Comment on Atmospheric Biosignatures and Habitability


The following post is based (in part) upon an article that appeared in the journal Astrobiology—Grenfell, et al. Volume 12, Number 12, 2012, Response of Atmospheric Biomarkers to NOx-Induced Photochemistry Generated by Stellar Cosmic Rays for Earth-like Planets in the Habitable Zone of M Dwarf Stars It is available behind a paywall.

A major reason for life on our planet is the interactions between the Earth and the Sun. And, I am certain that many of us ponder whether life would be the same if our Sun were slightly different. I will try to imagine how life may be different on a cooler and smaller star—an M class dwarf. (For anyone who desires more information about stars—I will provide links/references at the end of post)

Technically speaking, astronomers classify stars in many types of schemes—however I will attempt to simplify the scheme with only what is needed in the post.

Our Sun falls into the G spectral class—and our Sun is regarded as a main sequence yellow dwarf. The next diagram goes a long way in delineating what the term “main sequence” means.

Fig. 1 classifying stellar types in table format via Atlas of the Universe URL http://www.atlasoftheuniverse.com/wstartype.gif

It is estimated that M-class dwarf stars far outnumber its larger G-class dwarf—our Sun. A quick glance at the table above indicates that G-class stars account for 3.5% of the total stars and the M-class dwarf stars account for 80% of the total stars. One reason is their respective lifetimes—our Sun has a lifetime of 10 billion years while an M-class dwarf has a lifetime of 200 billion years.

However, back to the subject at hand, what would a planet in a habitable zone of an M-dwarf star be like? Could we possibly find a home for ourselves in the far future at in an M-class star solar system?

Let’s see what the current findings may say about it.

First, how big is a typical dwarf star relative to our Sun? Taking a look, an M-class dwarf is approximately 1/3 the size of a typical G class dwarf (our Sun).

Fig.2 Comparison of our Sun against typical M class dwarf star

Original Authors: RJHall (original source) / Kevin Heider (this version) Source Wikipedia

URL link http://en.wikipedia.org/wiki/File:SunGliese581.png#filelinks

 
From Source---

== Summary == {{Information |Description={{en|1=The relative size of the Sun and red dwarf Gliese 581.}} |Source=Based off of File:Alpha centauri size.png |Author=RJHall (original source) / [[:en:User:Kheider|Kevin

According to the cited journal which I am utilizing in the post, an M class dwarf star (i.e. Gliese 581) may bombard the planet in its “habitable zone” with an inordinate amount of stellar cosmic rays. Or, in short, there may not be a suitable “habitable zone” for us to populate. Strictly speaking the habitable zone within an M-class dwarf star may be too quickly rendered desolate and inhospitable for life. Why?

Let’s first try to envision this “goldilocks” planet as one which (similar to ours) possessed an ozone layer. Recalling that when our planet undergoes a bombardment by periodic stellar cosmic rays, the beautiful aurora appears. The ozone layer protects us from a great deal of harm from the coronal mass ejections and other harmful cosmic rays. The protective layer of ozone is slowly being depleted every time there is a coronal mass ejection (stellar cosmic rays). As the aforementioned journal article speaks of, there is the danger of completely losing the protective layer of ozone under certain adverse conditions. If the ozone cannot be replenished then there is the danger that it may completely disappear.

The caveat stems from a poor understanding of how or if all M-class dwarf stars produce copious amounts of harmful stellar cosmic rays. One study of a different M class dwarf, Gliese 388—also known as AD Leo undergoes approximately 36 coronal mass ejections per day. At that count, all protective ozone would be removed quickly.

What does that mean for our wayward settlers? Well—don’t go there? But, there is much more—it may not be known how the other M class dwarf (Gliese 581) would change or could change into one that emitted copious stellar rays. Or, for that matter any M class dwarf . . . at the current state of knowledge.

What may appear as a benign planet with a (?) breathable atmosphere may turn into one that was not so benign.

REFERENCES and LINKS

On the Sun and stars

There is a good deal on our Sun and our Solar System from the text: Encyclopedia of the Solar System 2nd ed, 2007, Academic Press Publishers. Edited by Lucy-Ann McFadden, Paul R. Weissman, and Torrence V. Johnson.

The following website from NASA is excellent

URL


http://solarsystem.nasa.gov/planets/

Wikipedia may present a good reference on the stars—URL

http://en.wikipedia.org/wiki/Star

In regards to M class dwarf stars and coronal mass ejections: (behind a paywall)

Khodachenko, M.L., Ribas, I., Lammer, H., Grießmeier, J.-M.,

Leitner, M., Selsis, F., Eiroa, C., Hanslmeier, A., Biernat, H.K.,

Farrugia, C.J., and Rucker, H.O. (2007) Coronal mass ejection

(CME) activity of low mass M stars as an important factor for

the habitability of terrestrial planets. I: CME impact on expected

magnetospheres of Earth-like exoplanets in close-in

habitable zones. Astrobiology 7:167–184.

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