"Alberto Carboni" <spacemail_at_tiscalinet.it> wrote:
>
>
>Fabrizio Guzzetta ha scritto nel messaggio ...
>>
>>Cosa ne pensate riguardo il corpo celeste scoperto recentemente nel nostro
>>sistema solare? E' possibile, come dicono, che sia una nana bruna? In tal
>>caso gli effetti gravitazionali non si sarebbero sentiti molto prima e
>molto
>>piu' forti?
>
>
>Ciao io non ne so niente, ma mi incuriosisce, in che punto del sistema
>solare si trova? a meno che non sia moltolontano l'ipotesi della nana nera,
>come dici tu � assurda, per quanto possa essere ridotta sarebbe certo pi�
>grande del nostro buon vecchio Giove che certo con i suoi effetti
>gravitazionali si fa sentire, oltre tutto a meno che non siano stati taciuti
>si sarebbero dovuti riscontrare incredibili problemi nella ricostruzione
>dell'evoluzione del sistema solare, una nana bruna ormai ha perso molta
>massa, ma un tempo doveva esserci una stella, e per fondere l'idrogeno di
>massa ce ne vuole parecchia
>
Qui di seguito riposto l'articolo. Io l'avvo riportato a prova che la
nube cometaria ipotizzata da Oort non solo non era certa come
pontificava un amico su questo NG, ma che era da tempo in discussione.
Ora Oort aveva supposto che le comete di lungo periodo avessero orbite
del tutto casuali e piombassero veso il Sole da ogni punto dello
spazio da qui l'ipotesi di una nube sferica di corpi, dapprima
ipotizzata a 50.000 UA poi portata fino a 100, poi divisa in due nubi,
una pi� vicina al Solo, che partiva appena dopo Nettuno, mentre
u'altra radissima sarebbe arrivata addirittura a 150.000 UA, met�
strada e passa per Proxima.
Studiando invece un maggior numero di comete, viene avanzata l'ipotesi
del pianetone o della brown dwarf a circa 32.000 UA.
Ma leggi l'articolo che spiega bene.
Ciao
Ernesto
------------------
ROYAL ASTRONOMICAL SOCIETY PRESS NOTICE
Date: 7 October 1999
E-mail: jmitton_at_dial.pipex.com
RAS Web:
http://www.ras.org.uk/ras/
* * * * * * * * * * * * * * * * * * * * * * * *
CONTACT FOR THIS RELEASE
Dr John B. Murray (j.b.murray_at_open.ac.uk)
Phone: 01908 652118
Dept. of Earth Sciences, The Open University,
Milton Keynes MK7 6AA
* * * * * * * * * * * * * * * * * * * * * * * * *
PUZZLE OF COMETARY ORBITS HINTS AT LARGE UNDISCOVERED OBJECT
Intrigued by the fact that long-period comets observed from Earth seem
to follow orbits that are *not randomly oriented in space*, a
scientist
at the Open University in the UK is arguing that these comets could be
influenced by the gravity of a large undiscovered object in orbit
around the Sun. Writing in the issue of the Monthly Notices of the
Royal Astronomical Society published on 11th October, Dr John Murray
sets out a case for an object orbiting the Sun 32,000 times farther
away than Earth. It would, however, be extremely faint and slow
moving,
and so would have escaped detection by present and previous searches
for distant planets.
Long-period comets are *believed* to originate in a vast 'reservoir'
of potential comets, known as the Oort cloud, surrounding the solar
system at distances between about 10,000 and 50,000 astronomical units
from the Sun. (One astronomical unit is approximately the average
distance between the Earth and the Sun.)
They reach Earth's vicinity in the
inner solar system when their usual, remote orbits are disturbed. Only
when near to the Sun do these icy objects grow the coma and tails that
give them the familiar form of a comet. Dr Murray notes that the
comets reaching the inner solar system include a group coming from
directions in space that are strung out along an arc across the sky.
He argues that this could mark the wake of some large body moving
through space in the outer part of the Oort cloud, giving
gravitational kicks to comets as it goes.
The object would have to be at least as massive as Jupiter to create a
gravitational disturbance large enough to give rise to the observed
effect, but currently favoured theories of how the solar system formed
cannot easily explain the presence of a large planet so far from the
Sun. If it were ten times more massive than Jupiter, it would be more
akin to a brown dwarf (the coolest kind of stellar object) than a
planet, brighter, and more likely to have been detected already.
So Dr Murray speculates that such an object, if it exists, will be
planetary in nature and will have been captured into its present orbit
since the solar system formed, even though the probability of such an
event seems low on the basis of current knowledge.
Though a large, distant planet is a fascinating possibility and the
evidence is suggestive, Dr Murray nevertheless stresses that he is not
ruling out other possible explanations for the observed clustering of
the comet orbits.
Received on Mon Oct 25 1999 - 00:00:00 CEST