Space Forum / Space Science / September 2005

But those are the ones that have been _deflected_ into the inner solar
system. There's supposed to be this cloud of a million million objects,
with aphelia of the order of tens of thousands of AU, but they
supposedly _originated_ within the orbit of Neptune.

-> But those are the ones that have been _deflected_ into the inner solar  
-> system. There's supposed to be this cloud of a million million objects,  
-> with aphelia of the order of tens of thousands of AU, but they  
-> supposedly _originated_ within the orbit of Neptune.
 
Nobody has yet seen any object actually in the Oort Cloud. The whole
thing is highly speculative.
 
                           dow

-> I tend to agree.  For this to work, you need another large object (at,
-> say, 100 AU or beyond) to perturb these mini planets' orbits again (that
-> is, after Jupiter/Saturn/Neptune have done their bit).  But, if there
-> is/was such a large object out there, it could have flung mini-planets
-> into inclined and highly elliptical orbits anyway, without them needing
-> to have formed closer to the Sun.

-> What might this object be/have been?  Well, as one possibility, suppose
-> our solar system came originally from a star-forming region which was at
-> least modestly well-populated, then it could have spent its early years
-> in some sort of open star cluster.  At some stage - possibly as part of
-> the process of leaving the cluster - our system could easily have passed
-> within 500 AU of another star.
 
It is generally thought that our solar system formed in a cloud of
material that contained debris from earlier stars that had exploded as
supernovas. It seems plausible to me that at least some of these
earlier stars had objects orbiting around them, and that at least some
of these objects might have survived the explosions and become
components of the cloud. When the solar system formed, the new giant
planets swept up or ejected the previously-existing objects from the
inner solar system, but, further out, some of these old objects became
captured into orbits in what we now call the Kuiper Belt or the Oort
Cloud.
 
If this is true, these objects should be older than the 4.6 billion
year age of the sun and inner planets. They should also show signs of
having been blasted by a supernova - although the traces of this may
have been buried somewhat below their present surfaces. I'd love to see
some sort of a probe that would land on a KBO and search for evidence
about this, but I doubt that I'll live long enough..
 
                              dow

-> I hope you live long enough to see more evidence of artefacts on Mars,  
-> if there is any.
 
And if there isn't?
 
                         dow

-> Are you saying we know (= "it is generally thought") that, at the time
-> of its birth, our solar system had debris from a more recent, more local
-> supernova explosion, in addition to the general background I just
-> described?  I understand this is not an unreasonable scenario, but do we
-> "know" this?
 
Not just one, but probably quite a lot of supernovas are thought to
have contributed material to the nascent solar system. The scanario is
that there was a cluster of stars, gas, etc., which was all
gravitationally bound. When a star went supernova, its material (and
also any objects that were in orbit around it) escaped from the star
itself, but not from the cluster. Shock waves from the explosions
propagated through the gas, initiating further star formation. The sun
is one of these second-generation stars, and the solar system contains
significant amounts of material that was ejected from the supernovas.
This includes all the elements with nuclear masses greater than that of
iron. Originally, it also included a lot of radioactive isotopes which
have since largely decayed. Uranium-235, for example, was much commoner
in the early Earth than it is now - so common, in fact, that there are
examples of uranium deposits that "went critical", undergoing rapid
fission of U-235, with the release of large amounts of energy. This
could never happen nowadays, since almost all of the original U-235 has
decayed away. Some other radioisotopes, including one of aluminum, I
believe, were common back then, and its daughter products can be found
in aluminum ores today.
 
These radioactive materials are evidence that the early Earth contained
stuff that had quite recently been in one or more supernovas.
 
-> I find this an interesting point that you make: the mass loss from a
-> star which goes supernova will mean that some large objects that were
-> orbiting the star will no longer be bound to its remnant.  I can see
-> that objects in substantially elliptical orbits will be easiest to set
-> free, if they are close to their perihelion when the explosion
occurs.
 
Yes. That sounds reasonable. However, if the explosion blows away more
than half the original mass of the star, then even an object that was
in circular orbit around it (at any distance) will find itself
travelling at more than the escape velocity of the diminished star, and
will therefore escape from it.
 
-> I find it difficult to envisage how most of this part of your scenario
-> could have occurred to any great extent.  How do substantial numbers of
-> these objects, scattered widely as they are released in various
-> directions from the gravitational hold of their original star, end up
-> concentrated within 200 AU of our Sun, let alone concentrated within the
-> inner solar system?  Objects in the Oort Cloud, OK, I'll buy that: (1) a
-> few captured by chance after the solar nebula has collapsed - this seems
-> very plausible; (2) large numbers which were ambling around the solar
-> nebula before its collapse (slowly enough to be gravitationally bound,
-> even at vast distances) - I'm sceptical that the pre-conditions for that
-> are likely.
 
Well, how many objects were there? Maybe there really aren't very many
of them. The "KBO's" that have found pretty close to the orbit of
Neptune, including Pluto, may well have been boosted out from the inner
solar system by interactions with the giant planets. Only a handful are
known out at much greater distances. Maybe there aren't many more.
 
We'll see...
 
                         dow