[Luis Dias]

Has anyone imagined this in science fiction? Apparently, it's quite possible for a planet in an habitable zone to dwell around a white dwarf, if they orbit at around 0.01 AU from it. At this distance, the angular aspect of the star is similar to our own sol, the temperature is the same (5000 K), and you have 3 billion years to develop life before the sun gets too cold (even then, "gaya hypothesis" type of feedbacks may enlarge the lifespan, I speculate).

http://www.centauri-dreams.org/?p=17182&utm_source=rss&utm_medium=rss&utm_campaign=white-dwarfs-and-habitable-planets

edit: most interesting part, search for these planets will begin with the introduction of the GAIA satellite. It'll be launched in 2012.

[headdie]

That is fascinating and a good spot.

It certainly throws open a new perspective on the search for habitable planets and story options in fiction

[Goober5000]

For a minute there, after reading the thread title, I thought this was going to be a judgefloro post.

[watsisname]

3GY of habitable zone stability at 0.01AU?  I'd be rather worried of such a planet becoming tidally locked during that period.  Granted that doesn't necessarily mean such a planet would be completely uninhabitable, but something to think about.

[Nuke]

wasnt there a made for tv movie about such a tidally locked planet orbiting a white dwarf. i remember watching it, it was bizzare.

[perihelion]

I've always thought tide-locking shouldn't really be a deal breaker.  I remember reading about some modeling that found that, while the dark side of the planet would undoubtedly be very cold, if there was sufficient atmospheric pressure for life to get much of a foothold in the first place, convection effects ought to be sufficient to keep the atmosphere from freezing out completely.  Add a large ocean and convection gets even more pronounced.

I'd be more worried about short-term volatility in solar output.  At a range of 0.01 AU, it wouldn't take much of a solar flare to cause an extra crispy extinction event.  Honestly, I'm not sure if that would be a problem or not.  I remember reading that red dwarfs are notorious for variable output.  Not as sure about white dwarfs.  Fusion has stopped.  Without that internal dynamo, they might be a lot more stable.  Quick googling is not giving me much.  I can see that there are some types called "pulsating white dwarfs" and "cataclysmic variables," but I cannot tell if those are the rule or exceptions.

[watsisname]

From what I can gather on white dwarf variability, this applies mainly to very young white dwarfs (which would be still extremely hot), or white dwarfs that are accreting material. 

The former scenario shouldn't be an issue since we're looking at older white dwarfs -- ones that have had enough time for a planet to form or migrate to the new habitable zone and evolve life on them.

The latter scenario would imply a semi-detached stellar companion, ie one that is orbiting close enough such that its outer atmosphere is being pulled off the star and accreting onto the white dwarf.  But having a stellar companion orbiting this close would pretty much guarantee no habitable zone around the dwarf to begin with, so there's no sense looking for such habitable planets in this type of system.

What we're left to look for are old, dim, solitary white dwarfs whose surfaces have cooled to perhaps <10000 Kelvin, probably more like 5000 or 6000 so that there isn't as much UV radiation.  (White dwarfs emit essentially as blackbodies, so a hotter surface implies more shortwave emission like UV and Xray).  You could also possibly have a white dwarf and a more distant companion, but you'd need to consider how much light is the planet getting from the companion in addition to the dwarf.