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ASTRO BOY
2013-Jan-17, 09:24 PM
Our star the Sun is destined to become a WD.
Earth will almost positively be scorched of all life forms by this time I would imagine.

But could life [1] re-establish itself on Earth in the course of time.

[2] What is the "Goldilocks zone" for a WD?

[3] How would Mars change when the WD stage is reached?

The above questions naturally apply to any WD in the galaxy/Universe.

transreality
2013-Jan-17, 09:42 PM
Our star the Sun is destined to become a WD.
Earth will almost positively be scorched of all life forms by this time I would imagine.

But could life [1] re-establish itself on Earth in the course of time.

[2] What is the "Goldilocks zone" for a WD?

[3] How would Mars change when the WD stage is reached?

The above questions naturally apply to any WD in the galaxy/Universe.

I guess that the zone where water would remain as liquid on the surface of the planet would move inwards, possibly to where planets have been stripped of surface water during the hot phase of the stars existance. So life would have difficulty attempting to reestablish, since there is no local water and no supply of incoming water from comets, and if the planet doesn't have tectonic activity water can't cycle from the interior. Mercury will never be conducive to abiogenesis. Then there may be problems with UV light as well if the planet doesn't already have an atmosphere or it was stripped away.

neilzero
2013-Jan-17, 10:14 PM
Not life as we know it. The radiation will be mostly X rays and ultra violet for the first 100 million years (assuming white dwarfs cool at the same rate as neutron stars = a recent thread/others have said the white dwarfs cool over many billions of years) In any case Earth will cool to about Pluto temperature in a million years, perhaps sooner. Even Mercury will possibly be too cold to evolve life.
I'm guessing one million to two million kilometers above the surface of the white dwarf, will have liquid water until the ionizing radiation decomposes most of the water (cause loss of the Hydrogen) = goldilocks zone.
Mars will also be badly scortched, then cool to about present Pluto temperatures in about one million years.
Mars, main belt asteroids and moons of Jupiter will have pleasent temperatures, during part of the earlier red giant stage of our sun, but it is unlikely any of these periods are long enough to evolve life, unless it already exists a kilometer or more below the surface of these bodies.
After the white dwarf has cooled enough to produce mostly infrared and visable light, there is a very slight chance a planet will be captured in a near circular orbit, where life could evolve. Likely true for about 1/2 of our galaxy and other galaxies. Neil

IsaacKuo
2013-Jan-17, 10:26 PM
A white dwarf starts off around 100,000K, according to this page: http://www.astronomynotes.com/evolutn/s11.htm

In comparison, the Sun's temperature is only 5800K.

Using the Stefan–Boltzmann formula, we see that luminosity is proportional to area and temperature to the fourth power. The white dwarf's radius is only about the radius of Earth, or about 1/100th of the Sun's radius. This reduces luminosity by a factor of around 10,000...but temperature is an incredible 17 times higher--increasing luminosity by a factor of around 90,000!

So, overall, the young white dwarf is 9 times brighter...keep in mind that we're talking really high end radiation, so it's not a pleasant thing to be around. But it does push the liquid water zone out by a factor of three. Mars would actually be quite a bit too hot. If Ceres manages to hold onto any of its volatiles, it would be right in the zone. Jupiter's moons might be the best bets, given a sufficiently deep atmosphere for a strong greenhouse effect.

Van Rijn
2013-Jan-18, 04:13 AM
There have been some ideas about the possibility of habitable planets around white dwarfs, in the case where the temperature wasn't too high (not too much UV) and the world was very close to the white dwarf. Earth wouldn't work (it's thought likely it will be destroyed during the red giant phase, but if not, it will be pretty nasty place anyway). As for other solar systems where there might be other options, here's a recent paper on some of the issues:

http://arxiv.org/pdf/1211.6467v1.pdf

They argue that the two big issues would be photodissociation causing most of the hydrogen to be lost before the star cools down sufficiently, and that very small variations in the distance to the white dwarf during orbit would cause massive tidal heating, both making habitability very unlikely. They think brown dwarfs (very different subject, of course) might be a little better.

eburacum45
2013-Jan-18, 09:11 AM
There are plenty of cool white dwarfs that would have about the same temperature as the Sun. They are very dim, so thy don't show up on many starlists, but they are there.The habitable zone is about a million km from the star or less, and curiously enough from a planet orbiting in the habitable zone the white dwarf would look the same size as the sun looks in our sky.

But I wouldn't anticipate finding any life-bearing planets orbiting white dwarfs; for a start, any planets at that distance wil have been destroyed during the red giant phase. Secondly white dwarfs start off incredibly hot, then get progressively cooler; so the planet starts off sterilised, then gradually gets frostier and frostier.

They might make good locations for artificial habitats, for a period, anyway.

ASTRO BOY
2013-Jan-18, 11:35 AM
There are plenty of cool white dwarfs that would have about the same temperature as the Sun. They are very dim, so thy don't show up on many starlists, but they are there.The habitable zone is about a million km from the star or less, and curiously enough from a planet orbiting in the habitable zone the white dwarf would look the same size as the sun looks in our sky.

But I wouldn't anticipate finding any life-bearing planets orbiting white dwarfs; for a start, any planets at that distance wil have been destroyed during the red giant phase. Secondly white dwarfs start off incredibly hot, then get progressively cooler; so the planet starts off sterilised, then gradually gets frostier and frostier.

They might make good locations for artificial habitats, for a period, anyway.

Exactly...and WD'S have a huge time period in which they radiate their tremendous heat.
Part of my question concerns the earth which in all probability would be just on the outer edge of the limit of the Sun in its giant red phase....
All Life would almost certainly be killed off, but could it re-establish itself.

eburacum45
2013-Jan-18, 12:02 PM
If the Earth survives the red giant phase of the Sun, it will be far outside the habitable zone of the white dwarf phase. Since the wd would have lost a lot of mass, the Earth will probably orbit way out at about 2 AU somewhere- but the wd will be only a thousandth as bright as the Sun, so the Earth will be bitterly cold. It would be a challenging place to live.

IsaacKuo
2013-Jan-18, 03:41 PM
I forgot about mass loss causing stuff to migrate outward. If Mars migrates out to around 3AU, then it would be in a good position to have liquid water conditions, for millions of years at the start. But as already noted, the spectrum is harsh.

Longer term, the best prospects for life would be outer system moons. If there isn't already life on them, the Sun's red giant phase might temporarily produce conditions more suitable for the development of life (thick atmosphere, surface liquid water conditions, plentiful visible wavelenth sunlight). Then, after the Sun enters its white dwarf phase, the biosphere could continue in subsurface oceans even after the upper surface freezes to ice. Such a biosphere could sustain itself on hydrothermal vents and cold seeps.

galacsi
2013-Jan-18, 06:06 PM
If a planet is spiralling down toward a red giant nucleus, slowed by the faint atmosphere and the nebula clouds , but not too fast , then may be if we are very lucky , it could arrive in the habitable zone of the white dwarf succeding the red giant. But I wonder what is the roche limit around a white dwarf.

And I found an interesting link inside the Centauri dreams site : http://www.centauri-dreams.org/?p=23911

The comments are great and they bring more info that the OP. They answer my question about roche limit :
The Roche Limit for an Earth density core around a solar mass WD is 600,000 km.

neilzero
2013-Jan-18, 06:43 PM
Shoulld that be an Earth density mantle (not core) around a solar mass? If the 600,000 kilometers still applies for roache limit that is possibly the inner edge of the goldilocks zone. We have however found some extra solar planets inside the roach limit = possible if the planet has sufficient cohesion and adhesion? My guess is the density gradient of the planet is also important for keeping the planet intact inside the roach limit?
Possibly Mars is only too hot for the first million years after our Sun becomes a white dwarf = at 100,000K the white dwarf should cool quickly the first thousand years. I agree the solar wind may double the radius of our inner planets, during red giant and very early white dwarf.
Also the white dwarf from our Sun may have as little mass as 0.7 solar mass which also increases the orbital radius of the planets, dwarf planets, asteroids and comets in our solar system. Please correct, embellish or refute. There seems to be a conflict: Decreasing orbital speed causes increase or decrease of orbital radius: Is spiraling in a different animal? Neil

eburacum45
2013-Jan-18, 07:31 PM
Here's a diagram of the habitable zone around various kinds of white dwarf;
http://www.solstation.com/images/wd2hz.jpg
note that the region where the habitable zone coincides with the tidal disruption zone is confined to the oldest and coolest stars.

I should also point out that the majority of the habitable orbits seem to fall in the range around 0.01 AU, which is around 1.5 million kilometers from the star; somewhat larger than I estimated earlier, and safely outside the Roche limit.

ASTRO BOY
2013-Jan-18, 07:44 PM
If a planet is spiralling down toward a red giant nucleus, slowed by the faint atmosphere and the nebula clouds , but not too fast , then may be if we are very lucky , it could arrive in the habitable zone of the white dwarf succeding the red giant. But I wonder what is the roche limit around a white dwarf.

And I found an interesting link inside the Centauri dreams site : http://www.centauri-dreams.org/?p=23911

The comments are great and they bring more info that the OP. They answer my question about roche limit :


Great link...Thatnks, explains and answers all my problematical questions.
The Roche limit question was one point I never considered.

neilzero
2013-Jan-18, 08:28 PM
It appears that a 5 hour orbit occurs at about 0.005 AU radius times 6.28 = 0.0314 AU circumfrence times 92 million = 2. 9 million miles, circumfrence; divide by 5 = 0.58 million miles per hour that the planet is colliding with the solar wind. Won't that cause some problems? Since the solar wind also has some speed at a 90 degree angle the impact with the upper atmosphere is at about one million kilometers per hour? Also micro meteorites in reverse orbit hit at almost 2 million kilometers per hour. Neil

galacsi
2013-Jan-18, 09:07 PM
It appears that a 5 hour orbit occurs at about 0.005 AU radius times 6.28 = 0.0314 AU circumfrence times 92 million = 2. 9 million miles, circumfrence; divide by 5 = 0.58 million miles per hour that the planet is colliding with the solar wind. Won't that cause some problems? Since the solar wind also has some speed at a 90 degree angle the impact with the upper atmosphere is at about one million kilometers per hour? Also micro meteorites in reverse orbit hit at almost 2 million kilometers per hour. Neil

I don't think there will be solar wind from a WD. See this explanation from this site : http://phys.org/news/2011-03-habitable-planets-white-dwarfs.html

With no nuclear reaction, there really shouldn't be a solar wind to maintain planetary orbits. And there is also the strength of the gravity , about a 10,000 times more than that of the sun, if I compute well.
But on the contrary if a comet or an asteroid hit the star (?) this will could generate an enormous flash of X rays.

So I don't think this a healthy location for a planet.

Van Rijn
2013-Jan-18, 11:19 PM
Here's a diagram of the habitable zone around various kinds of white dwarf;
http://www.solstation.com/images/wd2hz.jpg
note that the region where the habitable zone coincides with the tidal disruption zone is confined to the oldest and coolest stars.

I should also point out that the majority of the habitable orbits seem to fall in the range around 0.01 AU, which is around 1.5 million kilometers from the star; somewhat larger than I estimated earlier, and safely outside the Roche limit.

It's more than tidal disruption. Tidal heating is an issue as well, and this can cause a hot moist greenhouse on a world that would be in the insolation habitable zone. The orbital eccentricity is a factor - higher eccentricity will mean more tidal heating, and you need very circular orbits. See the article I previously linked to, it considers a number of scenarios.

IsaacKuo
2013-Jan-18, 11:48 PM
As for other solar systems where there might be other options, here's a recent paper on some of the issues:

http://arxiv.org/pdf/1211.6467v1.pdf

They argue that the two big issues would be photodissociation causing most of the hydrogen to be lost before the star cools down sufficiently, and that very small variations in the distance to the white dwarf during orbit would cause massive tidal heating, both making habitability very unlikely. They think brown dwarfs (very different subject, of course) might be a little better.
One factor they don't consider is water from comets. Even though Mercury lost all of its water, comet impacts gave it large amounts of water in the places which could accept them (polar craters). A hypothetical planet in a close orbit to a white dwarf would be 1:1 tide locked, meaning the entire dark hemisphere could accept water.

Glacial movement would cause some amount of water ice to migrate slowly into the light, but this would quickly sublimate and refreeze on the dark hemisphere. Some photodissociation will also occur, but this can actually be a good thing since it creates oxygen. Bits of oxygen gas get trapped and submerged into the ice as new ice is constantly layered on top.

Tidal heating can also be a good thing, powering geological activity for hydrothermal vents and cold seeps.

This sort of hot mercury has a dry hot light half but a Europa-like dark half. The dark hemisphere could be a nice place for a biosphere, with plentiful energy from hydrothermal vents and plentiful oxygen from photodissociation.