Brown and white dwarf stars are thought to have habitable zones but are very close orbits. Plus these planets will experience strong tidal forces and be tidally locked. But if you have a large Earth, maybe twice as large as Earth, around a brown dwarf but is outside but close to the habitable zone. And the brown dwarf is captured by a white dwarf but also just outside it's habitable zone. Can the combination allow the planet to be habitable? What issues would arise?
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2$\begingroup$ If it's a close binary system, habitable planet might be possible. In other type binary system - probably not. check out this question $\endgroup$– AlexanderJul 21, 2017 at 17:24
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$\begingroup$ Are you hoping to increase the size of the habitable zone, or what? I.e. a planet at a Lagrange point between a prestar and a postar? $\endgroup$– can-ned_foodJul 21, 2017 at 23:27
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$\begingroup$ Pretty much, more like create a unique habitable zone at the cold edges of two stars not commonly thought of for sustaining life. $\endgroup$– Brooks NelsonJul 22, 2017 at 18:47
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There are some minor problems:
1) Habitable zones shifts, and both objects that you use would on a bilion year scale become much, much dimmer. Thus either the planet would freeze, or it would be fried before. (the good thing is that fried from only one side ;) )
2) Habitable zone for billion years for brown dwarf is rather close to Roche limit. Would you mind if your planet would be torn in to pieces by tidal forces?
3) You are putting both stars and the planet really close, roughly counting 0.01 AU from each other. Such system are presumably not much stable and end up with some planets thrown out of system.
4) The only idea I could imagine it would be a trojan planet around 0,01 AU. That was damn lucky, that the preturbation of orbit moved it with time closer and closer to white dwarf as it dims...
EDIT:
"Too short" - less then 3 bilion years. Sources:
https://phys.org/news/2011-03-habitable-planets-white-dwarfs.html https://planetplanet.net/2014/10/09/real-life-sci-fi-world-4-earth-around-a-brown-dwarf/
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$\begingroup$ So even pushing the edge of too cold on both it'd still be very close together and only have a billion year life span. Maybe if the planet/brown system formed independently during the warmer period, then froze out with life living Europa style underneath, then joined the white dwarf later to warm up the planet for a second chance. Or just turn the brown dwarf into a red dwarf and give the orbits more breathing space. Cool charts, I wonder what red dwarves look like and what does too short mean in the white dwarf chart? $\endgroup$ Jul 21, 2017 at 21:05
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$\begingroup$ What does it mean “too short” in the distant–young corner of the white dwarf graph? $\endgroup$ Jul 21, 2017 at 23:10
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$\begingroup$ Could we please have a citation for the source of the diagrams and any papers with supporting analysis? This information would be greatly appreciated. It's too useful not to be shared. $\endgroup$ Jul 22, 2017 at 2:24
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1$\begingroup$ Such lucky catching (without loosing energy by ramming something) technically speaking do not violate laws of physics, but is damn unlikely. With red dwarfs your situation is much easier, as their durability beats the age of universe. $\endgroup$ Jul 22, 2017 at 13:30