It’s not just stars and galaxies that have habitable zones. Some regions of universe are more life-friendly than others
Gamma ray bursts are among the most powerful events in the universe. And although the SGR 1806 -20 flare was relatively mild, it was an unwelcome reminder that life on this planet is constantly threatened by events of unimaginable power.
But here’s the thing: gamma ray bursts are much more common in some parts of the universe than others. That raises the curious prospect that some parts of the universe ae much more inhospitable to life than others. So where are these death zones and what kind of constraints do they place on the origin of life?
Today, Tsvi Piran at The Hebrew University in Jerusalem, Israel and Raul Jimenez at Harvard University in Cambridge say they’ve worked out where in the universe gamma ray bursts are most deadly. As a result, they are able to work out for the first time the universe’s habitable zones.
Gamma ray bursts are a significant mystery—nobody is quite sure how or where the most powerful ones occur. But astronomers know that a burst could do significant damage to the Earth if it happened nearby. The gamma rays would rapidly strip the planet of its ozone layer, leaving the creatures on the surface vulnerable to ultraviolet light and other kinds of high-energy radiation. Indeed, several studies have examined the very real possibility that gamma ray bursts may have brought life on Earth to the edge of extinction on several occasions in the past.
Exactly how often a given planet would be hit by gamma ray bursts obviously depends on its neighbourhood. So the starting point for the work of Prian and Jimenez is to determine how common these powerful events are.
That is more complex than it sounds. Astronomers have found several different types of gamma ray burst, almost certainly produced by cataclysmic events of varying power, such as the death of stars, the collisions between black holes and so on. The 2004 event was yet another type caused by the violent reorganisation of the magnetic field around a neutron star.
The most powerful and therefore the most dangerous are known as long gamma ray bursts generated by the massive supernova explosions when very massive stars die. So the crucial factor in the calculation is the rate at which these explosions occur. This is complicated by the fact that different types of galaxies contain different populations of stars; even various regions in galaxies can contain different populations of stars.
Then there are so-called short gamma ray bursts, which astronomers think are caused when binary stars combine. These are much less powerful but occur more often than long gamma ray bursts.
Finally there are the low luminosity gamma ray bursts which are the least powerful of all and so only observed rarely. However, Prian and Jimenez think they are probably the most common of all gamma ray bursts and just difficult to observe on Earth.
Their main work is to calculate how often all these types of gamma ray burst occur in a given volume of space in a given unit of time. “We use the very recent determination of gamma ray burst rates and luminosity function to estimate the flux of Galactic gamma ray bursts on Earth and compare it with the flux needed to destroy the ozone layer,” they explain.
Their main conclusion is that gamma ray bursts are obviously more common in places where the density of stars is higher. “The stellar density is significantly larger towards the center of the Galaxy and hence the threat to life on most exoplanets,that reside in this region, are much larger,” they say.
Here, gamma ray bursts represent a severe threat to life. Prian and Jimenez calculate the chances of being hit by one lethal gamma ray burst every billion years or so and say this should happen with a likelihood of 95 percent for any star within about 2 kiloparsecs (6500 light-years) of the centre of the Milky Way. “Life can be preserved with certainty only in the outskirts of our Galaxy,” they say.
The Earth is about 8 kiloparsecs (26,000 light years) from the centre of the galaxy and so is within the galaxy’s habitable zone. And the greater the distance, the less risk there is from these types of events. “There are practically no lethal events with a distance larger than 30kpc,” they say.
This argument can be taken even further by looking at the density of galaxies. The Milky Way exists in a relatively sparse region of the universe with few neighbours. The Andromeda Galaxy is our nearest neighbou, and there are various dwarf galaxies nearby such as the two Magellanic clouds.
These objects turn out to be of little threat. Prian and Jimenez say that Andromeda is too far away to be a threat and that the nearer clouds do not have the right kind of star population to bathe the Milky Way in dangerous gamma rays.
However, there are plenty of regions of the universe where galaxies are distributed much more densely. And here, gamma ray bursts in one galaxy can easily prove lethal to life in their neighbours.
Astronomers even have a rough idea of the cosmic distribution of galaxies, saying that it follows a sponge-like structure. This consists of dense knots of galaxies connected by filamentary structures and surrounded by voids where there are a few galaxies. “Galaxies friendly to harbor and preserve life will preferably inhabit low density regions in voids and filaments of the cosmic web,” conclude Prian and Jimenez.
Interestingly, their analysis also places time limits on when life could have evolved. They point out that in the past, the universe was much more dense and galaxies much smaller. This would have significantly increased gamma ray lethality. “We conclude that it is impossible to harbour life [before the universe was about 1/50 of its current age] as long gamma ray bursts will always be sufficiently nearby to life-harbouring planets and thus cause life extinctions,” they say.
Of course, there are numerous uncertainties in this kind of calculation. Perhaps most significant is the question of how much radiation a gamma ray burst must produce to make life extinct on a planet. Prian and Jimenez have used life on Earth as a guide but obviously it is not beyond the realms of possibility that life may have evolved in other parts of the universe that is much more resilient to radiation.
Nevertheless, this is a fascinating study that extends the idea of habitable zones beyond solar systems and galaxies to the universe itself.
Astronomers have long known that the Earth occupies a unique position in the solar system that allows life to flourish. This idea of a habitable zone now allows them to focus search for exoplanets that might also have conditions that are right for life. Now they can take this further by excluding inhospitable regions of the galaxy, and searching only those stars and galaxies that exist in the universe’s habitable zones.
Read more at medium.com