Doctor Who "The Impossible Planet" / "The Satan Pit" ***Spoilers!

Sylvia Else wrote:
David Bromage wrote:

Sylvia Else wrote:

BTW, right at the beginning I was annoyed at the apparent assertion
that in the normal way of things, a planet could not be in orbit
about a black hole. There's no reason a planet cannot be.



Provided the planet is outside the event horizon and was captured
after the black hole was formed. The star's original planets would
have been destroyed when it went supernova. Even if a planet was
captured its orbit would certainly not be stable.


I take your point about surviving the supernova event (or perhaps
neutron star collision) that created tbe black hole. Given the absence
of existing planets, capture of one is problematic. I suppose it would
require a rather coincidental meeting of two objects in the vicinity of
the black hole. One is ejected or falls into the hole, and the other
ends up in orbit.

However, provided the object's orbit (which is hardly likely to be
circular) doesn't take it close to the black hole, I cannot see why its
orbit would be any less stable than that of an object in orbit around an
ordinary star of similar mass to the black hole.



And if it was far enough away not to be sucked it, it would be far
enough away to retain an atmosphere.


That's it if retained one whereever it was orginally created. If Mercury
were ejected from the solar system (by a freak event involving a large
mass) and then captured in orbit about by a black hole (by another freak
event involving a large mass) then it wouldn't have an atmosphere.

However this does rather suggest that the chances of finding a planet in
orbit about a black hole are pretty slim.

Sylvia.


It's certainly very unusual, but I like the reasoning! The problem isn't
just one of origin, although that makes it really unlikely. It's tidal
force, bit of a misnomer as there's no such force, just the observation
off the back of this planet's oceans that gravity pulls slightly more on
the forward side of a facing object. When dealing with gravity of this
order this translates into a lot of disproportionate pull. Astronomers
were amazed to find a planet in orbit around a neutron star, so the
Doctor's reaction that this system is "impossible" is justifiable hyperbole.

Martin...
--
http://sfsa.org.au/, the South Australian Doctor Who Fan Club, Inc.

Martin Dunne wrote:
It's certainly very unusual, but I like the reasoning! The problem isn't
just one of origin, although that makes it really unlikely. It's tidal
force, bit of a misnomer as there's no such force, just the observation
off the back of this planet's oceans that gravity pulls slightly more on
the forward side of a facing object. When dealing with gravity of this
order this translates into a lot of disproportionate pull.

And not on the ocean (there wasn't any) but on the rock, such that the
planet would be distorted to the extent that a Mt Everest could be
pulled up on the side closest to the black hole every rotation. The
friction alone would guarantee that there would be little if any solid
crust and the surface of the planet would be akin to our mantle.

Also the orbital period of a planet that close would be measured in
hours rather than days, weeks or months.

Cheers
David

David Bromage wrote:

Martin Dunne wrote:

It's certainly very unusual, but I like the reasoning! The problem
isn't just one of origin, although that makes it really unlikely. It's
tidal force, bit of a misnomer as there's no such force, just the
observation off the back of this planet's oceans that gravity pulls
slightly more on the forward side of a facing object. When dealing
with gravity of this order this translates into a lot of
disproportionate pull.


And not on the ocean (there wasn't any) but on the rock, such that the
planet would be distorted to the extent that a Mt Everest could be
pulled up on the side closest to the black hole every rotation. The
friction alone would guarantee that there would be little if any solid
crust and the surface of the planet would be akin to our mantle.

Also the orbital period of a planet that close would be measured in
hours rather than days, weeks or months.

Did the program ever say how close the planet was.

Also, it seems to me that if the tidal effects were enough to melt the
surface of the planet, then by the same token the planet's rotation
would be reducing rapidly. It would end up in tidal lock and cool down.

Sylvia.

Cheers
David

On Wed, 13 Sep 2006 14:30:18 +1000, Sylvia Else
wrote:

Also, it seems to me that if the tidal effects were enough to melt the
surface of the planet, then by the same token the planet's rotation
would be reducing rapidly. It would end up in tidal lock and cool down.

Jesus. Fracking, H. Christ.

....Kids, this is Doctor Who. This isn't Star Trek, or even Babylon 5.
Hell, it ain't even Lost in Space. It's *supposed* to have all
disbelieve put not only on suspension, but in total escrow awaiting
Supreme Court judiciary disposition. Just sit back, enjoy the ride,
and quit being anal-retentive pedantics. This is the *one* show you're
supposed to just let go and don't worry about whether or not it could
really happen.

[shakes head at those obviously still living in their parents'
basement]

OM
--
]=====================================[
] OMBlog - http://www.io.com/~o_m/omworld [
] Let's face it: Sometimes you *need* [
] an obnoxious opinion in your day! [
]=====================================[

"OM" wrote in message
...
...Kids, this is Doctor Who.

OM- try decafe.

David Bromage wrote in
:

Martin Dunne wrote:
It's certainly very unusual, but I like the reasoning! The problem
isn't just one of origin, although that makes it really unlikely.
It's tidal force, bit of a misnomer as there's no such force, just
the observation off the back of this planet's oceans that gravity
pulls slightly more on the forward side of a facing object. When
dealing with gravity of this order this translates into a lot of
disproportionate pull.

And not on the ocean (there wasn't any) but on the rock, such that the
planet would be distorted to the extent that a Mt Everest could be
pulled up on the side closest to the black hole every rotation. The
friction alone would guarantee that there would be little if any solid
crust and the surface of the planet would be akin to our mantle.

Also the orbital period of a planet that close would be measured in
hours rather than days, weeks or months.


Doesn't Jupiter's moon Io has active volcanoes because of tidal forces
from Jupiter's gravity? If that's case, how would a planet around a
black hole survive for long? Or at least, it may be able to have a solid
surface.

The Doctor wrote:

David Bromage wrote in
:


Martin Dunne wrote:

It's certainly very unusual, but I like the reasoning! The problem
isn't just one of origin, although that makes it really unlikely.
It's tidal force, bit of a misnomer as there's no such force, just
the observation off the back of this planet's oceans that gravity
pulls slightly more on the forward side of a facing object. When
dealing with gravity of this order this translates into a lot of
disproportionate pull.

And not on the ocean (there wasn't any) but on the rock, such that the
planet would be distorted to the extent that a Mt Everest could be
pulled up on the side closest to the black hole every rotation. The
friction alone would guarantee that there would be little if any solid
crust and the surface of the planet would be akin to our mantle.

Also the orbital period of a planet that close would be measured in
hours rather than days, weeks or months.



Doesn't Jupiter's moon Io has active volcanoes because of tidal forces
from Jupiter's gravity? If that's case, how would a planet around a
black hole survive for long? Or at least, it may be able to have a solid
surface.

It's all a question of distance. If the planet is suitably far from the
black hole, then it will not suffer such disruptive tidal effects.

People often seem under the impression that a black hole has a specially
high gravitational field. It doesn't. It has exactly the same
gravitational field as any other object of the same mass. The mass of
the black hole does not have to be particularly high by stellar standards.

It's a bad idea to get close to a black hole, but then it's a bad idea
to get close to any massive object.

Sylvia.

Sylvia Else wrote:


It's all a question of distance. If the planet is suitably far from
the black hole, then it will not suffer such disruptive tidal effects.


Also, the mass of the black hole- the larger the mass, the less intense
the tidal gravity gradient will be for a planet orbiting it at a
distance to have the same orbital period, assuming the planet's diameter
and mass are kept the same.

Pat