The paper (by Tsvi Piran and Raul Jiminez) was entitled “On the role of GRBs on life extinction in the universe,” and the title referred to gamma-ray bursts. Gamma ray bursts are tremendously strong bursts of gamma rays, the most intense type of radiation. Astronomers detect about one gamma ray burst a day, all from outside our galaxy. A satellite found that these bursts come equally from all directions of the sky, which indicates that they are coming from distant sources outside of our galaxy. The nearest detected gamma ray burst was about 100 million light years away, hundreds of times the length of our galaxy.
Being a very powerful form of energy, gamma ray bursts have probably been a significant trouble maker for the evolution of life in the universe. But how big an effect have they had? Estimating that requires that we estimate two huge unknowns: the rate at which planets get zapped by gamma rays, and the degree to which such radiation bursts might wipe out all or most of the life on a planet.
How Much of a Threat to Life Would Gamma Ray Bursts Be?
First, let us look at the biological effects of different intensities of gamma ray bursts. Following the paper of Piran and Jiminez, we can distinguish between a gamma ray burst that strikes a planet with an energy of 100 kilojoules per square meter (which I'll call for convenience a “class 100” burst), and a burst that strikes a planet with an energy of 1000 kilojoules per square meter (which I'll call for convenience a “class 1000” burst).
Piran and Jiminez say that the “class 100 burst” would cause a 91% depletion of the ozone layer, and that a “class 1000” burst would “wipe out nearly the whole atmosphere.” Piran and Jiminez did not do any independent calculations or research to reach the latter conclusion, because that would involve atmospheric science, which is not their field of study. To support this claim that the “class 1000” burst would “wipe out nearly the whole atmosphere,” they say they are “following Thomas,” and cite two papers by Brian C. Thomas and others, one here, and the other here.
But, in fact, this seems to be a faulty reference, a case of claiming that a scientific paper says something that it doesn't actually say. I examined these two papers by Brian C. Thomas and the rest, and I could find no statement that a gamma ray burst of the “class 1000” type (with an energy of 1000 kilojoules per square meter) would wipe out almost the whole atmosphere. To the contrary, on page 11 of the paper “Gamma-Ray Burst and the Earth: Exploration of Atmospheric, Biological, Climatic, and Biogeochemical Effects,” the authors say this: “Our model is probably not capable of correctly handling the chemistry, heating, and transport for a burst of much higher fluence than 100 kJ m2 [kilojoules per square meter].” That's a level of energy ten times less than the “class 1000” case.
The paper by Thomas and the rest discusses mainly lesser effects such as ozone depletion, and says that the ozone depletion would only last for a decade. The paper also makes clear that the ozone depletion would only be globally catastrophic in the worst way if the gamma ray burst occurred over the equator. Their paper says:
Note that for polar bursts, even in the long term, effects are isolated to the respective hemisphere. Similarly, even for bursts at 45 the opposite hemisphere experiences much less intense effects.
These points are not trivial to the matter being discussed here (whether gamma ray bursts make only 10% of galaxies habitable), because the case of Piran and Jiminez depend heavily on this claim that a “class 1000” burst would “wipe out nearly the whole atmosphere.” In fact, they do not supply any citation or research that backs up such a claim, which is not actually stated by the papers they cite.
I may also note that any galaxy would have plenty of planets or moons that could take the “class 1000” gamma ray hits with relatively little harm. Examples of such places would include planets with thicker atmospheres (which could afford to lose a large fraction of the original ozone and oxygen), and life-bearing moons rotating gas giants, which might be entirely shielded from gamma ray bursts by the gas giants they rotated around. There would also be many planets that would have an inclination or tilt that would result in the gamma ray burst not striking in the equatorial region but in some other region where much less damage would be done. Also, many planets would be protected by dust clouds lying between the planets and the gamma ray bursts.
Cosmic dust (the dark brown bands) tends to block gamma rays
What about the less lethal “class 100” gamma ray bursts? They would have the effect of removing some of the ozone layer in an atmosphere, which would temporarily increase the amount of ultraviolet radiation. The paper by Thomas and others says that this ozone depletion would only last for about a decade. That would be a setback for the evolution of life, but there is no reason to think that it would wipe out even 100 million years of evolutionary progress. Such a “class 100” gamma ray burst might cause a mass extinction, causing many species to become extinct. But such an event would still very probably leave many advanced species surviving. In fact, these “class 100” gamma ray bursts have little relevance to whether a galaxy is habitable.
How Often Would the “Class 1000” Bursts Occur?
Now, let's look at how often these events would occur. On page 5 of their paper, Piran and Jiminez say, “In total 90, 40 and 5% of the exoplanets in the MW would be exposed to a fluence of 10, 100, and 1000 kJ/m2 from GRBs within a period of 1 Gyr.” Here MW refers to the Milky Way galaxy, 100 refers to the “class 100” gamma ray bursts previously discussed, 1000 refers to the “class 1000” gamma ray bursts previously discussed, and 1Gyr refers to a period of a billion years. These numbers are guesses, since we don't really know for sure what causes gamma ray bursts.
These numbers are not particularly gloomy from the standpoint of the prospects of the evolution of life. According to these numbers, there is a chance of only 5% per billion years that a particular planet in our galaxy would get the “really bad” type of “class 1000” gamma ray burst during a billion year period. There would be a 40% chance of a planet in our galaxy getting a “fairly bad” blast from a “class 100” gamma ray burst, but such a blast would be unlikely to set back evolution on that planet by very much.
Let's also remember that Thomas and the others make clear (in the previously cited paper) that gamma ray bursts are only really devastating if they occur over the equator of the planet, and the chances are 80% that such a blast would not occur over the equator of any particular planet. That means the chance of a planet getting zapped really bad by gamma rays is not the 5% per billion years that Piran and Jiminez cite, but an actual net effective chance of only 1% per billion years.
Also, since different solar systems have different planes of inclination, and different planets have different tilts, even the worst type of gamma ray burst would never wipe out life throughout all the planets in a large fraction of a galaxy, because only some of those planets would experience the gamma ray burst above their equators.
Given also the fact (as I discussed before) that Piran and Jiminez have only a faulty reference to back up their insinuation that the “class 1000” gamma ray burst would wipe out most of the atmosphere of a planet, overall these conclusions fail to back up any conclusion that gamma ray bursts are an overwhelming obstruction to the evolution of life in this galaxy or any other large galaxy.
So given that Piran and Jiminez state in the abstract of their paper the gloomy conclusion that “life can exist in only ~10% of galaxies,” how do they justify that conclusion? Very simply, they don't. Oddly, this conclusion is made in their abstract at the beginning of their paper, but is never actually derived or deduced or mentioned in the body of their paper (which violates the rule that abstracts are only supposed to summarize conclusions made and justified in the paper). In their last paragraph, their paper merely says, “We have found that GRBs and in particular LGRBs are life threatening in a large part of the Milky Way as well as in many other locations in the Universe.” But that's a much, much weaker conclusion than the claim that such gamma ray bursts are such a big factor that that they prevent life in 90% of the universe's galaxies.
There is actually no basis for such a claim. Given the wide variety of different habitats that life could have (such as a planet with a thick atmosphere or a moon shielded from gamma rays by the gas giant it orbits or a planet protected by cosmic dust clouds), given the lack of any scientific papers showing that typical gamma ray bursts will wipe out a planet's atmosphere, given the quick recovery time of a decade following ozone depletion by a gamma ray burst, and given only about a 20% chance of a particular gamma ray burst occurring over any particular planet's equator, we have no good basis for concluding that any substantial fraction of the universe's larger galaxies cannot evolve life because of gamma ray bursts.
Trackback: http://arxiv.org/trackback/1409.2506
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