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