Radiobiological Damage in Circumstellar Disks
Published on Oct 9, 2006 at 2:11 pm.
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Filed under astrobiology, extrasolar planets.
Now, that’s a scary looking title!Â
A while back, I posted about the effects of cosmic radiation on airline crew members. You can read that post, but the gist of it is that the higher you go, the less that the atmosphere protects you from radiation. The universe is filled with radiation of all types. Some of that radiation that hits Earth comes from the Sun, and some from outside the Solar System. The galactic cosmic radiation is going to be pretty much the same anywhere in the galaxy unless near a supermassive star or if there has been a supernova nearby.
However, the amount of radiation that an individual planet receives from its parent star depends quite highly on the type of star that the planet is orbiting. Up until recently, no one really thought much about that. However, more and more planets are being found orbiting other stars. This naturally leads to the question, can life live there?
A team of researchers is working on that question right now. Now, most of their work is purely theoretical so far, but it supports some things that I’ve said for some time. The Earth holds a special place in the galaxy.
Stars can be classified in several ways. One of the most useful is based upon the spectra of a star. These spectral classes, in order of coolest to hottest, are T L M K G F A B O. Don’t worry about how they got their letters. I might blog on that sometime in the future. The Sun is a G type star. The T and L stars are some red dwarfs and some brown dwarfs (which are substellar). The B and O stars are unbelievably hot and short lived. M stars are red dwarfs that are very cool and long lived. In fact, M stars are so long lived that none have ever died — they have lifetimes many times longer than the current age of the universe! The hotter stars emit copius amounts of hard ultraviolet light. The cool stars are more gentle with their normal light output, but they tend to undergo massive flares that would blast their stellar systems with heavy radiation for the duration of the flare. The most well behaved stars are the G and K stars.
So, what does this have to do with extraterrestrial life? Well, all life that we know is based on carbon chemistry. There are very good reasons to believe that carbon is special in this regard, and that perhaps all life would be carbon based. Additionally, carbon is fairly common in the cosmos. Carbon in the final end product of nuclear fusion in most stars (after they are done fusing hydrogen into helium, they fuse helium into carbon). One of the things that makes carbon very attractive as a basis of life is that carbon bonds can be very strong, but they can also be easily remade, and bonds between carbon containing molecules can be easily formed and broken. So, it takes very little energy to get the chemistry of life going, and it works at reasonable temperatures. Organic molecules (carbon containing) also are often fairly easily transported using water as a solvent. So, liquid water becomes important to life. Even better, the temperatures at which water is liquid are also pretty much ideal for carbon chemistry necessary for life. It all works out great.
There is one problem, though. Carbon bonds can be broken with high heat and radiation. You use the high heat to break up the molecules and kill germs when you cook food. Radiation can also be used to sterilize food. However, this susceptability to heat and radiation is bad for life if you want it to keep going. A very high fever can damage your body beyond its ability to repair. And, radiation exposure can cause mutations that can ultimately lead to serious health problems. That was the basis of my previous posting (at least the second half of that posting).
But, radiation can also keep life from developing, or kill it off, on extrasolar planets. That is what this team of researchers is studying. They are looking at what types of stars may be suitable candidates for extraterrestrial life. The very hot stars, O and B stars, simply don’t live long enough for life to develop. In some cases, they don’t live long enough for planets to form! And in other cases, they are so bright that they blow away the circumstellar disks around them that would one day form planets, so the planets never get the chance to form. And, they emit so much UV that they’d completely sterilize the entire system anyway. In fact, their findings indicate that anything much hotter than G stars can produce so much UV that the radiobiological damage would be extreme enough to make life very improbable. The coolest F stars might be OK, but nothing hotter.
The very cool stars would be fine, as far as the radiation emited from normal stellar activity, but there is the danger of massive flares. According to their findings, though, the low level of damage normally present outweighs the effects of the flares, so life is just as likely to survive the radiation damage in M, K, and G stars, with slightly better odds in the K stars. However, these cooler stars are so cool that planets would have to orbit VERY close to the stars inorder for water to remain liquid. That complicates matters, because it makes the planets very close to the star when the flares light off, making the radiation exposure far more severe. That cancels the effects of the low normal radiation exposure. Worse, the planets that close to the star would be tidally locked, with one side facing the star (much like one side of the Moon always faces Earth). That would result is horrendous atmospheric conditions, likely preventing life.
So, the result is that you need a G type star, or a hot K or a cool F. In other words, you need a star like the Sun. Only a small percentage of the stars out there are like the Sun, so a life filled planet like Earth is probably not so common.
Oh, and the photo above is of an infrared image of Fomalhaut, showing the circumstellar disk around that star. As it turns out, Fomalhaut is a poor candidate to ever have life on any planets that may form around it. Fomalhaut is a rather hot A type star, and so it blasts its system with lots of ultraviolet light.
-Astroprof
(Image courtesy of NASA)
