Planets around a white dwarf

I'm told here's some reason to think a white dwarf might have planets
-- either survivors from its main sequence days (possibly somewhat,
er, ablated), or new planets that might condense after the transition
to a white dwarf.

WDs start out hot and bright, then gradually grow redder and dimmer.
The cooler they get, the slower they cool, so there are more
(relatively) cool ones around than hot bright ones.  The brighter ones
are about as bright as the bigger red dwarves (though the radiation
curve is very different, of course).

A white dwarf with a temperature similar to the Sun would be about
1/10,000 as bright, so a planet would have to be 100 times closer than
Earth to be comfortable.  Presumably a planet in the habitable zone
would be tidally locked.  (Or would it?  IMS tidal forces are affected
by the radius of the primary; does anyone know the exact
relationship?)  We can make the dwarf quite a lot brighter, but if
it's more than 1/500 as bright as the Sun it's probably pretty young
and very blue -- lots of hard UV and X-rays.

Still, if we can imagine a habitable tidally locked "ribbon world" in
a close orbit around a red dwarf, why not around a white dwarf?  Is
there anything that makes the idea ridiculous?


Doug M.