Why would Proxima b receive UV and X-ray 100 times stronger than on Earth?

[Piero]

I just read this article at lunch: http://phys.org/news/2016-10-planet-star-nearest-sun-oceans.html . It's about Proxima b, a Earth-like planet orbiting around the red dwarf star Proxima Centauri at an approximate distance of 7.5 million km.

The article is largely about the likelihood that this planet might have oceans. Besides this, there is a figure reporting that UV and X-rays would be 100 times stronger. I am not a physicist, so please I apologise if this may seem a stupid question. Could you explain me why, please? 

What it puzzles me is that although it is true that this planet is closer to its star "about a tenth the distance of innermost planet Mercury from the Sun" (which can explain why UV and X-rays are stronger), this star is also 0.12 times the mass of our Sun and "1000 time weaker" than our Sun (which would make its irradiation weaker..).

Piero

[Dave S]

Hi Piero

Can't go into the mathematics as I have to go out.  So I'll have to give the simple version.

It revolves around a principle called the inverse square law which says  the intensity of the energy equals 1/distance^2  (distance*distance)

Proxima B is 4.6million miles from the star and earth is 93 million miles from the sun which makes it a twentieth of the distance.

Using the inverse square law makes the intensity 400 times stronger IF IT ORBITED OUR SUN.

As Proxima centauri is 1000 times weaker it means it would receive 400/1000 the amount of energy which equates to 0.4 as much energy which is less than a half of what the earth gets.

However,  all starts have a core where the energy is produced surrounded by a shell of hydrogen.  The energy the core produces is all gamma, X-ray and UV.

This energy is absorbed by the hydrogen shell and a lot of it is then re-radiated as light and heat.  Our sun has a bigger shell of hydrogen so absorbs a lot more X-ray and UV than a smaller star like Proxima Centauri which has far less of a shell of hydrogen than our sun.

So even though Proxima B only receives about half the energy than the earth does. It receives many times the amount of X-ray and UV than the earth does because it is not converted to light and heat hence the quoted figure of 100 times more.

Hope I haven't confused you even more.

 

Dave...

 

[Piero]

@Dave S You haven't confused at all! I wasn't aware of the layer of hydrogen absorbing UV and X-rays on stars!

Thanks a lot for your explanation, it makes sense now!  

Piero   

[George Jones]

Piero, as an elaboration on the post by Dave S, you might try the "Characteristics" section of

https://en.wikipedia.org/wiki/Proxima_Centauri

[Piero]

Thanks @George Jones ! Interesting description!

I suppose that unless Proxima b has a very think layer of atmosphere to filter that large amount of irradiation from the star, it is impossible for that planet to host life, isn't it? There is also the problem of persistent and long flares from the star. Really intriguing!