Habitable Zones Around White Dwarfs

[renormalised]

Picked up a very interesting paper from a link at New Scientist magazine about the possibility of long term habitable zones around white dwarfs, and the possibility of habitable, water bearing Earth-like planets being in orbit about these stars.

http://arxiv.org/abs/1103.2791 (http://arxiv.org/abs/1103.2791)

Seems that the chances look quite good, depending on how the planets got there (be it after WD formation or through migration of the original system's planets). In order to detect transits by Earth sized planets around WD's, long term surveys of upto a minimum of 1000 WD's within 100pc of the Earth would have to be carried out. Only scopes of 1m aperture would be needed to undertake the work. The major advantage of these surveys is that the Earth-like planets would be a large fraction of the size of the star, so the transit depth would be quite pronounced, making their detection much easier than for transits of ordinary stars.

Seems like something that would be worth doing. Might make a good topic for PhD candidates in the field:)

[mswhin63]

Hi Carl,

I remember something during my course about Dwarf stars in that they produce very strong solar storms and thus would make it difficult to achieve life based on this.
I noticed though that these are non nuclear burning and curious that being non nuclear would cause excessive solar storm (limited gravity to hold the activity) or is this information not correct?

I am currently mid term now on holidays from studies and my account is frozen till assessment is complete so I can't access this paper via the library to find out more.

[mswhin63]

ooops forgot these are free downloads :)

[renormalised]

You're thinking of flare stars (late K and M class stars of young age). This is a white dwarf, not a flare star. Most of the stars they're looking at are class DG (4500-6000k, 0.55-0.7Ms) and are stable in their light output. Funny thing, at the temperature these WD's are at, their planets orbiting them will be orbiting a star that is literally rock hard...well, diamond hard:) Much of the structure of the star will be crystalline carbon and oxygen...basically oxygen doped diamond:)

[CraigS]

Sounds reasonable. Would be nice to get some spectroscopic evidence of water, maybe even before looking at light curves (??)
:)
On the other hand, just because a planet is within a zone which might now allow for the presence of liquid water, doesn't necessarily mean that planet actually possesses liquid water, though, eh? Let's face it, the environs around a WD have been through some fairly extreme conditions in order to get where it might be today (ie: un-water-friendly). What sort of probability figures can we assign to the presence of water (in any form) for WDs?
This idea of post WD planetary formation (or capture) might also a bit of a wild-card (read 'stretch').

On the other hand, it seems that every star has been through the protostar phase, where it now appears, is when the water is initially formed. (The latest discovery of huge water jets from L 1448-MM (http://www.physorg.com/news/2011-06-baby-star-blasts-jets-space.html) is one such example). It seems that Earth may have captured most of the water within 1AU of G2V ... but that's about all we presently really know.

Would water molecules linger after a star has gone WD .. that to me, is a key question …

Good idea to go looking for it all, though.
:)
Cheers

[renormalised]

Should be quite a bit of water in the system....even if all of it was lost from the star, any earth like planet should still be quite warm inside, even after 10-12Ga of existence, so it should still be outgassing some water from its interior.

A wildcard....maybe, but certainly not impossible. There's more than enough cast off material to form quite a number of earth-like planets out of a PN. And PN's are not devoid of water...most of the water that's put back into the ISM comes from PN's. So I can't see why water shouldn't linger around.

Most of the WD's they're looking at are way past the PN stage. Most would be at least 3Ga in age and even older. PN's only last 50-100Ka.

[CraigS]

Yep ..
Its interesting how the search is primarily guided by stellar and planetary evolution theory ..

A more empirical approach might be to ask the question .. 'Whereabouts can I detect abundant water molecules ?', and then work backwards (moving more towards where the theory tells us we might find HZ temps).

I wonder whether anyone has ever thought of doing a H2O molecule sky-survey ? :shrug:

Now that would be interesting … :)
:)
Cheers

[renormalised]

That's still open to debate. Given that the earth differentiated very early on in the piece means that there must've been a substantial amount of water present within the body of the planet. Despite all the bits and pieces that went into forming the body of the planet, it was never hot enough to melt right through, initially. Much of the heat which melted the planet came from the radioactive elements that were present in the materials which made it up. But even then, if the Earth were completely dry, it would take a tremendous amount of heat to melt it through and it would take a long time. The presence of water would lower the solidus temperature of the rocks, so they'd melt more easily and the process would be accelerated along because of this. The presence of water would aid in melting the planet and allowing it to differentiate.

Of course, some water would come in via comets etc, but it's how much that was initially present which is the subject of the question. Despite the Earth being where it is in the solar system and the temp here having been higher than the b.p. of water in this region of the solar nebula, that doesn't mean that water wasn't present. Being locked up in dust grains and rocky materials can, in fact, shield the water from the heat and radiation that was present. Plus, the speed at which it was incorporated into planetesimals and then the planets themselves would also become a factor in the survivability of the water.

[renormalised]

Yes, there have been a few and right at present, JCU is actually conducting one in cooperation with a number of other unis...

http://www.jcu.edu.au/eps/disciplines/astronomy/hops/index.htm

[renormalised]

What makes me wonder is what some poor inhabitants of a planet orbiting a WD would think of their little star. Their concept of the passage of time would be awfully skewed...just imagine how quick your birthdays would come around!!!!:):P

[CraigS]

No need to worry about that one … after all, this thread is about finding a zone to inhabit isn't it ? .. Once ya find one, there'd be no reason not to move in ! :P :)

Cheers

[CraigS]

There ya go … the direct approach, eh ?

I think I'll back this one !
:)
Cheers

[renormalised]

Mind you, I'd immediately become 16444 years old, in their years!!!:):P

Don't look so bad for an old fella, then:):P

[CraigS]

Yeah .. well here we go again … if the bulk of the water on Earth was present in this zone before the formation of the planet, then it must've been confined in an extremely tight zone ... given that Venus and, perhaps Mars, missed out on the similar volumes of it. (Ok .. ok .. I know .. there may have been big liquid water on Mars … that's open to debate, as far as I'm concerned, though .. ;) )

Cheers

[renormalised]

Actually, Venus didn't miss out on having a similar volume of water. The D/H ratio in the upper atmosphere of Venus points to it having a similar amount. It's basically lost most of it. Mars would've had a substantial amount as well. You also have to remember that the terrestrial planets also wandered about somewhat during their formation until they entered their present orbits.

It was most likely Venus losing its water that raised the surface temp up to where it is now. Creating a super greenhouse effect due to the presence of large amounts of water vapour in the atmosphere. Especially, once the Sun became hot enough at Venus' distance to evaporate the water quickly enough to drive the enhanced GH mechanism.

[mswhin63]

Thanks for the clarification on the type of dwarf star (red type) The topic was fleeted across and wasn't in the curriculum at that course so I didn't fully take in what was said.

Interesting article too, still reading it as I had to zip off.

[CraigS]

Hmm .. there's a lot more explanations for Venus' present overall temperature that I've read about … eg: no day/night temperature variation, no heat sinking effects due to the exo-thermic reaction between Sulphuric acid and what water exists in the atmosphere, the radiative heating at the surface (perhaps due to volcanism), and then the greenhouse effect.

Explanations which go back in time, would surely have to be debatable, especially if the key assumption is that water existed in some other state other than what we measure as vapour in present-day. This would be the linkage back into solar system formation theory again, eh ?

What's this D/H thing ?

Cheers

[renormalised]

You'll need to do a bit of catchup and further reading:)

Try this book....

Foundations of Astrophysics (http://www.fishpond.com.au/Books/Foundations-of-Astrophysics-Barbara-Ryden-Bradley-M-Peterson/9780321748058?cf=3&rid=2100837973&i=1&keywords=astrophysics), it's $85.56 at Fishpond.com.

[renormalised]

Deuterium to Hydrogen ratio. Depending on the amount of the ratio, the higher the water content of the body. Venus has a ratio of 10000:1. The only way to get a ratio that high is to have a very large content of water. The deuterium is preferentially concentrated in the atmosphere as the hydrogen is carried off after being separated from water molecules via UV radiation.

The build up of the oxygen gets to a point where the surface of the planet becomes superoxidised and that helps drive up the temperatures on the surface. With the large amount of CO2 that was present, it only exacerbated the GH effect, along with the vast amounts of water vapour that became part of the atmosphere of the planet.

There's a small day/night time temp variation on Venus (not enough to write home about....0.01 of a degree or less), but that's hardly enough to be the cause of the heat there. Most of the radiative heating of the planet's surface is due to the trapping of the long IR wavelengths by the atmosphere. Venus appears to be about as volcanically active as Earth...maybe a little less. No plate tectonics at present, either. It probably only contributes about 1% (at the very most) to the heating of the planet. Apart from the extra CO2 being pumped into the atmosphere.

However, it appears that every 500Ma or so, the planet completely resurfaces itself in a gigantic volcanic belch. That would heat the planet up!!!.

If there were any large scale exothermic reaction between what paltry amount of water is there and the sulphuric acid in the clouds, there would be heating of the cloud layers above the temps they presently show. That would be present in the temp profile of the atmosphere. There's most likely some reactions going on, but not enough to add substantially to the heating of the planet.

[ArcaneMagik]

Wien's Law says better wear your sunscreen on that planet.

[renormalised]

Sunscreen doesn't quite cut it:):)

[CraigS]

Done some reading on this aspect since yesterday. Here is a good, snappy paper (http://www.vanderbilt.edu/AnS/physics/astrocourses/AST101/readings/water_on_venus.html) explaining the theory (for those interested, and for the record).

Below is my interpretation of how this all works:

The above statements are valid if one starts with the key assumption that Venus once had a D/H ratio identical to that of Earth. If this ratio was identical, then it is logical to conclude that Venus may have lost 99.9% of its hydrogen. Since terrestrial planets don't have free hydrogen molecules or atoms in their atmospheres, it is also reasonable to conclude that they probably never did. On Earth, rocks rarely contain hydrogen unless the hydrogen is derived from water. In the atmosphere of Solar System planets so far sampled, hydrogen is contained mostly within methane (CH4), ammonia (NH3) and water (H2O). On Venus, neither CH4 or NH3 are abundant, so it is concluded that water is the likely reservoir for hydrogen. Since water vapour forms only about 0.002% of Venus' atmosphere, it is concluded that most of the hydrogen has escaped due to photodissociation (UV zapping) of water which, it is assumed, contains both deuterium and 'normal' hydrogen water in the same ratio as Earth, Asteroids, Comets and proto-planetary disks.

The deuterium atoms are left behind resulting in the disproportionate ratio, measured by the Venus Pioneer probes dropped into Venus' atmosphere. 'Disproportionate' when compared Earth, Asteroids, Comets and proto-planetary disks, that is.

Seeing as all this depends on the D/H ratio being fixed, how fixed is the ratio throughout the so far explored universe ?

As it turns out, they are finding that the D/H ratio may not be a unique criterion to discriminate between the different origins of water on Earth because new theories are explaining that it is possible for different ratios to have occurred in different temperature regions of proto-planetary disks.
On top of this, the remote measurement of deuterium via spectroscopic measurements is controversial, due to inaccurate assumptions in the models used in its determination.

So, my conclusion is that whilst there is rationale surrounding the theory of 'big water' in the past on Venus, this is all based on the assumption that the ratio of D/H on Venus started out the same as for Earth, which may or may not turn out to be as solid an assumption as first thought.

Cheers

[renormalised]

An assumption it is, but it's the best evidence that we have. There's no way at all to find any evidence in the rocks on the surface, because any sedimentary structures which may have been formed on the planet in the past are now long gone. The planet resurfaced itself, remember.

In any case, the bulk compositions of both planets (Earth and Venus) are much the same, so it can be reasonably assumed that they formed in the same, or near to same, parts of the solar nebula. From this, it can be reasonably assumed that the water content, hence the D/H ratios, of both planets was fairly similar.

[CraigS]

Hmm … well … the distribution of D/H ratios can vary by an order of magnitude (or up to almost two orders) throughout a fairly narrow radial distance. Jupiter is about 2.3 x 10^5, Venus is about 1 x 10^4, Earth 10^2 (or 10^3, depending on what paper ya read :) ), so there's quite a variance.

I'm not saying the initial formation of the universe resulted in the variance .. but what happened after that, certainly has resulted in considerable differences.

David Grinspoon (the seeming Godfather of exo-planet atmospheric physics), has also written papers questioning the beliefs about Venus having been wet. (http://www.funkyscience.net/documents/Venus_Wet_87.pdf) Admittedly, they're pretty old, but the recent spectroscopic investigations into protoplanetary disks has also shown quite a lot of variances with line of sight molecular column depths and densities. All this gives support to the cometary deposition theories ( … which I don't particularly swallow easily, either).

They've come up with quite a few chemical reactions which can bump up or destroy the deuterium levels in models, also.

The whole thing is very controversial, so some care is required in making definitive statements about the presence or absence of big water (when extrapolating into the past).

Most of these 'constants in physics' quoted in textbooks, can get outdated fairly rapidly during this 'Golden Age' of astrophysics, so gotta keep up with the trends ol' boy !! ;) :)

Cheers

[renormalised]

Ol' boy!!!!....if I'm old, you must be a fossil:):P

I might go and read what Sara Seager has to say on the matter. She's much better looking than Grinspoon:):P

Anyway, what would an astrobiologist know about exoplanet atmospheres....things might fly in them??!!:):P

[CraigS]

Its a state of mind, dude !! Surfin's the source !
… whaaa ??
:)

Thought that'd be right down your alley ? :P :)
Cheers

[renormalised]

Give me a rock any day....protoplasm's for people who like playing with mucus:):P