How dangerous is UV irradiation for life on extrasolar planets ?

[galacsi]

There are stars much hotter than the Sun that emit much more UV. I have often read that life would be impossible on a planet around such stars. But from the little of research I have done , it is not so sure.
If there is Oxygen in the atmosphere , the UV will generate Ozone and the more UV the more Ozone . I read photon whose wavelenght is < 240 nm are the one producing ozone. Ozone filter UV of wavelength < 320 nm , the more dangerous to us.

Clouds and aerosols also protect from UV , so may be , with an atmosphere thick enough ,even with a star light Sirius and for for an earth like planet orbiting it in the life zone , there could be no problem with UV ?

I don't know what to think. So I start this thread to listen to other opinion on the subject.

Galacsi.

PS : Sorry for my English.

[korjik]

Since we dont know for sure how and where life actually originated on Earth, it is hard to say without any certainty how a big UV flux would affect developing life.

Now if you are talking established life, then with the right type of skin and with eye protection, even a high UV flux would be livable.

[Ilya]

The biggest problem with stars much hotter than Sun is not UV radiation -- it is their lifespan. The more massive a star is the hotter it is, and the faster it uses up its hydrogen. A star which emits "much more UV than Sun" does not last long enough for complex life to evolve, as far as we know. Even though simple life in the oceans would be safe from UV -- several meters of water is an adequate shield.

[antoniseb]

You have noted that the answer depends on the atmosphere... which is right.
Imagine an Earthlike planet with 1000 times as much UV coming into the atmosphere. What would be the impact of that UV on life near the deep-sea volcanic smokers? Probably not much, even though any dryland creature with living skin would probably die of sunburn pretty quickly.

[DrRocket]

Now if you are talking established life, then with the right type of skin and with eye protection, even a high UV flux would be livable.


http://1.bp.blogspot.com/_hSuyrDde4nw/Rje4my3UjQI/AAAAAAAAAPo/HG7AaIsY7YA/s320/oh+deer,+nice+sunglasses.jpg

[galacsi]

Since we dont know for sure how and where life actually originated on Earth, it is hard to say without any certainty how a big UV flux would affect developing life.

Now if you are talking established life, then with the right type of skin and with eye protection, even a high UV flux would be livable.

Yes , I was talking about established life , like ours if we manage , one day , to colonize the stars.

[galacsi]

The biggest problem with stars much hotter than Sun is not UV radiation -- it is their lifespan. The more massive a star is the hotter it is, and the faster it uses up its hydrogen. A star which emits "much more UV than Sun" does not last long enough for complex life to evolve, as far as we know. Even though simple life in the oceans would be safe from UV -- several meters of water is an adequate shield.

But a star like Sirius have a life span of one Billion years and it is a long long period of time.

And it is true Rigel has a much shorter life span. From a colonization point of view this is not a problem. Would you really care that in one or two million years your star is going supernova ? Me , not at all . A million year or a billion years s it is just a scientific number without any signification on the human and even historic scale .

[galacsi]

You have noted that the answer depends on the atmosphere... which is right.
Imagine an Earthlike planet with 1000 times as much UV coming into the atmosphere. What would be the impact of that UV on life near the deep-sea volcanic smokers? Probably not much, even though any dryland creature with living skin would probably die of sunburn pretty quickly.

AS there is about 30% UV in the sun light , if I am not wrong , 1000 times the amount of UV is much too much even for the deep sea creatures ! Their seas are long gone , boiled into space.

[WayneFrancis]

If a star was going to go super nova within a million years I wouldn't want to be anywhere near it. I'm not sure how well our estimates of when a star will actually explode are and being anywhere in the neighbourhood of a star at the end of its life sound a bit risky.

[swampyankee]

I believe that there have been some papers (Kasting? Antigona Segura?) about the level of UV on the surface of Earth-like planets around other stars. iirc, the rather counter-intuitive result is that the UV flux on the surface of an Earth-like planet around an F or B star would be less than it would be on the same planet around a G or K star.

[George]

AS there is about 30% UV in the sun light , if I am not wrong , 1000 times the amount of UV is much too much even for the deep sea creatures ! Their seas are long gone , boiled into space.Where did you see that the Sun produces 30% UV? I think this is much to high, not that it changes things in this topic.

[George]

I believe that there have been some papers (Kasting? Antigona Segura?) about the level of UV on the surface of Earth-like planets around other stars. iirc, the rather counter-intuitive result is that the UV flux on the surface of an Earth-like planet around an F or B star would be less than it would be on the same planet around a G or K star.That does seem counter-intuitive. Assuming the same size and type of Earth atmosphere, and we want roughly 1000w m-2, then around F or B stars, the planet would have a more distant orbit. But, there would be a larger percentage of this wattage that would be in the UV since the Sun's peak is not in the UV. [It's photon flux peak is yellow (ug), but the peak is really just barely a pimple. :)]

[George]

http://1.bp.blogspot.com/_hSuyrDde4nw/Rje4my3UjQI/AAAAAAAAAPo/HG7AaIsY7YA/s320/oh+deer,+nice+sunglasses.jpg :cool: Oddly enough, white-tail deer are able to see things in near UV, but they can't see our reds, and probably not our oranges. Bananas, yes they can. ;)

[eburacum45]

I believe that there have been some papers (Kasting? Antigona Segura?) about the level of UV on the surface of Earth-like planets around other stars. iirc, the rather counter-intuitive result is that the UV flux on the surface of an Earth-like planet around an F or B star would be less than it would be on the same planet around a G or K star.

I'm having difficulty imagining how this could be so. Increased UV would destroy the water, drying the planet out somewhat but increasing the oxygen level and therefore the ozone. Perhaps that would cut down the UV enough to make the surface comfortable, if a little dry.

[galacsi]

I believe that there have been some papers (Kasting? Antigona Segura?) about the level of UV on the surface of Earth-like planets around other stars. iirc, the rather counter-intuitive result is that the UV flux on the surface of an Earth-like planet around an F or B star would be less than it would be on the same planet around a G or K star.

Very interesting Swampman ! This is exactly my question and what I was wondering about.

[galacsi]

Where did you see that the Sun produces 30% UV? I think this is much to high, not that it changes things in this topic.


Well the short answer , is I don't know , I thing I did a mistake , thinking ,the difference between the solar constant at top of the atmosphere and the solar constant at sea level , to be purely composed of UV.

The solar constant on top of the atmosphere is on average 1366.1 W·m-2. this contain UV , visible light and IR.
98,7% of the UV are blocked by the atmosphere but what is the part of UV in the sun productionof light ? I am currently browsing the net to find a figure or a composition table or something , but for the moment : no joy ! :mad:

On the bright side , that the UV part is 10 or 30% , does not change much what I was saying to Antoniseb.

PS : Wikipedia have a nice picture of the energy/wavelength composition and you can see the UV part is important.

http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

But if you want somenumbers you have to do the integration manually !

[swampyankee]

I found a paper about UV fluxes on the surface of Earth-like planets with the same overall level of solar insolation. If I find any papers on the same topic, either in arXiv or in a refereed journal, I'll try to post the links re.

It's by Antigona Segura and others (http://www.geosc.psu.edu/~kasting/PersonalPage/Pdf/Segura_et_al_Astrobiology_03.pdf).

(Sorry, but I just love her first name. I have fond memories of the play Antigone)

Added in edit: Once again, my memory played me wrong. Dr Segura's paper showed that an Earth-like planet orbiting a K2V or F2V star would have lower levels of UV flux given the same level of insolation as received by the Earth.

[George]

PS : Wikipedia have a nice picture of the energy/wavelength composition and you can see the UV part is important.

http://en.wikipedia.org/wiki/File:Solar_Spectrum.png

But if you want somenumbers you have to do the integration manually ! You can get many of the daily Solar SEDs from SORCE (http://lasp.colorado.edu/sorce/data/ssi_data.htm).

The Wiki graph is nice, but I see they do not accurately show the 98.7% UV extinction, which I think is a correct extinction value.

[galacsi]

I found a paper about UV fluxes on the surface of Earth-like planets with the same overall level of solar insolation. If I find any papers on the same topic, either in arXiv or in a refereed journal, I'll try to post the links re.

It's by Antigona Segura and others (http://www.geosc.psu.edu/~kasting/PersonalPage/Pdf/Segura_et_al_Astrobiology_03.pdf).

(Sorry, but I just love her first name. I have fond memories of the play Antigone)

Added in edit: Once again, my memory played me wrong. Dr Segura's paper showed that an Earth-like planet orbiting a K2V or F2V star would have lower levels of UV flux given the same level of insolation as received by the Earth.

Great link SY , I have bookmarked it.