Discovering Exoplanets: The Benefits of Studying White Dwarfs

[skydivephil]

What do you think is the best type to find planets in the habitabel zone? What do you think of the suggetsion of white dwarfs?
http://news.softpedia.com/news/White-Dwarfs-Are-Newest-Targets-for-Exoplanet-Research-192407.shtml
any better suggestions?

 

[qraal]

What do you think is the best type to find planets in the habitabel zone? What do you think of the suggetsion of white dwarfs?
http://news.softpedia.com/news/White-Dwarfs-Are-Newest-Targets-for-Exoplanet-Research-192407.shtml
any better suggestions?

White-dwarfs are unlikely places for Earths to form, but the long CHZ of 8 billion years is longer than our Sun's. Red dwarfs have longer lifespans - trillions of years - and are probably to be preferred.

 

[zhermes]

to "Find" planets in the habitable zone, brown-dwarfs are currently the best bet. Their small size and low-mass allows fairly easy detection via both the transit and radial-velocity techniques. Additionally, because they are colder and dimmer, the habitable zone is much closer in (again helping find them). The question of what type of star has the best chance of having planets in the habitable zone is much more complicated---and we don't really know for sure.

 

[Astronuc]

What is "Best star for exoplanet?" is a different question than "What do you think is the best type to find planets in the habitabel zone?" The qualifier of "habitable zone" is a significant constraint.

Now consider the following!

The AP (1/12, Borenstein) reports, "Three studies released Wednesday, in the journal Nature and at the American Astronomical Society's conference in Austin, Texas, demonstrate an extrasolar real estate boom. One study shows that in our Milky Way, most stars have planets." Arnaud Cassan of the Astrophysical Institute, who conducted the study, examined the amount of planets found using "several South American, African and Australian telescopes" and calculated that there should be about 1.6 planets per star, a figure that Cassan thought should be higher if combined with findings from the Kepler telescope.

. . . .

Bold font is mine for emphasis.

In addition to "habitable zone" around a star, one must consider the "habitability" of the planet - atmosphere (oxygen), water, solid (elements/minerals), . . . .

 

[Ophiolite]

I find most discussions relating to the habitable zone tend to gloss over the stability of the planetary orbit, ensuring it remains within the habitable zone, and the 'range' of the zone, given changes in stellar output over the lifetime of the star. These reduce, by some factor, the number of planets that will remain in the habitable zone long enough for complex life to develop.

As Astronuc points out just having a planet there is in itself insufficient. There are many other factors to consider. We do not know if the stabilising effect of the moon on variations in the Earth's axial tilt have been essential, important, or irrelevant to the origin and evolution of life. We do not know if plate tectonics is a necessary feature of a habitable planet and how common Earht style plate teconics may be. And so forth...

 

[Nik_2213]

There was a recent article (linky) which suggests that red-dwarf stars will quickly square-up the tilt of planets in their habitable zones. This suggests that stars cooler than K5 should be avoided.

At the other end, stars hotter than F5 rotate rapidly, (linky) suggesting that they have not off-loaded their spin to planets.

So, that leaves F5~~K5 as contenders.

 

[Super Luminal]

White-dwarfs are unlikely places for Earths to form, but the long CHZ of 8 billion years is longer than our Sun's. Red dwarfs have longer lifespans - trillions of years - and are probably to be preferred.

With all due respect, it is my understanding that the universe is approximately 14 billion years old, so it confuses me how you can state that red dwarfs (or ANYTHING for that matter) have life spans that extend into trillions of years...So am I wrong, or are you?

 

[Drakkith]

With all due respect, it is my understanding that the universe is approximately 14 billion years old, so it confuses me how you can state that red dwarfs (or ANYTHING for that matter) have life spans that extend into trillions of years...So am I wrong, or are you?

Based on the calculations of fuel burnup inside a star and how it is related to the mass and composition of the star along with its energy output let's us calculate the lifetime.
The short explanation is that more massive stars (compared to the sun) burn up their fuel exponentially faster, while lower mass stars do so exponentially slower. A star of 2 solar masses, such as Sirius, puts out 25 times more energy than our own sun. The star HD 179930 is 0.59 solar masses and only puts out 12% of the Suns energy.

 

[Chronos]

The fuel consumption rate of a star appears fairly irrelevant. I think any star class A, or higher, is hostile to any planet with biological life aspirations. And planets of M class stars [or lower] probably become tidally before life can evolve to the point of significant adaptive ability. Single cell organisms surely exist beyond earth, but, fascinate few - aside from biologists. Hell, we are looking for Yoda, not another refrigerator parasite.

 

[phyzguy]

The fuel consumption rate of a star appears fairly irrelevant. I think any star class A, or higher, is hostile to any planet with biological life aspirations. And planets of M class stars [or lower] probably become tidally before life can evolve to the point of significant adaptive ability. Single cell organisms surely exist beyond earth, but, fascinate few - aside from biologists. Hell, we are looking for Yoda, not another refrigerator parasite.

Why would tidal locking be hostile to life? As long as the temperatures stay reasonable, I don't see a problem with it. In fact, you could argue that a tidally locked planet, where the temperatures are more constant, is more suitable for life than a planet with a day-night cycle.

 

[lpetrich]

I'd posted on white-dwarf planet habitability in post 5 of Habitable exoplanets of white dwarfs.

A habitable planet would have to be awfully close to a white dwarf, about 0.01 AU, and its habitable time would be about 0.6 times its age at the middle of that time.

 

[Astronuc]

NASA's Kepler Announces 11 Planetary Systems Hosting 26 Planets
http://www.nasa.gov/mission_pages/kepler/news/new-multi-systems.html

The planets orbit close to their host stars and range in size from 1.5 times the radius of Earth to larger than Jupiter. Fifteen of them are between Earth and Neptune in size, and further observations will be required to determine which are rocky like Earth and which have thick gaseous atmospheres like Neptune. The planets orbit their host star once every six to 143 days. All are closer to their host star than Venus is to our sun.

"Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky," said Doug Hudgins, Kepler program scientist at NASA Headquarters in Washington. "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."

. . . .

NASA's Kepler Mission Finds Three Smallest Exoplanets
http://www.nasa.gov/mission_pages/kepler/news/smallest-exoplanets.html

 

[Astronuc]

Potentially Habitable Planet Detected Around Nearby Star
http://news.yahoo.com/potentially-habitable-planet-detected-around-nearby-star-050641876.html

A sun-like star in our solar system's backyard may host five planets, including one perhaps capable of supporting life as we know it, a new study reports.

Astronomers have detected five possible alien planets circling the star Tau Ceti, which is less than 12 light-years from Earth — a mere stone's throw in the cosmic scheme of things. One of the newfound worlds appears to orbit in Tau Ceti's habitable zone, a range of distances from a star where liquid water can exist on a planet's surface.

. . .

 

[DHF]

Why would tidal locking be hostile to life? As long as the temperatures stay reasonable, I don't see a problem with it. In fact, you could argue that a tidally locked planet, where the temperatures are more constant, is more suitable for life than a planet with a day-night cycle.

The spin of our planet is responsible for more then just the night and day cycle, our planet's spin determines a whole host of conditions which all add up for favorable conditions to life. There are many articles detailing the fairly hostile conditions our planet would sport if it slowly stopped spinning, here is one that sums things up pretty clearly.

http://science.howstuffworks.com/science-vs-myth/what-if/what-if-earth-stopped-spinning.htm

there is also a very interesting video on you tube that originally aired on the discovery channel I think. look for "AFTERMATH - When The Earth Stops Spinning "

Don

 

[fm.illuminatus]

White-dwarfs are unlikely places for Earths to form, but the long CHZ of 8 billion years is longer than our Sun's. Red dwarfs have longer lifespans - trillions of years - and are probably to be preferred.

Red dwarfs are too volatile, often experiencing huge changes in output or massive flares that would sterilize otherwise habitable worlds. Further, planets close enough to be in the habitable zone are usually tidally locked, which would create permanent temperature extremes that are difficult for habitability. Our best bet for habitable planets is something relatively close to a sun-like star, essentially for all the reasons that the sun itself is an ideal star for supporting a habitable world.

 

[vemvare]

Another problem with red dwarfs is that the proportion of its output that is visible light will be lower. IR-photons aren't terribly useful in driving chemical reactions.

If enough visible light to "run a biosphere" hits the planet, then it is likely to be too hot. If the star is also close enough for its tidal effects to significantly heat the interior of the planet then that will lead to higher volcanism, which means higher CO₂ output, then the problem is double-sided since the temperature will be even higher.

 

[snorkack]

Another problem with red dwarfs is that the proportion of its output that is visible light will be lower. IR-photons aren't terribly useful in driving chemical reactions.

If enough visible light to "run a biosphere" hits the planet, then it is likely to be too hot.

IR photons are not terribly useful, but visible may be.

Here on Earth, there are 2 major conditions with limited visible light.

For one, water at intermediate depths. Water is blue, red and yellow are strongly absorbed, blue penetrates to somewhat bigger depth.

That is where red algae grow.

For the other, the forest understory. The upper branches capture most light, and mostly green light passes below. The understory has various types of plants - grasses, flowers, moss, bushes, lower branches of the same trees whose upper branches capture the full sunlight, young tree saplings...

There is a curious contrast between how plants adapt to these two conditions.

In deep water, red is absent, but blue is relatively abundant. Plants adapt their photosynthesis: red algae make no use of red, and reflect it if it does reach them.

Under shadow of other plants, blue is in short supply, and green is relatively abundant.
What is curious is that plants do NOT adapt - the understory plants and leaves lay be slow growing but they are just as green as the plants in full light, making use of the little blue and red light that leaks through the canopy, instead of finding an use for the abundant green light rejected by other plants.

Here on Earth we do not have places with limited (but nonneglible) supply of blue light and abundance of red. How would plants adapt, or not adapt, on red dwarf planets?