed as light years rather than AUs, we are talking about
objects, held within the Sun’s gravitational field, that are more than three
light years away. The closest star to the Sun is Proxima Centauri, which is
4.25 light years away. However, the mass of Proxima Centauri is only about one
tenth that of the Sun, which is why the outermost Oort Cloud objects have not
yet moved house!
Even so, the gravitational influence of the Sun is extremely
weak at these vast distances, and the rotation of the Milky Way has its own
gravitational influence as well as bringing other stars closer at times to the
Sun – although such relative movements take place over thousands and millions
of years. It is these gravitational perturbations that cause smaller objects to
leave their million-year solar orbits and take a different course towards the
inner Solar System as comets.
Considering that we are still talking about the Solar
System, with just a passing reference to our next-door neighbor, the wise words
of Douglas Adams in “The Hitchhiker’s Guide to the Galaxy” seem more pertinent
than ever:
“Space is big. Really big. You just won't believe how
vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long
way down the road to the chemist, but that's just peanuts to space.”
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